面向对象三大特征
〇、三大特征总览
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一、封装（Encapsulation）
1.1 概念定义
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1.2 访问修饰符
| 修饰符 |
同类 |
同包 |
子类 |
不同包 |
使用场景 |
private |
✅ |
❌ |
❌ |
❌ |
属性、内部方法 |
(default) |
✅ |
✅ |
❌ |
❌ |
包内工具类 |
protected |
✅ |
✅ |
✅ |
❌ |
允许子类访问 |
public |
✅ |
✅ |
✅ |
✅ |
对外公开的接口 |
可见性范围图：
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</mxCell>
<mxCell id="5" value="public" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=top;fontSize=20;fontColor=#FFFFFF;fontStyle=1;fontFamily=Courier New;" vertex="1" parent="1">
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</mxCell>
<mxCell id="6" value="" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#F59E0B;strokeColor=#D97706;strokeWidth=4;shadow=1;" vertex="1" parent="1">
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</mxCell>
<mxCell id="7" value="protected" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=top;fontSize=20;fontColor=#FFFFFF;fontStyle=1;fontFamily=Courier New;" vertex="1" parent="1">
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</mxCell>
<mxCell id="8" value="" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#3B82F6;strokeColor=#1D4ED8;strokeWidth=4;shadow=1;" vertex="1" parent="1">
<mxGeometry x="140" y="200" width="620" height="300" as="geometry" />
</mxCell>
<mxCell id="9" value="default (包访问权限)" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=top;fontSize=20;fontColor=#FFFFFF;fontStyle=1;fontFamily=Courier New;" vertex="1" parent="1">
<mxGeometry x="160" y="210" width="280" height="30" as="geometry" />
</mxCell>
<mxCell id="10" value="" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#EF4444;strokeColor=#DC2626;strokeWidth=4;shadow=1;" vertex="1" parent="1">
<mxGeometry x="180" y="250" width="540" height="210" as="geometry" />
</mxCell>
<mxCell id="11" value="private" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=top;fontSize=20;fontColor=#FFFFFF;fontStyle=1;fontFamily=Courier New;" vertex="1" parent="1">
<mxGeometry x="200" y="260" width="120" height="30" as="geometry" />
</mxCell>
<mxCell id="12" value="🔒 只有本类可见" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;fontSize=18;fontColor=#FFFFFF;fontStyle=1;" vertex="1" parent="1">
<mxGeometry x="180" y="320" width="540" height="80" as="geometry" />
</mxCell>
<mxCell id="13" value="📦 同包可见" style="text;html=1;strokeColor=none;fillColor=none;align=right;verticalAlign=bottom;fontSize=16;fontColor=#FFFFFF;fontStyle=1;" vertex="1" parent="1">
<mxGeometry x="540" y="460" width="200" height="30" as="geometry" />
</mxCell>
<mxCell id="14" value="👨👩👧 同包 + 子类可见" style="text;html=1;strokeColor=none;fillColor=none;align=right;verticalAlign=bottom;fontSize=16;fontColor=#FFFFFF;fontStyle=1;" vertex="1" parent="1">
<mxGeometry x="540" y="500" width="240" height="30" as="geometry" />
</mxCell>
<mxCell id="15" value="🌍 所有地方可见" style="text;html=1;strokeColor=none;fillColor=none;align=right;verticalAlign=bottom;fontSize=16;fontColor=#FFFFFF;fontStyle=1;" vertex="1" parent="1">
<mxGeometry x="600" y="545" width="220" height="30" as="geometry" />
</mxCell>
<mxCell id="16" value="" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#1E293B;strokeColor=#475569;strokeWidth=2;" vertex="1" parent="1">
<mxGeometry x="60" y="600" width="780" height="60" as="geometry" />
</mxCell>
<mxCell id="17" value="" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#EF4444;strokeColor=none;" vertex="1" parent="1">
<mxGeometry x="80" y="615" width="30" height="30" as="geometry" />
</mxCell>
<mxCell id="18" value="private" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=12;fontColor=#FCA5A5;fontFamily=Courier New;" vertex="1" parent="1">
<mxGeometry x="115" y="615" width="70" height="30" as="geometry" />
</mxCell>
<mxCell id="19" value="" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#3B82F6;strokeColor=none;" vertex="1" parent="1">
<mxGeometry x="200" y="615" width="30" height="30" as="geometry" />
</mxCell>
<mxCell id="20" value="default" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=12;fontColor=#93C5FD;fontFamily=Courier New;" vertex="1" parent="1">
<mxGeometry x="235" y="615" width="70" height="30" as="geometry" />
</mxCell>
<mxCell id="21" value="" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#F59E0B;strokeColor=none;" vertex="1" parent="1">
<mxGeometry x="320" y="615" width="30" height="30" as="geometry" />
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<mxCell id="22" value="protected" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=12;fontColor=#FCD34D;fontFamily=Courier New;" vertex="1" parent="1">
<mxGeometry x="355" y="615" width="80" height="30" as="geometry" />
</mxCell>
<mxCell id="23" value="" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#10B981;strokeColor=none;" vertex="1" parent="1">
<mxGeometry x="450" y="615" width="30" height="30" as="geometry" />
</mxCell>
<mxCell id="24" value="public" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=12;fontColor=#6EE7B7;fontFamily=Courier New;" vertex="1" parent="1">
<mxGeometry x="485" y="615" width="60" height="30" as="geometry" />
</mxCell>
<mxCell id="25" value="范围：小 ◀─────────────────▶ 大" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;fontSize=14;fontColor=#94A3B8;fontStyle=2;" vertex="1" parent="1">
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</root>
</mxGraphModel>
</diagram>
</mxfile>
1.3 完整代码示例
public class BankAccount {
private String accountNumber;
private String ownerName;
private double balance;
private String password;
public BankAccount(String accountNumber, String ownerName, String password) {
this.accountNumber = accountNumber;
this.ownerName = ownerName;
this.password = password;
this.balance = 0.0;
}
public String getAccountNumber() {
return accountNumber.substring(0, 4) + "****" +
accountNumber.substring(accountNumber.length() - 4);
}
public String getOwnerName() {
return ownerName;
}
public double getBalance() {
return balance;
}
public void setPassword(String oldPassword, String newPassword) {
if (validatePassword(oldPassword)) {
this.password = newPassword;
System.out.println("密码修改成功");
} else {
System.out.println("原密码错误");
}
}
public void deposit(double amount) {
if (amount <= 0) {
System.out.println("存款金额必须大于0");
return;
}
balance += amount;
System.out.println("存款成功，当前余额：" + balance);
}
public boolean withdraw(double amount, String password) {
if (!validatePassword(password)) {
System.out.println("密码错误");
return false;
}
if (amount <= 0) {
System.out.println("取款金额必须大于0");
return false;
}
if (amount > balance) {
System.out.println("余额不足");
return false;
}
balance -= amount;
System.out.println("取款成功，当前余额：" + balance);
return true;
}
private boolean validatePassword(String inputPassword) {
return this.password.equals(inputPassword);
}
}
1.4 使用示例
public class Main {
public static void main(String[] args) {
BankAccount account = new BankAccount("6222021234567890", "张三", "123456");
account.deposit(1000);
account.withdraw(500, "123456");
System.out.println("账号：" + account.getAccountNumber());
System.out.println("余额：" + account.getBalance());
}
}
1.5 封装的好处
<mxfile>
<diagram id="encapsulation-benefits" name="封装的四大好处">
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<mxCell id="3" parent="1" style="text;html=1;strokeColor=none;fillColor=#6366F1;align=center;verticalAlign=middle;fontSize=24;fontStyle=1;fontColor=#FFFFFF;rounded=1;" value="🎯 封装的四大好处" vertex="1">
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<mxCell id="4" parent="1" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#1E293B;strokeColor=#3B82F6;strokeWidth=3;shadow=1;" value="" vertex="1">
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<mxCell id="5" parent="1" style="text;html=1;strokeColor=none;fillColor=#3B82F6;align=center;verticalAlign=middle;fontSize=24;fontColor=#FFFFFF;rounded=1;" value="1️⃣" vertex="1">
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</mxCell>
<mxCell id="6" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=20;fontColor=#3B82F6;fontStyle=1;" value="隐藏实现细节" vertex="1">
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</mxCell>
<mxCell id="7" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;fontSize=32;" value="👁️" vertex="1">
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</mxCell>
<mxCell id="8" parent="1" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#0F172A;strokeColor=#475569;strokeWidth=1;" value="" vertex="1">
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<mxCell id="9" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=14;fontColor=#E2E8F0;" value="用户只需知道怎么用，不需要知道内部实现" vertex="1">
<mxGeometry height="30" width="400" x="80" y="170" as="geometry" />
</mxCell>
<mxCell id="10" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=13;fontColor=#94A3B8;fontStyle=2;" value="💡 例：调用 withdraw() 即可取款，无需了解验证逻辑" vertex="1">
<mxGeometry height="25" width="450" x="80" y="200" as="geometry" />
</mxCell>
<mxCell id="11" parent="1" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#1E293B;strokeColor=#10B981;strokeWidth=3;shadow=1;" value="" vertex="1">
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</mxCell>
<mxCell id="12" parent="1" style="text;html=1;strokeColor=none;fillColor=#10B981;align=center;verticalAlign=middle;fontSize=24;fontColor=#FFFFFF;rounded=1;" value="2️⃣" vertex="1">
<mxGeometry height="50" width="50" x="60" y="275" as="geometry" />
</mxCell>
<mxCell id="13" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=20;fontColor=#10B981;fontStyle=1;" value="保护数据安全" vertex="1">
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</mxCell>
<mxCell id="14" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;fontSize=32;" value="🛡️" vertex="1">
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</mxCell>
<mxCell id="15" parent="1" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#0F172A;strokeColor=#475569;strokeWidth=1;" value="" vertex="1">
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</mxCell>
<mxCell id="16" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=14;fontColor=#E2E8F0;" value="防止外部随意修改数据" vertex="1">
<mxGeometry height="30" width="400" x="80" y="330" as="geometry" />
</mxCell>
<mxCell id="17" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=13;fontColor=#94A3B8;fontStyle=2;" value="💡 例：balance 只能通过 deposit/withdraw 修改" vertex="1">
<mxGeometry height="25" width="450" x="80" y="360" as="geometry" />
</mxCell>
<mxCell id="18" parent="1" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#1E293B;strokeColor=#F59E0B;strokeWidth=3;shadow=1;" value="" vertex="1">
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<mxCell id="19" parent="1" style="text;html=1;strokeColor=none;fillColor=#F59E0B;align=center;verticalAlign=middle;fontSize=24;fontColor=#FFFFFF;rounded=1;" value="3️⃣" vertex="1">
<mxGeometry height="50" width="50" x="60" y="435" as="geometry" />
</mxCell>
<mxCell id="20" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=20;fontColor=#F59E0B;fontStyle=1;" value="便于修改实现" vertex="1">
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</mxCell>
<mxCell id="21" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;fontSize=32;" value="🔧" vertex="1">
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</mxCell>
<mxCell id="22" parent="1" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#0F172A;strokeColor=#475569;strokeWidth=1;" value="" vertex="1">
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</mxCell>
<mxCell id="23" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=14;fontColor=#E2E8F0;" value="内部实现可以随时修改，不影响外部调用" vertex="1">
<mxGeometry height="30" width="400" x="80" y="490" as="geometry" />
</mxCell>
<mxCell id="24" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=13;fontColor=#94A3B8;fontStyle=2;" value="💡 例：密码从明文改为加密存储，外部代码无需修改" vertex="1">
<mxGeometry height="25" width="450" x="80" y="520" as="geometry" />
</mxCell>
<mxCell id="25" parent="1" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#1E293B;strokeColor=#EF4444;strokeWidth=3;shadow=1;" value="" vertex="1">
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</mxCell>
<mxCell id="26" parent="1" style="text;html=1;strokeColor=none;fillColor=#EF4444;align=center;verticalAlign=middle;fontSize=24;fontColor=#FFFFFF;rounded=1;" value="4️⃣" vertex="1">
<mxGeometry height="50" width="50" x="60" y="595" as="geometry" />
</mxCell>
<mxCell id="27" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=20;fontColor=#EF4444;fontStyle=1;" value="数据验证" vertex="1">
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</mxCell>
<mxCell id="28" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;fontSize=32;" value="✅" vertex="1">
<mxGeometry height="50" width="60" x="780" y="595" as="geometry" />
</mxCell>
<mxCell id="29" parent="1" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#0F172A;strokeColor=#475569;strokeWidth=1;" value="" vertex="1">
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</mxCell>
<mxCell id="30" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=14;fontColor=#E2E8F0;" value="在 setter / 业务方法中加入验证逻辑" vertex="1">
<mxGeometry height="30" width="400" x="80" y="650" as="geometry" />
</mxCell>
<mxCell id="31" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=13;fontColor=#94A3B8;fontStyle=2;" value="💡 例：存款金额必须 > 0，取款不能超过余额" vertex="1">
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</mxCell>
</root>
</mxGraphModel>
</diagram>
</mxfile>
1.6 JavaBean 规范
public class User {
private Long id;
private String username;
private String email;
private Integer age;
public User() {
}
public User(Long id, String username, String email, Integer age) {
this.id = id;
this.username = username;
this.email = email;
this.age = age;
}
public Long getId() {
return id;
}
public void setId(Long id) {
this.id = id;
}
public String getUsername() {
return username;
}
public void setUsername(String username) {
this.username = username;
}
public String getEmail() {
return email;
}
public void setEmail(String email) {
this.email = email;
}
public Integer getAge() {
return age;
}
public void setAge(Integer age) {
if (age != null && age >= 0 && age <= 150) {
this.age = age;
}
}
@Override
public String toString() {
return "User{id=" + id + ", username='" + username + "', email='" + email + "', age=" + age + "}";
}
}
二、继承（Inheritance）
2.1 概念定义
<mxfile>
<diagram id="inheritance-essence" name="继承的本质">
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<mxCell id="0" />
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<mxCell id="8" parent="1" style="text;html=1;strokeColor=none;fillColor=#6D28D9;align=center;verticalAlign=middle;fontSize=12;fontColor=#FFFFFF;rounded=1;" value="父类" vertex="1">
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<mxCell id="11" parent="1" style="rounded=0;whiteSpace=wrap;html=1;fillColor=#312E81;strokeColor=#8B5CF6;strokeWidth=2;" value="" vertex="1">
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+ sleep()" vertex="1">
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<mxGeometry height="30" width="80" x="460" y="380" as="geometry" />
</mxCell>
<mxCell id="18" parent="1" style="text;html=1;strokeColor=none;fillColor=#3B82F6;align=center;verticalAlign=middle;fontSize=18;fontColor=#FFFFFF;fontStyle=1;rounded=1;" value="Dog" vertex="1">
<mxGeometry height="40" width="200" x="150" y="450" as="geometry" />
</mxCell>
<mxCell id="19" parent="1" style="rounded=0;whiteSpace=wrap;html=1;fillColor=#1E3A5F;strokeColor=#3B82F6;strokeWidth=2;" value="" vertex="1">
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</mxCell>
<mxCell id="20" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=14;fontColor=#93C5FD;fontFamily=Courier New;spacingLeft=10;" value="+ bark()" vertex="1">
<mxGeometry height="50" width="200" x="150" y="490" as="geometry" />
</mxCell>
<mxCell id="21" parent="1" style="text;html=1;strokeColor=none;fillColor=#10B981;align=center;verticalAlign=middle;fontSize=18;fontColor=#FFFFFF;fontStyle=1;rounded=1;" value="Cat" vertex="1">
<mxGeometry height="40" width="200" x="550" y="450" as="geometry" />
</mxCell>
<mxCell id="22" parent="1" style="rounded=0;whiteSpace=wrap;html=1;fillColor=#064E3B;strokeColor=#10B981;strokeWidth=2;" value="" vertex="1">
<mxGeometry height="50" width="200" x="550" y="490" as="geometry" />
</mxCell>
<mxCell id="23" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=14;fontColor=#6EE7B7;fontFamily=Courier New;spacingLeft=10;" value="+ meow()" vertex="1">
<mxGeometry height="50" width="200" x="550" y="490" as="geometry" />
</mxCell>
<mxCell id="24" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=13;fontColor=#F59E0B;fontStyle=1;" value="← 子类扩展" vertex="1">
<mxGeometry height="50" width="90" x="760" y="490" as="geometry" />
</mxCell>
<mxCell id="25" parent="1" style="text;html=1;strokeColor=none;fillColor=#0F172A;align=center;verticalAlign=middle;fontSize=14;fontColor=#A78BFA;rounded=1;strokeWidth=1;" value="✅ Dog 和 Cat 自动拥有 name、age、eat()、sleep()" vertex="1">
<mxGeometry height="35" width="600" x="150" y="555" as="geometry" />
</mxCell>
<mxCell id="26" parent="1" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#1E293B;strokeColor=#F59E0B;strokeWidth=2;" value="" vertex="1">
<mxGeometry height="50" width="790" x="50" y="590" as="geometry" />
</mxCell>
<mxCell id="27" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;fontSize=28;" value="💡" vertex="1">
<mxGeometry height="50" width="50" x="60" y="585" as="geometry" />
</mxCell>
<mxCell id="28" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=15;fontColor=#FCD34D;fontStyle=2;" value="类比：孩子继承父母的基因（外貌、血型等），同时有自己的个性" vertex="1">
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</mxCell>
</root>
</mxGraphModel>
</diagram>
</mxfile>
2.2 基本语法
class 子类 extends 父类 {
}
2.3 完整代码示例
父类定义
public class Employee {
protected String name;
protected double salary;
private String id;
public Employee() {
System.out.println("Employee 无参构造");
}
public Employee(String name, double salary, String id) {
this.name = name;
this.salary = salary;
this.id = id;
System.out.println("Employee 有参构造");
}
public void work() {
System.out.println(name + " 正在工作");
}
public void showInfo() {
System.out.println("姓名：" + name + "，薪资：" + salary);
}
public String getId() {
return id;
}
public void setId(String id) {
this.id = id;
}
}
子类定义
public class Programmer extends Employee {
private String language;
public Programmer() {
super();
System.out.println("Programmer 无参构造");
}
public Programmer(String name, double salary, String id, String language) {
super(name, salary, id);
this.language = language;
System.out.println("Programmer 有参构造");
}
@Override
public void work() {
System.out.println(name + " 正在用 " + language + " 编程");
}
public void debug() {
System.out.println(name + " 正在调试代码");
}
public void code() {
System.out.println(name + "（薪资：" + salary + "）开始写代码");
System.out.println("工号：" + getId());
}
}
使用示例
public class Main {
public static void main(String[] args) {
Programmer p = new Programmer("张三", 15000, "P001", "Java");
p.showInfo();
p.work();
p.debug();
p.code();
}
}
输出结果：
Employee 有参构造
Programmer 有参构造
姓名：张三，薪资：15000.0
张三 正在用 Java 编程
张三 正在调试代码
张三（薪资：15000.0）开始写代码
工号：P001
2.4 super 关键字详解
<mxfile>
<diagram id="super-keyword-usage" name="super关键字用法">
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</mxCell>
<mxCell id="5" parent="1" style="ellipse;whiteSpace=wrap;html=1;fillColor=#3B82F6;strokeColor=none;fontSize=18;fontStyle=1;fontColor=#FFFFFF;" value="1" vertex="1">
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</mxCell>
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</mxCell>
<mxCell id="7" parent="1" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#0F172A;strokeColor=#475569;strokeWidth=1;" value="" vertex="1">
<mxGeometry height="100" width="780" x="60" y="165" as="geometry" />
</mxCell>
<mxCell id="8" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=14;fontColor=#93C5FD;fontFamily=Courier New;" value="public Programmer(String name, double salary) {" vertex="1">
<mxGeometry height="25" width="500" x="80" y="175" as="geometry" />
</mxCell>
<mxCell id="9" parent="1" style="text;html=1;strokeColor=none;fillColor=#1E3A5F;align=left;verticalAlign=middle;fontSize=14;fontColor=#FCD34D;fontFamily=Courier New;rounded=1;" value="super(name, salary);" vertex="1">
<mxGeometry height="25" width="220" x="100" y="200" as="geometry" />
</mxCell>
<mxCell id="10" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=13;fontColor=#F87171;fontFamily=Courier New;fontStyle=2;" value="// 必须在第一行" vertex="1">
<mxGeometry height="25" width="150" x="330" y="200" as="geometry" />
</mxCell>
<mxCell id="11" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=14;fontColor=#93C5FD;fontFamily=Courier New;" value="}" vertex="1">
<mxGeometry height="25" width="50" x="80" y="230" as="geometry" />
</mxCell>
<mxCell id="12" parent="1" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#1E293B;strokeColor=#10B981;strokeWidth=3;shadow=1;" value="" vertex="1">
<mxGeometry height="185" width="820" x="40" y="300" as="geometry" />
</mxCell>
<mxCell id="13" parent="1" style="ellipse;whiteSpace=wrap;html=1;fillColor=#10B981;strokeColor=none;fontSize=18;fontStyle=1;fontColor=#FFFFFF;" value="2" vertex="1">
<mxGeometry height="40" width="40" x="60" y="315" as="geometry" />
</mxCell>
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</mxCell>
<mxCell id="15" parent="1" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#0F172A;strokeColor=#475569;strokeWidth=1;" value="" vertex="1">
<mxGeometry height="100" width="780" x="60" y="365" as="geometry" />
</mxCell>
<mxCell id="16" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=14;fontColor=#6EE7B7;fontFamily=Courier New;" value="public void show() {" vertex="1">
<mxGeometry height="25" width="300" x="80" y="375" as="geometry" />
</mxCell>
<mxCell id="17" parent="1" style="text;html=1;strokeColor=none;fillColor=#064E3B;align=left;verticalAlign=middle;fontSize=14;fontColor=#FCD34D;fontFamily=Courier New;rounded=1;" value="System.out.println(super.name);" vertex="1">
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</mxCell>
<mxCell id="18" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=13;fontColor=#94A3B8;fontFamily=Courier New;fontStyle=2;" value="// 父类属性" vertex="1">
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</mxCell>
<mxCell id="19" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=14;fontColor=#6EE7B7;fontFamily=Courier New;" value="}" vertex="1">
<mxGeometry height="25" width="50" x="80" y="430" as="geometry" />
</mxCell>
<mxCell id="20" parent="1" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#1E293B;strokeColor=#F59E0B;strokeWidth=3;shadow=1;" value="" vertex="1">
<mxGeometry height="210" width="820" x="40" y="500" as="geometry" />
</mxCell>
<mxCell id="21" parent="1" style="ellipse;whiteSpace=wrap;html=1;fillColor=#F59E0B;strokeColor=none;fontSize=18;fontStyle=1;fontColor=#FFFFFF;" value="3" vertex="1">
<mxGeometry height="40" width="40" x="60" y="515" as="geometry" />
</mxCell>
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</mxCell>
<mxCell id="23" parent="1" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#0F172A;strokeColor=#475569;strokeWidth=1;" value="" vertex="1">
<mxGeometry height="125" width="780" x="60" y="565" as="geometry" />
</mxCell>
<mxCell id="24" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=14;fontColor=#A78BFA;fontFamily=Courier New;" value="@Override" vertex="1">
<mxGeometry height="25" width="120" x="80" y="575" as="geometry" />
</mxCell>
<mxCell id="25" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=14;fontColor=#FCD34D;fontFamily=Courier New;" value="public void work() {" vertex="1">
<mxGeometry height="25" width="250" x="80" y="600" as="geometry" />
</mxCell>
<mxCell id="26" parent="1" style="text;html=1;strokeColor=none;fillColor=#78350F;align=left;verticalAlign=middle;fontSize=14;fontColor=#FCD34D;fontFamily=Courier New;rounded=1;" value="super.work();" vertex="1">
<mxGeometry height="25" width="150" x="100" y="625" as="geometry" />
</mxCell>
<mxCell id="27" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=13;fontColor=#94A3B8;fontFamily=Courier New;fontStyle=2;" value="// 先执行父类的 work" vertex="1">
<mxGeometry height="25" width="200" x="260" y="625" as="geometry" />
</mxCell>
<mxCell id="28" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=14;fontColor=#FCD34D;fontFamily=Courier New;" value="System.out.println("然后写代码");" vertex="1">
<mxGeometry height="25" width="350" x="100" y="650" as="geometry" />
</mxCell>
<mxCell id="29" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=14;fontColor=#FCD34D;fontFamily=Courier New;" value="}" vertex="1">
<mxGeometry height="25" width="50" x="80" y="675" as="geometry" />
</mxCell>
<mxCell id="30" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;fontSize=24;" value="⚠️" vertex="1">
<mxGeometry height="40" width="40" x="770" y="115" as="geometry" />
</mxCell>
<mxCell id="31" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;fontSize=24;" value="📖" vertex="1">
<mxGeometry height="40" width="40" x="770" y="315" as="geometry" />
</mxCell>
<mxCell id="32" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;fontSize=24;" value="🔄" vertex="1">
<mxGeometry height="40" width="40" x="770" y="515" as="geometry" />
</mxCell>
</root>
</mxGraphModel>
</diagram>
</mxfile>
super vs this 对比
| 关键字 |
指向 |
用途 |
this |
当前对象 |
访问本类成员、调用本类构造 |
super |
父类部分 |
访问父类成员、调用父类构造 |
public class Child extends Parent {
private String name = "子类名";
public void show() {
System.out.println(this.name);
System.out.println(super.name);
}
public Child() {
super();
}
}
2.5 方法重写（Override）
<mxfile>
<diagram id="method-override-rules-fixed" name="方法重写规则">
<mxGraphModel dx="1040" dy="820" grid="1" gridSize="10" guides="1" tooltips="1" connect="1" arrows="1" fold="1" page="1" pageScale="1" pageWidth="760" pageHeight="880" math="0" shadow="0">
<root>
<mxCell id="0" />
<mxCell id="1" parent="0" />
<mxCell id="2" value="" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#fafafa;strokeColor=#424242;strokeWidth=2;" vertex="1" parent="1">
<mxGeometry x="30" y="30" width="700" height="820" as="geometry" />
</mxCell>
<mxCell id="3" value="Java 方法重写 (Override) 核心规则" style="text;html=1;strokeColor=none;fillColor=#5c6bc0;align=center;verticalAlign=middle;whiteSpace=wrap;rounded=1;fontSize=22;fontStyle=1;fontColor=#ffffff;" vertex="1" parent="1">
<mxGeometry x="30" y="30" width="700" height="55" as="geometry" />
</mxCell>
<mxCell id="4" value="" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#e8eaf6;strokeColor=#5c6bc0;strokeWidth=2;" vertex="1" parent="1">
<mxGeometry x="60" y="105" width="640" height="50" as="geometry" />
</mxCell>
<mxCell id="5" value="📖" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;whiteSpace=wrap;rounded=0;fontSize=24;" vertex="1" parent="1">
<mxGeometry x="75" y="113" width="40" height="35" as="geometry" />
</mxCell>
<mxCell id="6" value="方法重写 = 子类重新定义父类中已有的方法" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;whiteSpace=wrap;rounded=0;fontSize=15;fontColor=#3949ab;fontStyle=1;" vertex="1" parent="1">
<mxGeometry x="120" y="113" width="560" height="35" as="geometry" />
</mxCell>
<mxCell id="7" value="" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#ffffff;strokeColor=#7986cb;strokeWidth=2;" vertex="1" parent="1">
<mxGeometry x="60" y="175" width="640" height="420" as="geometry" />
</mxCell>
<mxCell id="8" value="📌 5 大核心规则" style="text;html=1;strokeColor=none;fillColor=#7986cb;align=center;verticalAlign=middle;whiteSpace=wrap;rounded=1;fontSize=16;fontStyle=1;fontColor=#ffffff;" vertex="1" parent="1">
<mxGeometry x="60" y="175" width="640" height="40" as="geometry" />
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<mxCell id="9" value="" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#e3f2fd;strokeColor=#1976d2;strokeWidth=1;" vertex="1" parent="1">
<mxGeometry x="80" y="230" width="600" height="50" as="geometry" />
</mxCell>
<mxCell id="10" value="1" style="ellipse;whiteSpace=wrap;html=1;fillColor=#1976d2;strokeColor=none;fontColor=#ffffff;fontSize=14;fontStyle=1;" vertex="1" parent="1">
<mxGeometry x="95" y="240" width="30" height="30" as="geometry" />
</mxCell>
<mxCell id="11" value="方法签名必须相同（<font color="#1565c0"><b>方法名 + 参数列表</b></font>）" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;whiteSpace=wrap;rounded=0;fontSize=14;" vertex="1" parent="1">
<mxGeometry x="135" y="240" width="530" height="30" as="geometry" />
</mxCell>
<mxCell id="12" value="" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#e8f5e9;strokeColor=#43a047;strokeWidth=1;" vertex="1" parent="1">
<mxGeometry x="80" y="290" width="600" height="50" as="geometry" />
</mxCell>
<mxCell id="13" value="2" style="ellipse;whiteSpace=wrap;html=1;fillColor=#43a047;strokeColor=none;fontColor=#ffffff;fontSize=14;fontStyle=1;" vertex="1" parent="1">
<mxGeometry x="95" y="300" width="30" height="30" as="geometry" />
</mxCell>
<mxCell id="14" value="返回类型：<font color="#2e7d32"><b>相同</b></font>或是<font color="#2e7d32"><b>子类型</b></font>（协变返回）" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;whiteSpace=wrap;rounded=0;fontSize=14;" vertex="1" parent="1">
<mxGeometry x="135" y="300" width="530" height="30" as="geometry" />
</mxCell>
<mxCell id="15" value="" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#fff3e0;strokeColor=#fb8c00;strokeWidth=1;" vertex="1" parent="1">
<mxGeometry x="80" y="350" width="600" height="90" as="geometry" />
</mxCell>
<mxCell id="16" value="3" style="ellipse;whiteSpace=wrap;html=1;fillColor=#fb8c00;strokeColor=none;fontColor=#ffffff;fontSize=14;fontStyle=1;" vertex="1" parent="1">
<mxGeometry x="95" y="360" width="30" height="30" as="geometry" />
</mxCell>
<mxCell id="17" value="访问权限：<font color="#e65100"><b>不能比父类更严格</b></font>" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;whiteSpace=wrap;rounded=0;fontSize=14;" vertex="1" parent="1">
<mxGeometry x="135" y="358" width="350" height="30" as="geometry" />
</mxCell>
<mxCell id="18" value="父类 public → 子类必须 public" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;whiteSpace=wrap;rounded=0;fontSize=12;fontColor=#666666;fontFamily=Consolas;" vertex="1" parent="1">
<mxGeometry x="155" y="388" width="250" height="22" as="geometry" />
</mxCell>
<mxCell id="19" value="父类 protected → 子类可以 protected 或 public" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;whiteSpace=wrap;rounded=0;fontSize=12;fontColor=#666666;fontFamily=Consolas;" vertex="1" parent="1">
<mxGeometry x="155" y="410" width="350" height="22" as="geometry" />
</mxCell>
<mxCell id="20" value="" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#fce4ec;strokeColor=#e91e63;strokeWidth=1;" vertex="1" parent="1">
<mxGeometry x="80" y="450" width="600" height="50" as="geometry" />
</mxCell>
<mxCell id="21" value="4" style="ellipse;whiteSpace=wrap;html=1;fillColor=#e91e63;strokeColor=none;fontColor=#ffffff;fontSize=14;fontStyle=1;" vertex="1" parent="1">
<mxGeometry x="95" y="460" width="30" height="30" as="geometry" />
</mxCell>
<mxCell id="22" value="异常：<font color="#c2185b"><b>不能抛出比父类更多/更广的异常</b></font>" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;whiteSpace=wrap;rounded=0;fontSize=14;" vertex="1" parent="1">
<mxGeometry x="135" y="460" width="530" height="30" as="geometry" />
</mxCell>
<mxCell id="23" value="" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#f3e5f5;strokeColor=#9c27b0;strokeWidth=1;" vertex="1" parent="1">
<mxGeometry x="80" y="510" width="600" height="70" as="geometry" />
</mxCell>
<mxCell id="24" value="5" style="ellipse;whiteSpace=wrap;html=1;fillColor=#9c27b0;strokeColor=none;fontColor=#ffffff;fontSize=14;fontStyle=1;" vertex="1" parent="1">
<mxGeometry x="95" y="525" width="30" height="30" as="geometry" />
</mxCell>
<mxCell id="25" value="不能重写：" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;whiteSpace=wrap;rounded=0;fontSize=14;" vertex="1" parent="1">
<mxGeometry x="135" y="525" width="80" height="30" as="geometry" />
</mxCell>
<mxCell id="26" value="static" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#9c27b0;strokeColor=none;fontColor=#ffffff;fontSize=13;fontStyle=1;fontFamily=Consolas;" vertex="1" parent="1">
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重写 vs 重载 对比
| 对比项 |
重写（Override） |
重载（Overload） |
| 发生位置 |
子类与父类之间 |
同一个类中 |
| 方法名 |
必须相同 |
必须相同 |
| 参数列表 |
必须相同 |
必须不同 |
| 返回类型 |
相同或子类型 |
无关 |
| 访问修饰符 |
不能更严格 |
无关 |
| 目的 |
改变行为 |
提供多种调用方式 |
2.6 继承的内存结构
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2.7 继承的要点与限制
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三、多态（Polymorphism）
3.1 概念定义
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3.2 多态的三个必要条件
<mxfile>
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3.3 完整代码示例
定义类结构
public class Animal {
protected String name;
public Animal(String name) {
this.name = name;
}
public void makeSound() {
System.out.println(name + " 发出声音");
}
public void eat() {
System.out.println(name + " 正在吃东西");
}
}
public class Dog extends Animal {
public Dog(String name) {
super(name);
}
@Override
public void makeSound() {
System.out.println(name + " 汪汪汪！");
}
public void guardHouse() {
System.out.println(name + " 正在看家");
}
}
public class Cat extends Animal {
public Cat(String name) {
super(name);
}
@Override
public void makeSound() {
System.out.println(name + " 喵喵喵~");
}
public void catchMouse() {
System.out.println(name + " 正在抓老鼠");
}
}
public class Bird extends Animal {
public Bird(String name) {
super(name);
}
@Override
public void makeSound() {
System.out.println(name + " 叽叽喳喳！");
}
public void fly() {
System.out.println(name + " 正在飞翔");
}
}
多态使用示例
public class PolymorphismDemo {
public static void main(String[] args) {
Animal animal1 = new Dog("旺财");
Animal animal2 = new Cat("小花");
Animal animal3 = new Bird("小鸟");
animal1.makeSound();
animal2.makeSound();
animal3.makeSound();
Animal[] animals = {
new Dog("大黄"),
new Cat("咪咪"),
new Bird("鹦鹉")
};
for (Animal animal : animals) {
animal.makeSound();
}
feedAnimal(new Dog("小白"));
feedAnimal(new Cat("小黑"));
}
public static void feedAnimal(Animal animal) {
animal.eat();
animal.makeSound();
}
}
3.4 多态的执行原理
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</mxCell>
<mxCell id="20" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=13;fontColor=#60A5FA;" value="不看实际对象类型" vertex="1">
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<mxCell id="21" edge="1" parent="1" style="endArrow=classic;html=1;strokeColor=#F59E0B;strokeWidth=4;flowAnimation=1;" value="">
<mxGeometry relative="1" as="geometry">
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<mxCell id="22" parent="1" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#1E293B;strokeColor=#22C55E;strokeWidth=3;shadow=1;" value="" vertex="1">
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<mxCell id="23" parent="1" style="text;html=1;strokeColor=none;fillColor=#22C55E;align=center;verticalAlign=middle;fontSize=18;fontColor=#FFFFFF;fontStyle=1;rounded=1;" value="🚀 运行时" vertex="1">
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</mxCell>
<mxCell id="24" parent="1" style="text;html=1;strokeColor=none;fillColor=#14532D;align=center;verticalAlign=middle;fontSize=14;fontColor=#86EFAC;rounded=1;fontStyle=1;" value="JVM 查看" vertex="1">
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<mxCell id="25" parent="1" style="text;html=1;strokeColor=none;fillColor=#0F172A;align=center;verticalAlign=middle;fontSize=15;fontColor=#BBF7D0;rounded=1;strokeColor=#14532D;strokeWidth=2;" value="animal 实际指向什么对象？" vertex="1">
<mxGeometry height="45" width="260" x="210" y="560" as="geometry" />
</mxCell>
<mxCell id="26" edge="1" parent="1" style="endArrow=classic;html=1;strokeColor=#4ADE80;strokeWidth=3;flowAnimation=1;" value="">
<mxGeometry relative="1" as="geometry">
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</mxCell>
<mxCell id="27" parent="1" style="ellipse;whiteSpace=wrap;html=1;fillColor=#F97316;strokeColor=#FB923C;strokeWidth=2;fontSize=15;fontColor=#FFFFFF;fontStyle=1;" value="🐕 Dog 对象" vertex="1">
<mxGeometry height="50" width="130" x="275" y="645" as="geometry" />
</mxCell>
<mxCell id="28" edge="1" parent="1" style="endArrow=classic;html=1;strokeColor=#4ADE80;strokeWidth=3;flowAnimation=1;" value="">
<mxGeometry relative="1" as="geometry">
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</mxCell>
<mxCell id="29" parent="1" style="text;html=1;strokeColor=none;fillColor=#0F172A;align=center;verticalAlign=middle;fontSize=14;fontColor=#FCD34D;rounded=1;strokeColor=#78350F;strokeWidth=2;" value="调用 Dog 类的 makeSound() 方法" vertex="1">
<mxGeometry height="35" width="280" x="170" y="735" as="geometry" />
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<mxCell id="30" edge="1" parent="1" style="endArrow=classic;html=1;strokeColor=#4ADE80;strokeWidth=3;flowAnimation=1;" value="">
<mxGeometry relative="1" as="geometry">
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<mxPoint x="520" y="752" as="targetPoint" />
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</mxCell>
<mxCell id="31" parent="1" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#7C2D12;strokeColor=#F97316;strokeWidth=2;fontSize=16;fontColor=#FED7AA;fontStyle=1;" value="🔊 "旺财 汪汪汪！"" vertex="1">
<mxGeometry height="45" width="180" x="530" y="730" as="geometry" />
</mxCell>
<mxCell id="32" parent="1" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#052E16;strokeColor=#22C55E;strokeWidth=2;" value="" vertex="1">
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</mxCell>
<mxCell id="33" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;fontSize=24;" value="⚠️" vertex="1">
<mxGeometry height="40" width="40" x="530" y="575" as="geometry" />
</mxCell>
<mxCell id="34" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=14;fontColor=#86EFAC;fontStyle=1;" value="运行时看实际对象类型" vertex="1">
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<mxCell id="35" parent="1" style="text;html=1;strokeColor=none;fillColor=#EA580C;align=center;verticalAlign=middle;fontSize=14;fontColor=#FFFFFF;fontFamily=Courier New;rounded=1;fontStyle=1;" value="Dog" vertex="1">
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</mxCell>
<mxCell id="36" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=13;fontColor=#4ADE80;" value="动态绑定机制" vertex="1">
<mxGeometry height="25" width="150" x="570" y="608" as="geometry" />
</mxCell>
<mxCell id="37" parent="1" style="text;html=1;strokeColor=none;fillColor=#2563EB;align=center;verticalAlign=middle;fontSize=12;fontColor=#FFFFFF;rounded=1;fontStyle=1;" value="左边" vertex="1">
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<mxCell id="38" parent="1" style="text;html=1;strokeColor=none;fillColor=#EA580C;align=center;verticalAlign=middle;fontSize=12;fontColor=#FFFFFF;rounded=1;fontStyle=1;" value="右边" vertex="1">
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</mxCell>
</root>
</mxGraphModel>
</diagram>
</mxfile>
3.5 向上转型与向下转型
<mxfile>
<diagram id="type-casting" name="类型转换">
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<mxCell id="3" value="类型转换" style="text;html=1;strokeColor=none;fillColor=#5c6bc0;align=center;verticalAlign=middle;whiteSpace=wrap;rounded=1;fontSize=24;fontStyle=1;fontColor=#ffffff;" vertex="1" parent="1">
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<mxCell id="4" value="" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#e8f5e9;strokeColor=#43a047;strokeWidth=2;" vertex="1" parent="1">
<mxGeometry x="50" y="110" width="620" height="330" as="geometry" />
</mxCell>
<mxCell id="5" value="向上转型" style="text;html=1;strokeColor=none;fillColor=#43a047;align=center;verticalAlign=middle;whiteSpace=wrap;rounded=1;fontSize=20;fontStyle=1;fontColor=#ffffff;" vertex="1" parent="1">
<mxGeometry x="50" y="110" width="620" height="45" as="geometry" />
</mxCell>
<mxCell id="6" value="(Upcasting)" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;whiteSpace=wrap;rounded=0;fontSize=14;fontColor=#2e7d32;fontStyle=2;" vertex="1" parent="1">
<mxGeometry x="50" y="155" width="620" height="25" as="geometry" />
</mxCell>
<mxCell id="7" value="⬆️" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;whiteSpace=wrap;rounded=0;fontSize=36;" vertex="1" parent="1">
<mxGeometry x="600" y="115" width="50" height="50" as="geometry" />
</mxCell>
<mxCell id="8" value="" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#ffffff;strokeColor=#a5d6a7;strokeWidth=1;" vertex="1" parent="1">
<mxGeometry x="70" y="190" width="580" height="90" as="geometry" />
</mxCell>
<mxCell id="9" value="Animal" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#1976d2;strokeColor=none;fontSize=14;fontFamily=Consolas;fontColor=#ffffff;fontStyle=1;" vertex="1" parent="1">
<mxGeometry x="90" y="205" width="70" height="28" as="geometry" />
</mxCell>
<mxCell id="10" value="animal = new" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;whiteSpace=wrap;rounded=0;fontSize=14;fontFamily=Consolas;" vertex="1" parent="1">
<mxGeometry x="165" y="205" width="110" height="28" as="geometry" />
</mxCell>
<mxCell id="11" value="Dog("旺财")" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#43a047;strokeColor=none;fontSize=14;fontFamily=Consolas;fontColor=#ffffff;fontStyle=1;" vertex="1" parent="1">
<mxGeometry x="275" y="205" width="110" height="28" as="geometry" />
</mxCell>
<mxCell id="12" value=";" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;whiteSpace=wrap;rounded=0;fontSize=14;fontFamily=Consolas;" vertex="1" parent="1">
<mxGeometry x="385" y="205" width="20" height="28" as="geometry" />
</mxCell>
<mxCell id="13" value="// 自动转型" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;whiteSpace=wrap;rounded=0;fontSize=13;fontFamily=Consolas;fontColor=#757575;fontStyle=2;" vertex="1" parent="1">
<mxGeometry x="410" y="205" width="100" height="28" as="geometry" />
</mxCell>
<mxCell id="14" value="" style="endArrow=classic;html=1;rounded=0;strokeWidth=2;strokeColor=#1976d2;" edge="1" parent="1">
<mxGeometry relative="1" as="geometry">
<mxPoint x="125" y="255" as="sourcePoint" />
<mxPoint x="125" y="235" as="targetPoint" />
</mxGeometry>
</mxCell>
<mxCell id="15" value="父类型" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;whiteSpace=wrap;rounded=0;fontSize=12;fontColor=#1976d2;fontStyle=1;" vertex="1" parent="1">
<mxGeometry x="100" y="255" width="50" height="20" as="geometry" />
</mxCell>
<mxCell id="16" value="" style="endArrow=classic;html=1;rounded=0;strokeWidth=2;strokeColor=#43a047;" edge="1" parent="1">
<mxGeometry relative="1" as="geometry">
<mxPoint x="330" y="255" as="sourcePoint" />
<mxPoint x="330" y="235" as="targetPoint" />
</mxGeometry>
</mxCell>
<mxCell id="17" value="子类对象" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;whiteSpace=wrap;rounded=0;fontSize=12;fontColor=#43a047;fontStyle=1;" vertex="1" parent="1">
<mxGeometry x="300" y="255" width="60" height="20" as="geometry" />
</mxCell>
<mxCell id="18" value="特点：" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;whiteSpace=wrap;rounded=0;fontSize=15;fontStyle=1;fontColor=#2e7d32;" vertex="1" parent="1">
<mxGeometry x="70" y="290" width="60" height="30" as="geometry" />
</mxCell>
<mxCell id="19" value="" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#c8e6c9;strokeColor=none;" vertex="1" parent="1">
<mxGeometry x="70" y="320" width="580" height="110" as="geometry" />
</mxCell>
<mxCell id="20" value="✓" style="text;html=1;strokeColor=none;fillColor=#43a047;align=center;verticalAlign=middle;whiteSpace=wrap;rounded=1;fontSize=14;fontColor=#ffffff;fontStyle=1;" vertex="1" parent="1">
<mxGeometry x="85" y="330" width="25" height="25" as="geometry" />
</mxCell>
<mxCell id="21" value="自动进行，无需强制转换" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;whiteSpace=wrap;rounded=0;fontSize=14;fontColor=#1b5e20;" vertex="1" parent="1">
<mxGeometry x="120" y="330" width="200" height="25" as="geometry" />
</mxCell>
<mxCell id="22" value="✓" style="text;html=1;strokeColor=none;fillColor=#43a047;align=center;verticalAlign=middle;whiteSpace=wrap;rounded=1;fontSize=14;fontColor=#ffffff;fontStyle=1;" vertex="1" parent="1">
<mxGeometry x="85" y="360" width="25" height="25" as="geometry" />
</mxCell>
<mxCell id="23" value="安全，不会失败" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;whiteSpace=wrap;rounded=0;fontSize=14;fontColor=#1b5e20;" vertex="1" parent="1">
<mxGeometry x="120" y="360" width="150" height="25" as="geometry" />
</mxCell>
<mxCell id="24" value="✓" style="text;html=1;strokeColor=none;fillColor=#43a047;align=center;verticalAlign=middle;whiteSpace=wrap;rounded=1;fontSize=14;fontColor=#ffffff;fontStyle=1;" vertex="1" parent="1">
<mxGeometry x="85" y="390" width="25" height="25" as="geometry" />
</mxCell>
<mxCell id="25" value="只能访问父类成员，不能访问子类特有成员" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;whiteSpace=wrap;rounded=0;fontSize=14;fontColor=#1b5e20;" vertex="1" parent="1">
<mxGeometry x="120" y="390" width="350" height="25" as="geometry" />
</mxCell>
<mxCell id="26" value="" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#fff3e0;strokeColor=#fb8c00;strokeWidth=2;" vertex="1" parent="1">
<mxGeometry x="50" y="460" width="620" height="370" as="geometry" />
</mxCell>
<mxCell id="27" value="向下转型" style="text;html=1;strokeColor=none;fillColor=#fb8c00;align=center;verticalAlign=middle;whiteSpace=wrap;rounded=1;fontSize=20;fontStyle=1;fontColor=#ffffff;" vertex="1" parent="1">
<mxGeometry x="50" y="460" width="620" height="45" as="geometry" />
</mxCell>
<mxCell id="28" value="(Downcasting)" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;whiteSpace=wrap;rounded=0;fontSize=14;fontColor=#e65100;fontStyle=2;" vertex="1" parent="1">
<mxGeometry x="50" y="505" width="620" height="25" as="geometry" />
</mxCell>
<mxCell id="29" value="⬇️" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;whiteSpace=wrap;rounded=0;fontSize=36;" vertex="1" parent="1">
<mxGeometry x="600" y="465" width="50" height="50" as="geometry" />
</mxCell>
<mxCell id="30" value="" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#ffffff;strokeColor=#ffcc80;strokeWidth=1;" vertex="1" parent="1">
<mxGeometry x="70" y="540" width="580" height="120" as="geometry" />
</mxCell>
<mxCell id="31" value="Animal animal = new Dog("旺财");" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;whiteSpace=wrap;rounded=0;fontSize=14;fontFamily=Consolas;" vertex="1" parent="1">
<mxGeometry x="90" y="555" width="300" height="25" as="geometry" />
</mxCell>
<mxCell id="32" value="Dog dog =" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;whiteSpace=wrap;rounded=0;fontSize=14;fontFamily=Consolas;" vertex="1" parent="1">
<mxGeometry x="90" y="585" width="80" height="25" as="geometry" />
</mxCell>
<mxCell id="33" value="(Dog)" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#e91e63;strokeColor=none;fontSize=14;fontFamily=Consolas;fontColor=#ffffff;fontStyle=1;" vertex="1" parent="1">
<mxGeometry x="170" y="585" width="55" height="25" as="geometry" />
</mxCell>
<mxCell id="34" value="animal;" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;whiteSpace=wrap;rounded=0;fontSize=14;fontFamily=Consolas;" vertex="1" parent="1">
<mxGeometry x="230" y="585" width="70" height="25" as="geometry" />
</mxCell>
<mxCell id="35" value="// 强制转换" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;whiteSpace=wrap;rounded=0;fontSize=13;fontFamily=Consolas;fontColor=#757575;fontStyle=2;" vertex="1" parent="1">
<mxGeometry x="310" y="585" width="100" height="25" as="geometry" />
</mxCell>
<mxCell id="36" value="" style="endArrow=classic;html=1;rounded=0;strokeWidth=2;strokeColor=#e91e63;" edge="1" parent="1">
<mxGeometry relative="1" as="geometry">
<mxPoint x="197" y="635" as="sourcePoint" />
<mxPoint x="197" y="612" as="targetPoint" />
</mxGeometry>
</mxCell>
<mxCell id="37" value="必须显式转换" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;whiteSpace=wrap;rounded=0;fontSize=12;fontColor=#e91e63;fontStyle=1;" vertex="1" parent="1">
<mxGeometry x="155" y="635" width="90" height="20" as="geometry" />
</mxCell>
<mxCell id="38" value="特点：" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;whiteSpace=wrap;rounded=0;fontSize=15;fontStyle=1;fontColor=#e65100;" vertex="1" parent="1">
<mxGeometry x="70" y="670" width="60" height="30" as="geometry" />
</mxCell>
<mxCell id="39" value="" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#ffe0b2;strokeColor=none;" vertex="1" parent="1">
<mxGeometry x="70" y="700" width="580" height="120" as="geometry" />
</mxCell>
<mxCell id="40" value="!" style="text;html=1;strokeColor=none;fillColor=#fb8c00;align=center;verticalAlign=middle;whiteSpace=wrap;rounded=1;fontSize=14;fontColor=#ffffff;fontStyle=1;" vertex="1" parent="1">
<mxGeometry x="85" y="710" width="25" height="25" as="geometry" />
</mxCell>
<mxCell id="41" value="需要显式强制转换" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;whiteSpace=wrap;rounded=0;fontSize=14;fontColor=#e65100;" vertex="1" parent="1">
<mxGeometry x="120" y="710" width="180" height="25" as="geometry" />
</mxCell>
<mxCell id="42" value="⚠" style="text;html=1;strokeColor=none;fillColor=#f44336;align=center;verticalAlign=middle;whiteSpace=wrap;rounded=1;fontSize=14;fontColor=#ffffff;fontStyle=1;" vertex="1" parent="1">
<mxGeometry x="85" y="740" width="25" height="25" as="geometry" />
</mxCell>
<mxCell id="43" value="可能失败，抛出" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;whiteSpace=wrap;rounded=0;fontSize=14;fontColor=#e65100;" vertex="1" parent="1">
<mxGeometry x="120" y="740" width="120" height="25" as="geometry" />
</mxCell>
<mxCell id="44" value="ClassCastException" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#f44336;strokeColor=none;fontSize=12;fontFamily=Consolas;fontColor=#ffffff;fontStyle=1;" vertex="1" parent="1">
<mxGeometry x="240" y="742" width="150" height="22" as="geometry" />
</mxCell>
<mxCell id="45" value="✓" style="text;html=1;strokeColor=none;fillColor=#43a047;align=center;verticalAlign=middle;whiteSpace=wrap;rounded=1;fontSize=14;fontColor=#ffffff;fontStyle=1;" vertex="1" parent="1">
<mxGeometry x="85" y="775" width="25" height="25" as="geometry" />
</mxCell>
<mxCell id="46" value="转换后可以访问子类特有成员" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;whiteSpace=wrap;rounded=0;fontSize=14;fontColor=#e65100;" vertex="1" parent="1">
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</mxCell>
<mxCell id="47" value="" style="shape=flexArrow;endArrow=classic;startArrow=classic;html=1;rounded=0;fillColor=#7c4dff;strokeColor=none;width=15;endSize=5;startSize=5;" edge="1" parent="1">
<mxGeometry relative="1" as="geometry">
<mxPoint x="350" y="440" as="sourcePoint" />
<mxPoint x="350" y="460" as="targetPoint" />
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</mxCell>
</root>
</mxGraphModel>
</diagram>
</mxfile>
代码示例
public class CastDemo {
public static void main(String[] args) {
Animal animal = new Dog("旺财");
animal.makeSound();
animal.eat();
if (animal instanceof Dog) {
Dog dog = (Dog) animal;
dog.guardHouse();
}
Animal animal2 = new Cat("小花");
if (animal2 instanceof Dog) {
Dog dog2 = (Dog) animal2;
dog2.guardHouse();
} else {
System.out.println("animal2 不是 Dog 类型");
}
if (animal instanceof Dog dog) {
dog.guardHouse();
}
}
}
3.6 instanceof 关键字
<mxfile>
<diagram id="instanceof-usage" name="instanceof 用法">
<mxGraphModel dx="1281" dy="581" grid="1" gridSize="10" guides="1" tooltips="1" connect="1" arrows="1" fold="1" page="1" pageScale="1" pageWidth="900" pageHeight="700" math="0" shadow="0">
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<mxCell id="0" />
<mxCell id="1" parent="0" />
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<mxCell id="3" parent="1" style="text;html=1;strokeColor=none;fillColor=#8B5CF6;align=center;verticalAlign=middle;fontSize=24;fontStyle=1;fontColor=#FFFFFF;rounded=1;" value="🔍 instanceof 用法" vertex="1">
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</mxCell>
<mxCell id="4" parent="1" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#1E293B;strokeColor=#6366F1;strokeWidth=2;" value="" vertex="1">
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3.7 多态的实际应用
应用1：统一处理不同类型
public class Zoo {
private List<Animal> animals = new ArrayList<>();
public void addAnimal(Animal animal) {
animals.add(animal);
}
public void feedAll() {
for (Animal animal : animals) {
animal.eat();
}
}
public void performAll() {
for (Animal animal : animals) {
animal.makeSound();
}
}
}
Zoo zoo = new Zoo();
zoo.addAnimal(new Dog("旺财"));
zoo.addAnimal(new Cat("咪咪"));
zoo.addAnimal(new Bird("鹦鹉"));
zoo.performAll();
应用2：策略模式
interface PaymentStrategy {
void pay(double amount);
}
class WeChatPay implements PaymentStrategy {
@Override
public void pay(double amount) {
System.out.println("微信支付：" + amount + " 元");
}
}
class AliPay implements PaymentStrategy {
@Override
public void pay(double amount) {
System.out.println("支付宝支付：" + amount + " 元");
}
}
class CreditCardPay implements PaymentStrategy {
@Override
public void pay(double amount) {
System.out.println("信用卡支付：" + amount + " 元");
}
}
class ShoppingCart {
private PaymentStrategy paymentStrategy;
public void setPaymentStrategy(PaymentStrategy strategy) {
this.paymentStrategy = strategy;
}
public void checkout(double amount) {
paymentStrategy.pay(amount);
}
}
ShoppingCart cart = new ShoppingCart();
cart.setPaymentStrategy(new WeChatPay());
cart.checkout(100);
cart.setPaymentStrategy(new AliPay());
cart.checkout(200);
3.8 多态的优缺点
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<mxCell id="31" parent="1" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#172554;strokeColor=#3B82F6;strokeWidth=2;" value="" vertex="1">
<mxGeometry height="60" width="220" x="550" y="350" as="geometry" />
</mxCell>
<mxCell id="32" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;fontSize=14;fontColor=#93C5FD;fontStyle=1;" value="符合开闭原则" vertex="1">
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<mxCell id="34" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;fontSize=20;" value="💡" vertex="1">
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</mxCell>
<mxCell id="35" parent="1" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#450A0A;strokeColor=#DC2626;strokeWidth=3;shadow=1;" value="" vertex="1">
<mxGeometry height="235" width="820" x="40" y="480" as="geometry" />
</mxCell>
<mxCell id="36" parent="1" style="text;html=1;strokeColor=none;fillColor=#DC2626;align=center;verticalAlign=middle;fontSize=20;fontColor=#FFFFFF;fontStyle=1;rounded=1;" value="❌ 缺点" vertex="1">
<mxGeometry height="45" width="120" x="40" y="480" as="geometry" />
</mxCell>
<mxCell id="37" parent="1" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#7F1D1D;strokeColor=#F87171;strokeWidth=2;" value="" vertex="1">
<mxGeometry height="80" width="780" x="60" y="540" as="geometry" />
</mxCell>
<mxCell id="38" parent="1" style="ellipse;whiteSpace=wrap;html=1;fillColor=#DC2626;strokeColor=none;fontSize=16;fontColor=#FFFFFF;fontStyle=1;" value="1" vertex="1">
<mxGeometry height="35" width="35" x="75" y="550" as="geometry" />
</mxCell>
<mxCell id="39" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=18;fontColor=#F87171;fontStyle=1;" value="无法直接访问子类特有成员" vertex="1">
<mxGeometry height="35" width="300" x="120" y="550" as="geometry" />
</mxCell>
<mxCell id="40" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=14;fontColor=#FCA5A5;" value="需要向下转型才能访问子类独有的属性和方法" vertex="1">
<mxGeometry height="25" width="400" x="120" y="585" as="geometry" />
</mxCell>
<mxCell id="41" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;fontSize=28;" value="🔒" vertex="1">
<mxGeometry height="50" width="50" x="780" y="555" as="geometry" />
</mxCell>
<mxCell id="42" parent="1" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#172554;strokeColor=#3B82F6;strokeWidth=2;" value="" vertex="1">
<mxGeometry height="60" width="230" x="530" y="550" as="geometry" />
</mxCell>
<mxCell id="43" parent="1" style="text;html=1;strokeColor=none;fillColor=#3B82F6;align=center;verticalAlign=middle;fontSize=12;fontColor=#FFFFFF;fontStyle=1;rounded=1;" value="解决方案" vertex="1">
<mxGeometry height="22" width="70" x="530" y="550" as="geometry" />
</mxCell>
<mxCell id="44" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;fontSize=13;fontColor=#93C5FD;fontFamily=Courier New;" value="instanceof + 强制转型" vertex="1">
<mxGeometry height="30" width="220" x="530" y="575" as="geometry" />
</mxCell>
<mxCell id="45" parent="1" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#7F1D1D;strokeColor=#F87171;strokeWidth=2;" value="" vertex="1">
<mxGeometry height="65" width="780" x="60" y="635" as="geometry" />
</mxCell>
<mxCell id="46" parent="1" style="ellipse;whiteSpace=wrap;html=1;fillColor=#DC2626;strokeColor=none;fontSize=16;fontColor=#FFFFFF;fontStyle=1;" value="2" vertex="1">
<mxGeometry height="35" width="35" x="75" y="645" as="geometry" />
</mxCell>
<mxCell id="47" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=18;fontColor=#F87171;fontStyle=1;" value="运行时才确定调用方法" vertex="1">
<mxGeometry height="35" width="260" x="120" y="645" as="geometry" />
</mxCell>
<mxCell id="48" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=14;fontColor=#FCA5A5;" value="略有性能开销（动态绑定）" vertex="1">
<mxGeometry height="20" width="250" x="120" y="675" as="geometry" />
</mxCell>
<mxCell id="49" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;fontSize=28;" value="⏱️" vertex="1">
<mxGeometry height="50" width="50" x="780" y="645" as="geometry" />
</mxCell>
<mxCell id="50" parent="1" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#14532D;strokeColor=#22C55E;strokeWidth=2;" value="" vertex="1">
<mxGeometry height="45" width="360" x="400" y="645" as="geometry" />
</mxCell>
<mxCell id="51" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;fontSize=14;fontColor=#86EFAC;fontStyle=1;" value="✅ 现代 JVM 优化后可忽略不计" vertex="1">
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</root>
</mxGraphModel>
</diagram>
</mxfile>
四、三大特征综合对比与总结
4.1 对比表
| 特征 |
核心思想 |
关键字/机制 |
主要作用 |
| 封装 |
隐藏细节 |
private/getter/setter |
保护数据安全 |
| 继承 |
代码复用 |
extends/super |
建立类层次结构 |
| 多态 |
统一接口 |
重写/向上转型 |
灵活扩展 |
4.2 三者关系图
<mxfile>
<diagram id="three-features-relationship" name="三大特征的关系">
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<root>
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<mxGeometry height="780" width="860" x="20" y="20" as="geometry" />
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<mxGeometry height="55" width="860" x="20" y="20" as="geometry" />
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<mxCell id="4" parent="1" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#1E293B;strokeColor=#475569;strokeWidth=2;" value="" vertex="1">
<mxGeometry height="480" width="820" x="40" y="90" as="geometry" />
</mxCell>
<mxCell id="5" parent="1" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#172554;strokeColor=#3B82F6;strokeWidth=3;shadow=1;" value="" vertex="1">
<mxGeometry height="90" width="240" x="330" y="110" as="geometry" />
</mxCell>
<mxCell id="6" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;fontSize=28;fontColor=#60A5FA;fontStyle=1;" value="📦 封装" vertex="1">
<mxGeometry height="45" width="240" x="330" y="115" as="geometry" />
</mxCell>
<mxCell id="7" parent="1" style="text;html=1;strokeColor=none;fillColor=#3B82F6;align=center;verticalAlign=middle;fontSize=14;fontColor=#FFFFFF;rounded=1;fontStyle=1;" value="基础" vertex="1">
<mxGeometry height="28" width="70" x="415" y="160" as="geometry" />
</mxCell>
<mxCell id="8" edge="1" parent="1" style="endArrow=classic;html=1;strokeColor=#60A5FA;strokeWidth=4;flowAnimation=1;" value="">
<mxGeometry relative="1" as="geometry">
<mxPoint x="450" y="200" as="sourcePoint" />
<mxPoint x="450" y="270" as="targetPoint" />
</mxGeometry>
</mxCell>
<mxCell id="9" parent="1" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#0C4A6E;strokeColor=#0EA5E9;strokeWidth=2;dashed=1;" value="" vertex="1">
<mxGeometry height="35" width="380" x="260" y="225" as="geometry" />
</mxCell>
<mxCell id="10" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;fontSize=14;fontColor=#7DD3FC;" value="隐藏实现细节，提供公开接口" vertex="1">
<mxGeometry height="35" width="380" x="260" y="225" as="geometry" />
</mxCell>
<mxCell id="11" parent="1" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#14532D;strokeColor=#22C55E;strokeWidth=3;shadow=1;" value="" vertex="1">
<mxGeometry height="90" width="240" x="330" y="280" as="geometry" />
</mxCell>
<mxCell id="12" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;fontSize=28;fontColor=#4ADE80;fontStyle=1;" value="🔗 继承" vertex="1">
<mxGeometry height="45" width="240" x="330" y="285" as="geometry" />
</mxCell>
<mxCell id="13" parent="1" style="text;html=1;strokeColor=none;fillColor=#22C55E;align=center;verticalAlign=middle;fontSize=14;fontColor=#FFFFFF;rounded=1;fontStyle=1;" value="手段" vertex="1">
<mxGeometry height="28" width="70" x="415" y="330" as="geometry" />
</mxCell>
<mxCell id="14" edge="1" parent="1" style="endArrow=classic;html=1;strokeColor=#4ADE80;strokeWidth=4;flowAnimation=1;" value="">
<mxGeometry relative="1" as="geometry">
<mxPoint x="450" y="370" as="sourcePoint" />
<mxPoint x="450" y="440" as="targetPoint" />
</mxGeometry>
</mxCell>
<mxCell id="15" parent="1" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#052E16;strokeColor=#16A34A;strokeWidth=2;dashed=1;" value="" vertex="1">
<mxGeometry height="35" width="420" x="240" y="395" as="geometry" />
</mxCell>
<mxCell id="16" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;fontSize=14;fontColor=#86EFAC;" value="子类复用父类代码，建立 is-a 关系" vertex="1">
<mxGeometry height="35" width="420" x="240" y="395" as="geometry" />
</mxCell>
<mxCell id="17" parent="1" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#581C87;strokeColor=#A855F7;strokeWidth=3;shadow=1;" value="" vertex="1">
<mxGeometry height="90" width="240" x="330" y="450" as="geometry" />
</mxCell>
<mxCell id="18" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;fontSize=28;fontColor=#C084FC;fontStyle=1;" value="✨ 多态" vertex="1">
<mxGeometry height="45" width="240" x="330" y="455" as="geometry" />
</mxCell>
<mxCell id="19" parent="1" style="text;html=1;strokeColor=none;fillColor=#A855F7;align=center;verticalAlign=middle;fontSize=14;fontColor=#FFFFFF;rounded=1;fontStyle=1;" value="目的" vertex="1">
<mxGeometry height="28" width="70" x="415" y="500" as="geometry" />
</mxCell>
<mxCell id="20" parent="1" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#3B0764;strokeColor=#9333EA;strokeWidth=2;dashed=1;" value="" vertex="1">
<mxGeometry height="35" width="420" x="240" y="550" as="geometry" />
</mxCell>
<mxCell id="21" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;fontSize=14;fontColor=#D8B4FE;" value="同一接口不同实现，实现灵活扩展" vertex="1">
<mxGeometry height="35" width="420" x="240" y="550" as="geometry" />
</mxCell>
<mxCell id="22" parent="1" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#1E293B;strokeColor=#F59E0B;strokeWidth=3;shadow=1;" value="" vertex="1">
<mxGeometry height="170" width="820" x="40" y="595" as="geometry" />
</mxCell>
<mxCell id="23" parent="1" style="text;html=1;strokeColor=none;fillColor=#F59E0B;align=center;verticalAlign=middle;fontSize=16;fontColor=#FFFFFF;fontStyle=1;rounded=1;" value="⚠️ 缺一不可" vertex="1">
<mxGeometry height="35" width="130" x="40" y="595" as="geometry" />
</mxCell>
<mxCell id="24" parent="1" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#172554;strokeColor=#3B82F6;strokeWidth=2;" value="" vertex="1">
<mxGeometry height="55" width="250" x="60" y="645" as="geometry" />
</mxCell>
<mxCell id="25" parent="1" style="text;html=1;strokeColor=none;fillColor=#3B82F6;align=center;verticalAlign=middle;fontSize=12;fontColor=#FFFFFF;fontStyle=1;rounded=1;" value="没有封装" vertex="1">
<mxGeometry height="22" width="70" x="60" y="645" as="geometry" />
</mxCell>
<mxCell id="26" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;fontSize=13;fontColor=#93C5FD;" value="❌ 数据不安全，类设计混乱" vertex="1">
<mxGeometry height="25" width="250" x="60" y="670" as="geometry" />
</mxCell>
<mxCell id="27" parent="1" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#14532D;strokeColor=#22C55E;strokeWidth=2;" value="" vertex="1">
<mxGeometry height="55" width="250" x="325" y="645" as="geometry" />
</mxCell>
<mxCell id="28" parent="1" style="text;html=1;strokeColor=none;fillColor=#22C55E;align=center;verticalAlign=middle;fontSize=12;fontColor=#FFFFFF;fontStyle=1;rounded=1;" value="没有继承" vertex="1">
<mxGeometry height="22" width="70" x="325" y="645" as="geometry" />
</mxCell>
<mxCell id="29" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;fontSize=13;fontColor=#86EFAC;" value="❌ 多态无法实现（需父子关系）" vertex="1">
<mxGeometry height="25" width="250" x="325" y="670" as="geometry" />
</mxCell>
<mxCell id="30" parent="1" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#581C87;strokeColor=#A855F7;strokeWidth=2;" value="" vertex="1">
<mxGeometry height="55" width="250" x="590" y="645" as="geometry" />
</mxCell>
<mxCell id="31" parent="1" style="text;html=1;strokeColor=none;fillColor=#A855F7;align=center;verticalAlign=middle;fontSize=12;fontColor=#FFFFFF;fontStyle=1;rounded=1;" value="没有多态" vertex="1">
<mxGeometry height="22" width="70" x="590" y="645" as="geometry" />
</mxCell>
<mxCell id="32" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;fontSize=13;fontColor=#D8B4FE;" value="❌ 继承的价值大打折扣" vertex="1">
<mxGeometry height="25" width="250" x="590" y="670" as="geometry" />
</mxCell>
<mxCell id="33" parent="1" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#0F172A;strokeColor=#10B981;strokeWidth=2;" value="" vertex="1">
<mxGeometry height="35" width="780" x="60" y="715" as="geometry" />
</mxCell>
<mxCell id="34" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;fontSize=14;fontColor=#6EE7B7;fontStyle=1;" value="💡 封装是基石，继承是桥梁，多态是目标 —— 三者相辅相成，共同构建面向对象编程体系" vertex="1">
<mxGeometry height="35" width="780" x="60" y="715" as="geometry" />
</mxCell>
<mxCell id="35" parent="1" style="shape=curlyBracket;whiteSpace=wrap;html=1;rounded=1;flipH=1;labelPosition=right;verticalLabelPosition=middle;align=left;verticalAlign=middle;strokeColor=#F59E0B;strokeWidth=3;size=0.5;" value="" vertex="1">
<mxGeometry height="420" width="30" x="580" y="120" as="geometry" />
</mxCell>
<mxCell id="36" parent="1" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#1E293B;strokeColor=#F59E0B;strokeWidth=2;" value="" vertex="1">
<mxGeometry height="100" width="220" x="620" y="280" as="geometry" />
</mxCell>
<mxCell id="37" parent="1" style="text;html=1;strokeColor=none;fillColor=#F59E0B;align=center;verticalAlign=middle;fontSize=14;fontColor=#FFFFFF;fontStyle=1;rounded=1;" value="🎯 层层递进" vertex="1">
<mxGeometry height="28" width="100" x="620" y="280" as="geometry" />
</mxCell>
<mxCell id="38" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=13;fontColor=#93C5FD;" value="封装 → 保护数据" vertex="1">
<mxGeometry height="20" width="180" x="635" y="315" as="geometry" />
</mxCell>
<mxCell id="39" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=13;fontColor=#86EFAC;" value="继承 → 复用代码" vertex="1">
<mxGeometry height="20" width="180" x="635" y="335" as="geometry" />
</mxCell>
<mxCell id="40" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;fontSize=13;fontColor=#D8B4FE;" value="多态 → 灵活扩展" vertex="1">
<mxGeometry height="20" width="180" x="635" y="355" as="geometry" />
</mxCell>
<mxCell id="41" parent="1" style="shape=curlyBracket;whiteSpace=wrap;html=1;rounded=1;labelPosition=right;verticalLabelPosition=middle;align=left;verticalAlign=middle;strokeColor=#F59E0B;strokeWidth=3;size=0.5;" value="" vertex="1">
<mxGeometry height="420" width="30" x="90" y="120" as="geometry" />
</mxCell>
<mxCell id="42" parent="1" style="rounded=1;whiteSpace=wrap;html=1;fillColor=#1E293B;strokeColor=#EC4899;strokeWidth=2;" value="" vertex="1">
<mxGeometry height="100" width="160" x="60" y="280" as="geometry" />
</mxCell>
<mxCell id="43" parent="1" style="text;html=1;strokeColor=none;fillColor=#EC4899;align=center;verticalAlign=middle;fontSize=12;fontColor=#FFFFFF;fontStyle=1;rounded=1;" value="🏗️ 设计原则" vertex="1">
<mxGeometry height="25" width="90" x="60" y="280" as="geometry" />
</mxCell>
<mxCell id="44" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;fontSize=12;fontColor=#F9A8D4;" value="高内聚" vertex="1">
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</mxCell>
<mxCell id="45" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;fontSize=12;fontColor=#F9A8D4;" value="低耦合" vertex="1">
<mxGeometry height="20" width="60" x="130" y="310" as="geometry" />
</mxCell>
<mxCell id="46" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;fontSize=12;fontColor=#F9A8D4;" value="开闭原则" vertex="1">
<mxGeometry height="20" width="70" x="70" y="335" as="geometry" />
</mxCell>
<mxCell id="47" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;fontSize=12;fontColor=#F9A8D4;" value="里氏替换" vertex="1">
<mxGeometry height="20" width="70" x="140" y="335" as="geometry" />
</mxCell>
<mxCell id="48" parent="1" style="text;html=1;strokeColor=none;fillColor=none;align=center;verticalAlign=middle;fontSize=12;fontColor=#F9A8D4;" value="依赖倒置" vertex="1">
<mxGeometry height="20" width="70" x="100" y="358" as="geometry" />
</mxCell>
</root>
</mxGraphModel>
</diagram>
</mxfile>
4.3 综合代码示例
abstract class Shape {
private String color;
private boolean filled;
public Shape(String color, boolean filled) {
this.color = color;
this.filled = filled;
}
public String getColor() { return color; }
public void setColor(String color) { this.color = color; }
public boolean isFilled() { return filled; }
public abstract double getArea();
public abstract double getPerimeter();
@Override
public String toString() {
return "Shape[color=" + color + ", filled=" + filled + "]";
}
}
class Circle extends Shape {
private double radius;
public Circle(String color, boolean filled, double radius) {
super(color, filled);
this.radius = radius;
}
@Override
public double getArea() {
return Math.PI * radius * radius;
}
@Override
public double getPerimeter() {
return 2 * Math.PI * radius;
}
}
class Rectangle extends Shape {
private double width;
private double height;
public Rectangle(String color, boolean filled, double width, double height) {
super(color, filled);
this.width = width;
this.height = height;
}
@Override
public double getArea() {
return width * height;
}
@Override
public double getPerimeter() {
return 2 * (width + height);
}
}
class Canvas {
private List<Shape> shapes = new ArrayList<>();
public void addShape(Shape shape) {
shapes.add(shape);
}
public double getTotalArea() {
double total = 0;
for (Shape shape : shapes) {
total += shape.getArea();
}
return total;
}
public void printAllShapes() {
for (Shape shape : shapes) {
System.out.println(shape + ", Area=" + shape.getArea());
}
}
}
public class Main {
public static void main(String[] args) {
Canvas canvas = new Canvas();
canvas.addShape(new Circle("红色", true, 5));
canvas.addShape(new Rectangle("蓝色", false, 3, 4));
canvas.addShape(new Circle("绿色", true, 3));
canvas.printAllShapes();
System.out.println("总面积：" + canvas.getTotalArea());
}
}
4.4 速查表
| 特征 |
核心要点 |
实现方式 |
| 封装 |
隐藏数据，暴露接口 |
private + getter/setter |
| 继承 |
子类复用父类 |
extends + super |
| 多态 |
同一行为多种形态 |
继承 + 重写 + 向上转型 |
4.5 记忆口诀
┌─────────────────────────────────────────────────────────────────┐
│ 记忆口诀 │
├─────────────────────────────────────────────────────────────────┤
│ │
│ 封装：藏好属性露接口，getter/setter 来帮忙 │
│ │
│ 继承：extends 连接父与子，super 调用不能忘 │
│ │
│ 多态：父类引用指向子，编译看左运行右 │
│ │
│ ───────────────────────────────────────────────── │
│ │
│ 多态三要素： │
│ ① 有继承 ② 有重写 ③ 父类引用指向子类对象 │
│ │
│ 方法重写规则（两同两小一大）： │
│ • 两同：方法名、参数列表相同 │
│ • 两小：返回类型 ≤ 父类、异常 ≤ 父类 │
│ • 一大：访问权限 ≥ 父类 │
│ │
└─────────────────────────────────────────────────────────────────┘
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