Java二次分发

java 复制代码

package com.example.demo;

public class UserDemo {
  public static void main(String[] args) {
    Driver a = new FemaleDriver();
    Driver b = new MaleDriver();
    Vehicle x = new Car();
    Vehicle y = new Bus();
    a.drives(x);
    b.drives(y);
  }
}

abstract class Vehicle {
  private String type;
  public Vehicle() {};
  public Vehicle(String s) {
    type = s;
  };
  public abstract void driveByFemaleDriver();
  public abstract void driveByMaleDriver();
}

class Bus extends Vehicle {
  public Bus() {}
  public void driveByFemaleDriver(){
    System.out.println("bus drive by female driver");
  }
  public void driveByMaleDriver() {
    System.out.println("bus drive by male driver");
  }
}

class Car extends Vehicle {
  public Car() {}
  public void driveByFemaleDriver(){
    System.out.println("bus drive by female driver");
  }
  public void driveByMaleDriver() {
    System.out.println("bus drive by male driver");
  }
}

abstract class Driver{
  public Driver() {}
  public abstract void drives(Vehicle v);
}

// 二次分发
class FemaleDriver extends Driver {
  public FemaleDriver() {}
  public void drives(Vehicle v) {
    v.driveByFemaleDriver();
  }
}
class MaleDriver extends Driver {
  public MaleDriver() {}
  public void drives(Vehicle v) {
    v.driveByMaleDriver();
  }
}

这段代码演示了一个经典的设计模式------双分派（Double Dispatch），我来逐层解释：

📐 整体类结构

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.lollipop{fill:#ECECFF!important;stroke:#333333!important;stroke-width:1;}#mermaid-svg-ik2oUyw5tiUM60vP .edgeTerminals{font-size:11px;line-height:initial;}#mermaid-svg-ik2oUyw5tiUM60vP .classTitleText{text-anchor:middle;font-size:18px;fill:#333;}#mermaid-svg-ik2oUyw5tiUM60vP .label-icon{display:inline-block;height:1em;overflow:visible;vertical-align:-0.125em;}#mermaid-svg-ik2oUyw5tiUM60vP .node .label-icon path{fill:currentColor;stroke:revert;stroke-width:revert;}#mermaid-svg-ik2oUyw5tiUM60vP :root{--mermaid-font-family:"trebuchet ms",verdana,arial,sans-serif;} <<abstract>>
Vehicle
-String type
+driveByFemaleDriver()
+driveByMaleDriver()
<<abstract>>
Driver
+drives(Vehicle v)
Bus
Car
FemaleDriver
MaleDriver

🔍 逐段解析

第一层：抽象类 `Vehicle`（载具）

java 复制代码

abstract class Vehicle {
    private String type;
    public Vehicle() {};
    public Vehicle(String s) { type = s; }

    // 定义两个抽象方法：谁来开，由子类实现
    public abstract void driveByFemaleDriver();
    public abstract void driveByMaleDriver();
}

abstract 表示这是抽象类，不能直接 new
声明了两个抽象方法，强制子类（Car、Bus）必须实现

第二层：`Car` 和 `Bus` 实现具体行为

java 复制代码

class Bus extends Vehicle {
    public void driveByFemaleDriver() {
        System.out.println("bus drive by female driver");
    }
    public void driveByMaleDriver() {
        System.out.println("bus drive by male driver");
    }
}

class Car extends Vehicle {
    // 同理，实现自己的驾驶行为
}

第三层：抽象类 `Driver`（司机）

java 复制代码

abstract class Driver {
    public abstract void drives(Vehicle v);  // 接收一个载具
}

第四层：`FemaleDriver` 和 `MaleDriver` 实现分派

java 复制代码

class FemaleDriver extends Driver {
    public void drives(Vehicle v) {
        v.driveByFemaleDriver();  // 调用载具上"女性驾驶"方法
    }
}

class MaleDriver extends Driver {
    public void drives(Vehicle v) {
        v.driveByMaleDriver();    // 调用载具上"男性驾驶"方法
    }
}

第五层：主方法执行

java 复制代码

Driver a = new FemaleDriver();
Driver b = new MaleDriver();
Vehicle x = new Car();
Vehicle y = new Bus();

a.drives(x);  // FemaleDriver.drives(Car)  → Car.driveByFemaleDriver()
b.drives(y);  // MaleDriver.drives(Bus)    → Bus.driveByMaleDriver()

🎯 核心：什么是双分派？

Java 本身只支持单分派 （只根据调用者的类型决定执行哪个方法），但这段代码通过两次多态调用模拟了双分派：

复制代码

第一次分派：根据 Driver 的实际类型（Female / Male）
     ↓
a.drives(x)  →  FemaleDriver.drives()

第二次分派：根据 Vehicle 的实际类型（Car / Bus）
     ↓
v.driveByFemaleDriver()  →  Car.driveByFemaleDriver()

用表格表示最终会调用哪个方法：

	`Car`	`Bus`
FemaleDriver	`Car.driveByFemaleDriver()`	`Bus.driveByFemaleDriver()`
MaleDriver	`Car.driveByMaleDriver()`	`Bus.driveByMaleDriver()`

💡 这个模式解决了什么问题？

不用双分派的写法（反例）：

java 复制代码

// ❌ 用 if-else 判断类型，难以扩展
public void drives(Vehicle v) {
    if (v instanceof Car) {
        System.out.println("female drives car");
    } else if (v instanceof Bus) {
        System.out.println("female drives bus");
    }
    // 每次新增车辆类型都要修改这里 ❌
}

用双分派的写法：

java 复制代码

// ✅ 新增一种车辆，只需新建子类并实现两个方法，无需改动 Driver
class Truck extends Vehicle {
    public void driveByFemaleDriver() { ... }
    public void driveByMaleDriver() { ... }
}

📝 总结

概念	说明
抽象类	`Vehicle` 和 `Driver` 定义骨架，不能直接实例化
多态	父类引用指向子类对象（`Driver a = new FemaleDriver()`）
双分派	通过两次虚方法调用，根据司机类型 + 载具类型共同决定最终行为
设计目的	避免 `instanceof` 判断，新增类型时只需扩展，不修改已有代码（开闭原则）

📐 整体类结构

🔍 逐段解析

第一层：抽象类 Vehicle（载具）

第二层：Car 和 Bus 实现具体行为

第三层：抽象类 Driver（司机）

第四层：FemaleDriver 和 MaleDriver 实现分派

第五层：主方法执行

🎯 核心：什么是双分派？

💡 这个模式解决了什么问题？

📝 总结

第一层：抽象类 `Vehicle`（载具）

第二层：`Car` 和 `Bus` 实现具体行为

第三层：抽象类 `Driver`（司机）

第四层：`FemaleDriver` 和 `MaleDriver` 实现分派