java多态实现示例

Posted 2020-11-27 innndown

多态

多态存在的三个必要条件：继承，重写，父类引用指向子类对象，
当使用多态方式调用方法时，首先检查父类中是否有该方法，如果没有，则error；如果有，调用子类的同名方法。
example:

package demo;

public class people {
    public people(){

    }
    public void run(){
        System.out.println("10");
    }
}

package demo;

public class player extends people{
    player(){

    }

    @Override
    public void run() {
        //super.run();
        System.out.println("100");
    }

    public static void main(String[] args) {
        people bob = new player();
        bob.run();
    }
}

输出结果为100

多态的缺陷

• "重写私有方法"

//: polymorphism/PrivateOverride.java
// Trying to override a private method.
package polymorphism;
import static net.mindview.util.Print.*;

public class PrivateOverride {
  private void f() { print("private f()"); }
  public static void main(String[] args) {
    PrivateOverride po = new Derived();
    po.f();
  }
}

class Derived extends PrivateOverride {
  public void f() { print("public f()"); }
} /* Output:
private f()
*///:~

期望输出的是public f(),但是父类中的private方法被自动认为是final方法，对子类是屏蔽的，所以不能被重载；所以在这种情况下，子类的f()被当作一个全新的方法。

• 域（类变量成员变量或参数）

//: polymorphism/FieldAccess.java
// Direct field access is determined at compile time.

class Super {
  public int field = 0;
  public int getField() { return field; }
}

class Sub extends Super {
  public int field = 1;
  public int getField() { return field; }
  public int getSuperField() { return super.field; }
}

public class FieldAccess {
  public static void main(String[] args) {
    Super sup = new Sub(); // Upcast
    System.out.println("sup.field = " + sup.field +
      ", sup.getField() = " + sup.getField());
    Sub sub = new Sub();
    System.out.println("sub.field = " +
      sub.field + ", sub.getField() = " +
      sub.getField() +
      ", sub.getSuperField() = " +
      sub.getSuperField());
  }
} /* Output:
sup.field = 0, sup.getField() = 1
sub.field = 1, sub.getField() = 1, sub.getSuperField() = 0
*///:~

所有的域操作都不是多态的，对于子类，域有不同的存储空间。

• 静态方法
  静态方法属于类，不能被重写

子类重写父类方法调用父类方法

在c++中，可以通过向上提升的类型操作实现对父类行为的调用，而在Java中，无论对其进行什么样的类型转换，其类型实际上是不变的，只能通过super调用。
example:

package demo;

public class people {
    public people(){

    }
    public void run(){
        System.out.println("10");
    }
}

package demo;

public class player extends people{
    player(){

    }

    @Override
    public void run() {
        super.run();
        System.out.println("100");
    }

    public static void main(String[] args) {
        people bob = new player();
        bob.run();
    }
}

多态示例1：

函数参数实现多态性

public class people {
    public people(){

    }
    public void run(){
        System.out.println("10");
    }
}

public class player extends people{
    player(){

    }

    @Override
    public void run() {
        //super.run();
        System.out.println("100");
    }


}

public class running {
    public void competition(people p1,people p2){//父类引用指向子类对象
        p1.run();
        p2.run();
    }
    public static void main(String[] args) {
        people bob = new player();
        people sam = new player();
        running round1 = new running();
        round1.competition(bob,sam);
    }
}

多态示例2：

接口实现多态性，解耦

public interface runrunrun {
    public void run();
}

public class people implements runrunrun {
    public people(){

    }
    public void run(){
        System.out.println("10");
    }
}


public class player implements runrunrun{
    player(){

    }
    //@Override
    public void run() {
        //super.run();
        System.out.println("100");
    }
}


public class running {
    public void competition(runrunrun p1,runrunrun p2){
        p1.run();
        p2.run();
    }
    public static void main(String[] args) {
        player bob = new player();
        player sam = new player();
        running round1 = new running();
        round1.competition(bob,sam);
    }
}