java - 静态工厂方法

Question

静态工厂方法的一个优点是：

与构造函数不同，它们可以返回其返回类型的任何子类型的对象，这为您选择返回对象的类提供了极大的灵活性。

这到底是什么意思？有人可以用代码解释一下吗？

Answer 1

public class Foo {
    public Foo() {
        // If this is called by someone saying "new Foo()", I must be a Foo.
    }
}

public class Bar extends Foo {
    public Bar() {
        // If this is called by someone saying "new Bar()", I must be a Bar.
    }
}

public class FooFactory {
    public static Foo buildAFoo() {
        // This method can return either a Foo, a Bar,
        // or anything else that extends Foo.
    }
}

Answer 2

让我将您的问题分为两部分
（1）与构造函数不同，它们可以返回其返回类型的任何子类型的对象
（2），这为您选择返回对象的类提供了极大的灵活性。
假设您有两个扩展类，Player它们是PlayerWithBall和PlayerWithoutBall

public class Player{
  public Player(boolean withOrWithout){
    //...
  }
}

//...

// What exactly does this mean?
Player player = new Player(true);
// You should look the documentation to be sure.
// Even if you remember that the boolean has something to do with a Ball
// you might not remember whether it specified withBall or withoutBall.

to

public class PlayerFactory{
  public static Player createWithBall(){
    //...
  }

  public static Player createWithoutBall(){
    //...
  }
}

// ...

//Now its on your desire , what you want :)
Foo foo = Foo.createWithBall(); //or createWithoutBall();

在这里你得到两个答案 灵活性和不同于构造函数的行为 现在你可以通过这些工厂方法看到你需要哪种类型的玩家