Polymorphic and static method

class Parent {
    int num = 4;

    protected void foo() {
        System.out.println("foo() of Parent");
    }

    static protected void bar() {
        System.out.println("bar() of Parent");
    }
}

class Child extends Parent {
    int num = 5;

    protected void foo() {
        System.out.println("foo() of Child");
    }

    static protected void bar() {
        System.out.println("bar() of Child");
    }
}

public class Test {
    public static void main(String[] args) {
        Parent f1 = new Parent();
        System.out.println(f1.num);

        Parent f2 = new Child();
        System.out.println(f2.num);

        Child c = new Child();
        System.out.println(c.num);

        f1.foo();
        f2.foo();
        c.foo();

        f1.bar();
        f2.bar();
        c.bar();
    }
}

输出如下：

4
4
5
foo() of Parent
foo() of Child
foo() of Child
bar() of Parent
bar() of Parent
bar() of Child

 

1.关于输出4 4 5：需要明确多态的定义，多态是针对方法的，和属性无关，f2引用的类型是Parent，所以输出4

2.关于f2.bar()：static方法是没有多态的，static、final、private都是前期绑定（编译期绑定），多态是晚绑定（动态绑定）。扩展:当被定义为private，会被隐式指定为final

补充：

1.final：

There are three distinct meanings for the final keyword in Java.

• A final class cannot be extended.

• A final method cannot be overridden.

• A final variable cannot be assigned to after it has been initialized.

• 总结：子类可以获得父类的final方法，但无法覆盖（override）

2. All methods that are accessible are inherited by subclasses.The only difference with inherited static (class) methods and inherited non-static (instance) methods is that when you write a new static method with the same signature, the old static method is just hidden, not overridden.

在本例中，子类的static bar隐藏了父类的static bar，也就是说就算没有子类的bar方法，子类依然可以调用bar方法