为什么以下代码的输出为x = 0 x = 20？

Question

class Parent 

{

protected int x = 10;

public Parent() 

{

    foo();
    }
    public void foo()
    {
         System.out.printf("x = %d ", x);
    }
}

class Child extends Parent 

{

private int x = 20;

public void foo() 

{

    System.out.printf("x = %d ", x);

    }

}
public class Main

{

    public static void main(String[] args)

    {

        Parent p=new Child();

        p.foo();

        }

}

Answer 1

You get 0 because that is the default value of an int , and your constructor hasn't yet run . You are also shadowing your variable x in Child . Further, there isn't an implicit super invocation for anything except constructors. I think you wanted / expected something like,

public class Parent {
    private int x = 10; // <-- this also works if protected

    public void foo() {
        System.out.printf("x = %d ", x);        
    }
}

public class Child extends Parent {
    private int x = 20;

    @Override
    public void foo() {
        super.foo();
        System.out.printf("x = %d ", x);
    }
}

With those changes I get

x = 10 x = 20 

Note that the Override annotation is optional (but a good practice when you override an implementation).

Answer 2

Parent p = new Child();

Will call the constructor in the parent.

The constructor in the parent calls the method foo() The method foo() is in both the class Parent and Child

Since you have instantiated the Child class, the method call foo() in the parent calls the method foo() in the child class.

After you call p.foo() , x is assigned to 20 While the child class is still not instantiated which has an undeclared integer x which prints 0 .