jsonfry jsonfry - 5 months ago 8
Java Question

Java: Calling a super method which calls an overridden method

public class SuperClass
{
public void method1()
{
System.out.println("superclass method1");
this.method2();
}

public void method2()
{
System.out.println("superclass method2");
}

}

public class SubClass extends SuperClass
{
@Override
public void method1()
{
System.out.println("subclass method1");
super.method1();
}

@Override
public void method2()
{
System.out.println("subclass method2");
}
}



public class Demo
{
public static void main(String[] args)
{
SubClass mSubClass = new SubClass();
mSubClass.method1();
}
}


my expected output:


subclass method1

superclass method1

superclass method2


actual output:


subclass method1

superclass method1

subclass method2


I know technically I have overriden a public method, but I figured that because I was calling the super, any calls within the super would stay in the super, this isn't happening. Any ideas as to how I can make it happen?

Answer

The keyword super doesn't "stick". Every method call is handled individually, so even if you got to SuperClass.method1() by calling super, that doesn't influence any other method call that you might make in the future.

That means there is no direct way to call SuperClass.method2() from SuperClass.method1() without going though SubClass.method2() unless you're working with an actual instance of SuperClass.

You can't even achieve the desired effect using Reflection (see the documentation of java.lang.reflect.Method.invoke(Object, Object...)).

[EDIT] There still seems to be some confusion. Let me try a different explanation.

When you invoke foo(), you actually invoke this.foo(). Java simply lets you omit the this. In the example in the question, the type of this is SubClass.

So when Java executes the code in SuperClass.method1(), it eventually arrives at this.method2();

Using super doesn't change the instance pointed to by this. So the call goes to SubClass.method2() since this is of type SubClass.

Maybe it's easier to understand when you imagine that Java passes this as a hidden first parameter:

public class SuperClass
{
    public void method1(SuperClass this)
    {
        System.out.println("superclass method1");
        this.method2(this); // <--- this == mSubClass
    }

    public void method2(SuperClass this)
    {
        System.out.println("superclass method2");
    }

}

public class SubClass extends SuperClass
{
    @Override
    public void method1(SubClass this)
    {
        System.out.println("subclass method1");
        super.method1(this);
    }

    @Override
    public void method2(SubClass this)
    {
        System.out.println("subclass method2");
    }
}



public class Demo 
{
    public static void main(String[] args) 
    {
        SubClass mSubClass = new SubClass();
        mSubClass.method1(mSubClass);
    }
}

If you follow the call stack, you can see that this never changes, it's always the instance created in main().