How does inheritance in Java work?

Question

We have next classes:

class Super {
    void foo() {
        System.out.println("Super");
    }
}

class Sub extends Super {
    void foo() {
        super.foo();
        System.out.println("Sub");
    }
}

public class Clazz {
    public static void main(String[] args) {
        new Sub().foo();
    }
}

Output is:

Super

Sub

Questions:

What does super present? Is it object of parent class, which child keeps as field?

• If it is, how does inheritance of abstract classes work? You can not create instance of abstract class.
• If it is not, where are overridden method hold?

I tried to Google, but all I found is common information about how to inherit classes and so on.

Update:

You are still telling me obvious things. Maybe my question was little misleading, but I'll try to rephrase it:

• When we are calling method with super, you say, we are accessing parent's method. But how can we call this method without parent's object?
• Is super same as this? this is a reference to concrete object, as you know.

Answer 1

The child class does not maintain any special field representing its parent. You may be thinking something along the lines of inner classes, which do maintain a reference to their outer class. This is a special case and does not represent how super-sub classes relate to each other.

Internally, the JVM maintains a 'method table', associating each class its loaded with the methods available for that class. The JVM also knows about the relationships between all classes its loaded, including super-sub relationships.

When you invoke a super function, the JVM actually does a couple of things:

• Determines the parent of the class that the method is is being called from
• Determines the method on the parent that will be called
• Invokes the method, with a special instruction (invokespecial)

If you were to examine the class file for your Sub class, you would see something like this for the foo method:

void foo();
    flags: 
    Code:
        stack=2, locals=1, args_size=1
            0: aload_0       
            1: invokespecial #2        // Method Super.foo:()V
            4: getstatic     #3        // Field java/lang/System.out:Ljava/io/PrintStream;
            7: ldc           #4        // String Sub
            9: invokevirtual #5        // Method java/io/PrintStream.println:(Ljava/lang/String;)V

Line 1 in the listing shows the special instruction that invokes the superclass method.

A good source of reading would be the Java Virtual Machine Specification, particularly Section 2.11.8.