How bridge method works?

Question

I am new to java.I still feel I have to understand a lot so if this question seems stupid forgive me. Now I was going through http://docs.oracle.com/javase/tutorial/java/generics/bridgeMethods.html

Here I found a lot of confusion .

public class Node<T> {

    public T data;

    public Node(T data) {
        this.data = data;
    }

    public void setData(T data) {
        System.out.println("Node.setData");
        this.data = data;
    }
}


public class MyNode extends Node<Integer> {
    public MyNode(Integer data) {
        super(data);
    }

    public void setData(Integer data) {
        System.out.println("MyNode.setData");
        super.setData(data);
    }

    public static void main(String[] args) {
        MyNode mn = new MyNode(5);
        Node n = mn; // A raw type - compiler throws an unchecked warning
        n.setData("Hello"); // Causes a ClassCastException to be thrown.
        Integer x = mn.data;

    }
}

When I ran this code I got below error

Exception in thread "main" java.lang.ClassCastException: java.lang.String cannot be cast to java.lang.Integer
at MyNode.setData(MyNode.java:1)
at MyNode.main(MyNode.java:14)

Below are the confusions
1) Why is it showing line number 1
2) If you read through the blog they say that the type erasure will replace type parameter will replace above code as below

public class Node {

    private Object data;

    public Node(Object data) { this.data = data; }

    public void setData(Object data) {
        System.out.println("Node.setData");
        this.data = data;
    }
}

public class MyNode extends Node {

    public MyNode(Integer data) { super(data); }

    public void setData(Integer data) {
        System.out.println(data);
        super.setData(data);
    }

    public static void main(String[] args) {
        MyNode mn = new MyNode(5);
        Node n = mn;            // A raw type - compiler throws an unchecked warning
        n.setData("Hello");     // Causes a ClassCastException to be thrown.
        //Integer x = mn.data

    }
}

when I run the above code I get no error , code runs fine

from the documentation both r same ? why this different in behavior

now other most important question on oop is when we extend one class and when we call super constructor though super object is not created then what is the use of calling super. Please explain.

Answer 1

from the documentation both r same ? why this different in behavior

The reason there is different behavior between the two code samples is because when you replaced the generic T with Object, you indirectly caused setData() to no longer override setData() in the super class. Since the parameter types are different, you are simply overloading it in the second example. So in the second example you are calling the super class directly which takes Object. In the first example you are calling the subclass which takes Integer.

when we call super constructor though super object is not created then what is the use of calling super

The super class and the sub-class are the same object (with its code split between two classes). So the super class constructor is called to initialize any fields defined in the super class. In your example, if you never called super(data) then this.data would never have been set.