Why does this provide me with a stack overflow error?

The following code yields a stack overflow error, can someone tell me why? (I know how to fix the error, if I reference 'super' instead of 'this' in the add method, but I am not sure why that works.)

package subclassingVector;

import java.util.Vector;

public class MyVectorSubclass extends Vector<MyModelClass> {

    public MyVectorSubclass(){
        MyModelClass m = new MyModelClass();
        m.setId(2);
        this.add(m);
        if(this.contains(m)){
        System.out.println("contains is true");
    }
    }


    public boolean add(MyModelClass o){
            this.add(o);
            return true;
    }

    public boolean contains(Object o){
            for(subclassingVector.MyModelClass mmc: this){
                if(mmc.equals((MyModelClass) o)){
                    return true;

                }
            }
        return false;
    }


    public static void main(String[] args) {

        String s = new String("SSEE");
        MyVectorSubclass m = new MyVectorSubclass();

    }

    public class MyModelClass {

        private Integer id = null;

        public MyModelClass() {
            // TODO Auto-generated constructor stub
        }

        public Integer getId() {
            return id;
        }

        public void setId(Integer id) {
            this.id = id;
        }

    }

}

Solution

This code calls itself, forever, hence the StackOverflowError:

public boolean add(MyModelClass o){
    this.add(o);
    return true;
}

The this represents the instance of the object that the current method is being called on. In fact, the above code, in this context, is precisely the same as:

public boolean add(MyModelClass o){
    add(o);
    return true;
}

This code does something entirely different:

public boolean add(MyModelClass o){
    super.add(o);
    return true;
}

That doesn't make a recursive call. That simply calls the implementation of add() in the base class.

Cory Kendall's comment on the question is very valuable:

If you get a StackOverflowError, looking at the stack trace of the Error should be informative; walk through the logic using the given line numbers.

The stack traces of an exception can give you a lot of useful information about the origin of the problem. In this case, the stack trace would clearly show that MyVectorSubClass.add() is repeatedly calling itself in the case of this.add(o).

I suggest taking a look at: