Question

In below class I am adding a String to a Vector(v1) and then adding that Vector(v1) to a new Vector(v2). I then re-initialise Vector v1.

How does java retain the reference to v1 ? When I re-initalise v1 is java maintaining a reference under the hood ?

The output of below is “1”.

public class VectorTest {

    public static void main(String args[]){
        new VectorTest().testVector();
    }

    private void testVector(){

        Vector v1 = new Vector();
        Vector v2 = new Vector();

        v1.add("1");
        v2.add(v1);
        v1 = new Vector();
        Vector v3 = (Vector)v2.get(0);
        System.out.println(v3.elementAt(0));
    }

}

Answer 1

private void testVector(){

    Vector v1 = new Vector(); // v1 references some vector created on the heap
    Vector v2 = new Vector(); 

    v1.add("1"); // vector referenced by v1 gets a "1" added
    v2.add(v1); // make v2[0] the second referrer to that Vector you created

    v1 = new Vector(); // Change what v1 refers to, this is not the same as destroying the original Vector
    // v2[0] still holds a reference to the original vector

    Vector v3 = (Vector)v2.get(0); // v3 is now another referrer to the same vector
    System.out.println(v3.elementAt(0));
}

You’re changing what v1 refers to, in this case some other Vector. But v2 still holds a reference to the original Vector.

Objects in Java can have multiple variables referring to it. Vector objects are also mutable, meaning that if one of its referrers modifies it, like you do with:

v1.add("1");

Then every other referrer will now reference that same vector that has a "1" in it.

You can also change what objects a variable refers to, like you do here:

v1 = new Vector();

So after that happens, v1 holds a different reference to a different Vector object, while v2[0] (excuse the array notation) still holds a reference to the original Vector you created and assigned to v1 at the beginning.

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Vector v3 = (Vector)v2.get(0);

When you do this, you’re making v3 refer to the same Vector that you created at the beginning, so there are two referrers to this object at this point:

1. v2[0]
2. v3

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I’d try to draw some crude images to demonstrate this, but I think it’s easier just to read this page from the Java tutorial about objects and references:

http://download.oracle.com/javase/tutorial/java/javaOO/objectcreation.html