Weird Object/class behaviour in Java

Question

I have an array of Group. When I call increment, to increment the id of the object (in A) all the IDs of all the object in the array are being incremented. Anyone know why please?

 Group [] groups = new Group [g];
    groups[0] = group;
    for (int i=1; i<g;i++){
        groups[i] = groups[i-1];
        groups[i].increment();              .......... A

    }


    public void increment() {
         this.groupid = this.groupid++;
    }

Answer 1

for (int i=1; i<g;i++){
    groups[i] = groups[i-1];
    groups[i].increment();
}

Every index of your array refers to the same Group object.

Answer 2

Because you're simply copying references to all the elements of the array. All the elements contain the same instance of Group but different references.

You should either create a new Group object each time in the loop or use a copy constructor.

Answer 3

There are two problems.

First the increment method does not actually work. It should probably be:

 public void increment() {
     this.groupid++;
 }

Otherwise it wouldn't actually change.

The second problem was already mentioned by other answers, that is, you actually have only one Group object and many references to that one object.