java collections stores values or references? [duplicate]

Question

Beginner Java question: when I have

Integer i = 6;
ArrayList<Integer> ar = ArrayList<Integer>();
ar.add(i);

then I write i = 8, ar.get(0) returns 6.

But if I trie the same thing with a class of mine:

class MyC
{
    Integer i;
}
MyC myc = new MyC();
myc.i = 6;
ArrayList<MyC> ar = ArrayList<MyC>();
ar.add(myc);

then do myc.i = 8, ar.get(0) returns 8.

Can you please explain this behavior?

Answer 1

The problem has nothing to do with autoboxing, as some answers say.

In the first example you create an Integer and put it in the ArrayList. Then you change the pointer to the Integer, so that i is pointing to another Integer. This don't affects the Integer of the ArrayList.

In the second example you create an object and put it in the ArrayList. Then you change the state of this object by myc.i = 8. This way the object in the ArrayList is changed.