How can Java assignment be made to point to an object instead of making a copy?

Question

In a class, I have:

private Foo bar;
public Constructor(Foo bar)
{
    this.bar = bar;
}

Instead of creating a copy of bar from the object provided in the parameter, is it possible to include a pointer to bar in the constructor such that changing the original bar changes the field in this object?

Another way of putting it:

int x = 7;
int y = x;
x = 9;
System.out.print(y); //Prints 7.

It is possible to set it up so that printing y prints 9 instead of 7?

Answer 1

Java never copies objects. It's easiest to think of in terms of for each "new" you will have one object instance--never more.

People get REALLY CONFUSING when they discuss this in terms of pass by reference/pass by value, if you aren't amazingly familiar with what these terms mean, I suggest you ignore them and just remember that Java never copies objects.

So java works exactly the way you wanted your first example to work, and this is a core part of OO Design--the fact that once you've instantiated an object, it's the same object for everyone using it.

Dealing with primitives and references is a little different--since they aren't objects they are always copied--but the net effect is that java is just about always doing what you want it to do without extra syntax or confusing options.