Question

I have to assume that the following method doesn’t leak memory:

public final void setData(final Integer p_iData)
{
    data = p_iData;
}

Where data is a property of some class.

Every time the method gets called, a new Integer is replacing the currently existing data reference. So what’s happening with the current/old data?

Java has to be doing something under the hood; otherwise we’d have to null-out any objects every time an object is assigned.

Answer 1

Simplistic explanation:

Periodically the garbage collector looks at all the objects in the system, and sees which aren’t reachable any more from live references. It frees any objects which are no longer reachable.

Note that your method does not create a new Integer object at all. A reference to the same Integer object could be passed in time and time again, for example.

The reality of garbage collection is a lot more complicated than this:

• Modern GCs tend to be generational, assuming that most objects are short-lived, so it doesn’t need to check the whole (possibly large) heap as often; it can just check “recent” objects for liveness frequently
• Objects can have finalizers – code to be run before they’re garbage collected. This delays garbage collection of such objects by a cycle, and the object could even “resurrect” itself by making itself reachable
• Modern GCs can collect in parallel, and have numerous tweaking options