What happens when an object is dereferenced but its member object is still referenced?

Question

class ClassA {
    ClassB mem1 = new ClassB();
    ClassB mem2 = new ClassB();
}

class ClassB {
}

public class Sample {
    public static void main(String[] args)          


        

Answer 1

You have 3 instances of classes: One of ClassA (I call it classA_1) and two of ClassB (classB_1, classB_2). Now look at all the reference paths that exist to those 3 instances.

Before obj1 = null it looks smth. like this:

classA_1 <- Sample.main.obj1
classB_1 <- classA_1.mem1 <- Sample.main.obj1
classB_1 <- Sample.main.obj2
classB_2 <- classA_1.mem2 <- Sample.main.obj1

After obj1 = null it looks like this, where no ref. means, it is eligible for garbage collection, because it has no reference pointing to it.

classA_1 <- no ref.
classB_1 <- classA_1.mem1 <- no ref.
classB_1 <- Sample.main.obj2
classB_2 <- classA_1.mem2 <- no ref.

The only path that remains is this: classB_1 <- Sample.main.obj2. There a no paths with valid references to objects classB_2 and classA_1. So these objects can be collected.

Answer 2

Automatic garbage collection is the process of looking at heap memory, identifying which objects are in use and which are not, and deleting the unused objects. An in use object, or a referenced object, means that some part of your program still maintains a pointer to that object. An unused object, or unreferenced object, is no longer referenced by any part of your program. So the memory used by an unreferenced object can be reclaimed.

Java Garbage Collection Basics

In this context, when obj1 = null; is processed there are no references to the object of type ClassA to which it points, and so it is eligable for garbage collection. The ClassB object, mem1 however still has a reference in the form of obj2 and so is kept at least until the line obj2 = null; is executed.

Answer 3

After obj1 is no longer referenced, it becomes eligible for garbage collection. mem1 still has a reference, so if java were to garbage-collect at this point, obj1 and mem2 it points too would be freed, but mem1 would remain untouched as obj2 still points to it.

Of course, after obj2 = null, it can also be collected.