java garbage collection

I was going through this question on an SCJP preparation site . How answer A is correct?

What is true about objects referenced by a, b, aa at the line labeled "// some code goes here"?

class A {
    private B b;
    public A() {
        this.b = new B(this);
    }
}

class B {
    private A a;
    public B(A a) {
        this.a = a;
    }
}

public class Test { 
    public static void main(String args[]) {
        A aa = new A();
        aa = null;
        // some code goes here
    }
}


A) The objects referenced by a and b are eligible for garbage collection.
B) None of these objects are eligible for garbage collection.
C) Only the object referenced by "开发者_如何转开发a" is eligible for garbage collection.
D) Only the object referenced by "b" is eligible for garbage collection.
E) Only the object referenced by "aa" is eligible for garbage collection.

Answer: A

Java doesn't only use a simple reference-counting garbage collector.

When the JVM does a full GC run, it walks the entire object graph, marking each item it finds. Any items that aren't marked are eligible for cleanup.

Since neither a nor b are reachable from your main code anymore, they won't be marked and are thus eligible for cleanup.

well, for something to not be garbage collectable, it has to be impossible to reach it from anywhere on the stack. Considering that the stack at that point only has a null value for aa and a single string array (args) there is no way to get to a or b.