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

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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.

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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.