If I run

public class Stacks {
    public static void main(String... args) {
        System.gc();
    }
}

It reports I exceed the memory limit with a usage of 4885 KB.

public class Stacks {
    public static void main(String... args) {
        System.gc();
    }
}

Reports 326 KB used.

How is the memory usage determined.  I assume you cannot have your managed memory trigger a GC which is a bit restrictive for Java.