public static void assertNull(Object o)
public static void assertNull(String message, Object o)
e.g. testing for memory leak:
public void testMemoryLeak() throws InterruptedException {
Fraction a = new Fraction(1, 2);
WeakReference ref = new WeakReference(a);
a = null;
System.gc();
System.gc();
System.gc();
Thread.sleep(1000);
assertNull(ref.get());
}