Weakly referenced object won't get garbage collected

Question

My concern is about an instance of an object that was once strongly referenced, but after an explicit null assignment to its strong reference and after an explicit System.gc() call, the instance is still reachable via the weak reference. If I understand correctly, when a referred object has only weak references left, the referent is guaranteed to be cleared in the next GC session. What am I missing?

Reference code:

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

        Example strongReferenceWrappedInWeak = new Example(42);
        strongReferenceWrappedInWeak.printA();

        WeakReference<Example> exampleWeakReference = new WeakReference<>(strongReferenceWrappedInWeak);

        System.gc();

        Example retrievedExample = exampleWeakReference.get();
        retrievedExample.printA(); //this works, because a strong reference is present to the instance, hence it's not cleared

        strongReferenceWrappedInWeak = null; //eligible for garbage collection

        System.gc();

        Example retrievedExampleTwo = exampleWeakReference.get(); //should be null
        retrievedExampleTwo.printA(); //should throw NPE
    }
}

class Example {
    private int a;

    Example(int a) {
        this.a = a;
    }

    void printA() {
        System.out.println(this.a);
    }
}

Answer 1

strongReferenceWrappedInWeak = null does not make the Example object instance eligible for garbage collection, because retrievedExample still maintains a strong reference to it.

To fix, add retrievedExample = null;

Example strongReferenceWrappedInWeak = new Example(42);
strongReferenceWrappedInWeak.printA();

WeakReference<Example> exampleWeakReference = new WeakReference<>(strongReferenceWrappedInWeak);

System.gc();

Example retrievedExample = exampleWeakReference.get();
retrievedExample.printA(); //this works, because a strong reference is present to the instance, hence it's not cleared

retrievedExample = null;
strongReferenceWrappedInWeak = null; //now eligible for garbage collection

System.gc();

Example retrievedExampleTwo = exampleWeakReference.get(); //will be null
retrievedExampleTwo.printA(); //will throw NPE

Alternatively, don't create a strong reference with a local variable, just call the method directly off the weak reference. That way you don't accidentally leave a strong reference behind, as you did. *(During the printA() call, the this reference is a strong reference, so object cannot be GC'd during the call)*

Example strongReferenceWrappedInWeak = new Example(42);
strongReferenceWrappedInWeak.printA();

WeakReference<Example> exampleWeakReference = new WeakReference<>(strongReferenceWrappedInWeak);

System.gc(); //does not collect object, since strong reference still exists

exampleWeakReference.get().printA(); //works

strongReferenceWrappedInWeak = null; //eligible for garbage collection
System.gc(); //collects object, since it is now weakly referenced only

exampleWeakReference.get().printA(); //throws NPE

Output (from both)

42
42
Exception in thread "main" java.lang.NullPointerException
    at Test.main(Test.java:**)

^{Tested on Java 13}