Reachability and references

Definition

The collector frees only objects that nothing can reach any more. A normal (strong) reference keeps an object alive. A WeakReference does not: once nothing else refers to the object, the collector may free it, which is useful for caches.

Example code

A weakly referenced object disappears after a collection

import java.lang.ref.WeakReference;

class Demo {
    public static void main(String[] args) throws InterruptedException {
        Object strong = new Object();
        WeakReference<Object> weakOfStrong = new WeakReference<>(strong);

        WeakReference<Object> weak = new WeakReference<>(new Object());   // nothing else refers to it

        System.gc();                                                        // a request to collect now
        Thread.sleep(100);

        System.out.println("still reachable object: " + (weakOfStrong.get() != null));
        System.out.println("unreachable object:     " + (weak.get() != null));
        System.out.println(strong != null);                                 // keeps 'strong' alive until here
    }
}

// Output:
// still reachable object: true
// unreachable object:     false
// true