public class ReferenceTest {

    public static void main(String[] args) {

        SoftReference<Person> softReference = new SoftReference<Person>(new Person("huhx", "123456"));

        System.out.println(softReference.get()); // username: huhx, password: 123456

        System.gc();

        System.runFinalization();

        System.out.println(softReference.get()); // username: huhx, password: 123456

        WeakReference<Person> weakReference = new WeakReference<Person>(new Person("linux", "12345"));

        System.out.println(weakReference.get()); // username: linux, password: 12345

        System.gc();

        System.runFinalization();

        System.out.println(weakReference.get()); // null

        ReferenceQueue<String> refQueue = new ReferenceQueue<String>();

        PhantomReference<String> referent = new PhantomReference<String>(new String("T"), refQueue);

        System.out.println(referent.get());// null

        System.gc();

        System.runFinalization();

        System.out.println(refQueue.poll() == referent); // true

    }

}