Understanding Java's Reference classes: SoftReference, WeakReference, and PhantomReference

Question

Can someone explain the difference between the three Reference classes (or post a link to a nice explanation)? SoftReference > WeakReference >

Answer 1

String str = new String("hello, world");
WeakReference ref = new WeakReference(str);
str = null;

if (ref != null) {                 
    System.gc(); 
    System.out.println(ref.get());
}

In this case, it will output null. The call to System.gc() is important here.