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Java 1.2 in 1998 introduced<ref>{{cite web|url=http://docs.oracle.com/javase/7/docs/api/java/lang/ref/WeakReference.html|title=WeakReference (Java Platform SE 7 )|website=docs.oracle.com}}</ref> two kinds of weak references, one known as a “soft reference” (intended to be used for maintaining GC-managed in-memory caches, but which doesn’t work very well in practice<ref>{{cite web|url=https://developer.android.com/reference/java/lang/ref/SoftReference.html|title=SoftReference - Android Developers|website=developer.android.com}}</ref>) and the other simply as a “weak reference”. It also added a related experimental mechanism dubbed “phantom references” as an alternative to the dangerous and inefficient finalize() mechanism.<ref>{{cite web|url=http://docs.oracle.com/javase/7/docs/api/java/lang/ref/PhantomReference.html|title=PhantomReference (Java Platform SE 7 )|website=docs.oracle.com}}</ref>
If a weak reference is created, and then elsewhere in the code <code>get()</code> is used to get the actual object, the weak reference isn't strong enough to prevent garbage collection, so it may be (if there are no strong references to the object) that <code>get()</code> suddenly starts returning null.<ref>https://web.archive.org/web/20110303225354/http://weblogs.java.net/blog/2006/05/04/understanding-weak-references Java Examples</ref>
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