Editing Weak reference (section)

===Java===

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 on some platforms with dynamic heap like Android<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 is not 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>

<syntaxhighlight lang=java>
import java.lang.ref.WeakReference;

public class ReferenceTest {
    public static void main(String[] args) throws InterruptedException {
        WeakReference r = new WeakReference("I'm here");
        StrongReference sr = new StrongReference("I'm here");
        System.out.println("Before gc: r=" + r.get() + ", static=" + sr.get());
        System.gc();
        Thread.sleep(100);

        // Only r.get() becomes null.
        System.out.println("After gc: r=" + r.get() + ", static=" + sr.get());
    }
}
</syntaxhighlight>
Another use of weak references is in writing a [[cache (computing)|cache]]. Using, for example, a weak [[hash map]], one can store in the cache the various referred objects via a weak reference. When the garbage collector runs &mdash; when for example the application's memory usage gets sufficiently high &mdash; those cached objects which are no longer directly referenced by other objects are removed from the cache.