null : Java Glossary

null
null is the reserved constant used in Java to represent a void reference i.e a pointer to nothing. Internally it is just a binary 0, but in the high level Java language, it is a magic constant, quite distinct from zero, that internally could have any representation.

If

x.doSomething();
and x is null, you will raise an NPE (Null Pointer Exception). Note NullPointerException, not NullReferenceException.

The difference between a null reference and a reference to an empty object, (e.g. a freshly minted default object, empty collection or empty Iterator) is a major headache in Java. When dealing with other people’s code I want to strangle them. Why won’t they tell me when a method might return null or empty? Why won’t they tell me if a method can’t accept

if ( x != null )
   {
   x.doSomething();
   }
must be the most common Java code fragment. I wish there were some more elegant way to handle this, that gives you the speed and space conservation of null, without the coding overhead bubblgum of checking for it at every turn. I have five solutions:
  1. Don’t return Lists from your methods; return Iterators instead. This way you can return the empty Iterator. The Iterator’s consumer does not need to deal specially with the null case. You don’t need the overhead of cloned empty List objects.
  2. Create a singleton empty prototype object for each class that you pass around to act like null. The catch is, it may actually need to be a subclass so that its methods don’t actually do anything. You have to make sure nobody modifies the empty object. You can safely call methods on this object and nothing will happen. You can safely retrieve its empty/default-value fields. You would need an amanuensis to compose the code for such objects. The effort of composing them manually would be even more work than dealing with null. Using this technique would be more robust however. You would make all methods final so that they would inline down to nothing. This would be even faster than checking explicitly for null! Null is typeless. Empty objects are typed. This may or may not be considered a Good Thing™.
  3. New Java syntax to insert if (x != null) more tersely: e.g. t!.doit(); is the same as if (t != null) t.doit();.
  4. Use rigid Javadoc conventions to track whether a method consumes/produces null references, empty objects, or blank Strings.
  5. Simply ignore java.lang.NullPointerExceptions. The catch is, there is no way to do that in Java. You can’t just ignore them and return to the code just after the call. You can’t turn them off. This would require a new language where NullPointerExceptions are simply treated as no-ops.

Null and Garbage Collection

Obviously, if you null a reference it becomes a candidate for GC (Garbage Collection) sooner. In ordinary Java code, it is most common that you would exit the method almost immediately after such a nulling, so it buys you nothing for local variables allocated during the method invocation.

However, if you had some very complicated method, it could pay to null half-way through execution. But that being true is a symptom that you probably would want to refactor the routine in two, which would then most likely obviate the nulling.


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