Question

I have a Set class (This is J2ME, so I have limited access to the standard API; just to explain my apparent wheel-reinvention). I am using my set class to create constant sets of things in classes and subclasses. It sort of looks like this…

class ParentClass {     protected final static Set THE_SET = new Set() {{         add('one');         add('two');         add('three');     }}; }   class SubClass extends ParentClass {     protected final static Set THE_SET = new Set() {{         add('four');         add('five');         add('six');         union(ParentClass.THE_SET); /* [1] */     }}; } 

All looks fine, except the line at [1] causes a null pointer exception. Presumably this means that the static initialiser in the subclass is being run before that of the parent class. This surprised me because I’d have thought it would run the static blocks in any new imports first, before running any in the instatiated subclass.

Am I right in this assumption? Is there any way to control or work around this behaviour?

Update:

Things are even stranger. I tried this instead (Note the ‘new ParentClass()’ line):

class ParentClass {     public ParentClass()     {         System.out.println(THE_SET);     }      protected final static Set THE_SET = new Set() {{         add('one');         add('two');         add('three');     }}; }   class SubClass extends ParentClass {     protected final static Set THE_SET = new Set() {{         System.out.println('a');         new ParentClass();         System.out.println('b');         add('four');         System.out.println('c');         add('five');         System.out.println('d');         add('six');         System.out.println('e');         union(ParentClass.THE_SET); /* [1] */         System.out.println('f');     }}; } 

And the output is strange:

a ['one', 'two', 'three'] b c d e Exception in thread 'main' java.lang.ExceptionInInitializerError Caused by: java.lang.NullPointerException 

So ParentClass is initialised, but the subclass doesn’t have access to it in its static initializer.

Answer 1

Is this what you are trying to accomplish? Or do you need a local implementation of the Set interface?

class ParentClass {     protected final static Set THE_SET;      static {         THE_SET = new HashSet();         THE_SET.add('one');         THE_SET.add('two');         THE_SET.add('three');     } }   class SubClass extends ParentClass {     protected final static Set THE_SECOND_SET;      static {         THE_SECOND_SET = new HashSet();         THE_SECOND_SET.add('four');         THE_SECOND_SET.add('five');         THE_SECOND_SET.add('six');         union(ParentClass.THE_SET); /* [1] */     } }