When should null values of Boolean be used?

Question

Java boolean allows values of true and false while Boolean allows true, false, and null. I have

Answer 1

ANSWER TO OWN QUESTION: I thought it would be useful to answer my own question as I have learnt a lot from the answers. This answer is intended to help those - like me - who do not have a complete understanding of the issues. If I use incorrect language please correct me.

• The null "value" is not a value and is fundamentally different from true and false. It is the absence of a pointer to objects. Therefore to think that Boolean is 3-valued is fundamentally wrong

• The syntax for Boolean is abbreviated and conceals the fact that the reference points to Objects:

  Boolean a = true;

conceals the fact that true is an object. Other equivalent assignments might be:

Boolean a = Boolean.TRUE;

or

Boolean a = new Boolean(true);

• The abbreviated syntax

  if (a) ...

is different from most other assignments and conceals the fact that a might be an object reference or a primitive. If an object it is necessary to test for null to avoid NPE. For me it is psychologically easier to remember this if there is an equality test:

if (a == true) ...

where we might be prompted to test for null. So the shortened form is only safe when a is a primitive.

For myself I now have the recommendations:

• Never use null for a 3-valued logic. Only use true and false.
• NEVER return Boolean from a method as it could be null. Only return boolean.
• Only use Boolean for wrapping elements in containers, or arguments to methods where objects are required