When should I use Boolean instead of boolean? [duplicate]

Question

When should I use Boolean instead of boolean?. I mean, why would I want to have a null value in a variable which should contain either "true" or "false".. One spontaneous answer (of most people) would be if the value is unknown. i.e, if we don't know whether the value is true or false. But from a programming perspective, I think using Boolean may break the code as we will not know whats inside. So, I think using the primitive type is better than the wrapper.. Correct me if I am wrong.

Answer 1

Generally speaking, the wrapper classes are used in cases where an object is required or strongly preferred. Outside of these situations, it's better to use the primitive types, since they have lower overhead, you can use ==, etc. There are two and a half major situations where this is frequently seen:

• Collections. This is now a subset of the next case, but even before Java 5 the Collections classes only supported objects as keys and values, and this hasn't changed.
• Generics. Generic types can only work with objects, not primitives, and so if you're using "boolean" as a type parameter, it has to be the wrapper class. For example, if you're using a Future, you have to use a Boolean instead of a boolean. (HT @user949300)
• ORM. JPA and other ORM systems technically can use primitive fields, but it's customary to use the wrapper classes, since the overhead is high enough that that doesn't really matter anyway, and the wrapper classes can represent a NULL value that might be present in the database. It's usually still better to forbid nulls and use a primitive for booleans, though, since semantically a default is usually better than "undefined".

Since boolean values are restricted to either true or false, it's uncommon to see them used in Collections or Generics; generally speaking, if you'd have a boolean as a value, you'll just use Collection#contains instead.