If a variable is dereferenced, and the variable has a null value on all possible execution paths leading to the dereferencing, the dereferencing is guaranteed to result in a NullPointerException.

A variable may also be implicitly dereferenced if its type is a boxed primitive type, and the variable occurs in a context in which implicit unboxing occurs. Note that the conditional operator unboxes its second and third operands when one of them is a primitive type and the other is the corresponding boxed type.

Ensure that the variable does not have a null value when it is dereferenced.

In the following examples, the condition !dir.exists() is only executed if dir is null. The second example guards the expression correctly by using && instead of ||.

Java Tutorial: Autoboxing and Unboxing.

Java Specification: Conditional Operator ? :.