Question

In Java, the instanceof operator will only return false if comparing null. If you test a reference variable that isn’t of the required type, the program fails to compile.
Does this operator effectively only test if a reference is null or not? Or is there another use I’m not thinking of?

If not, why not call it isNull or something more descriptive?

Answer 1

It fails to compile if the comparison can never be true. For example:

"string" instanceof Number

This will not compile because the compiler knows that a string can never be a number. The operator does exactly what one would expect it to do: it tests if the operand is an instance of the specified type.

Returning false when testing a null reference is meant to make the code simpler and more readable. It prevents you from needing to do this:

if (obj != null && obj instanceof Number) {
    ...
}

Read this to learn about the operator: http://java.sun.com/docs/books/jls/third_edition/html/expressions.html#15.20.2