int instanceof Integer

Question

Why this is a compile-time error when Java does the Autoboxing? Am I missing something?

int primitiveIntVariable = 0;

if (primitiveIntVariable instanceof Integer) {

}

I get

Inconvertible types; cannot cast 'int' to 'java.lang.Integer'

Answer 1

int cannot be anything but an int , so the entire concept of using instanceof is meaningless, and misguided.

You only use instanceof if you need to check if the actual object is a particular type different from the type of the variable in question (or known supertypes hereof).

Eg if the declared type of the variable (or the return value, or the compile-type of expression), is Integer , then it makes no sense whatsoever to check it is an instanceof of Integer . Given that Java is type-safe, you already know that it is.

Similarly, it makes no sense whatsoever to check if a known Integer is an instanceof of Number . You already know that it is.

And it makes even less sense to check if a known int is an instanceof of Integer . It's a primitive , and you know it is, so there is absolutely no way it can be an instance of any object type.

The last will generate a compiler error. The first two examples are compiler warnings, which is very evident if you use any good IDE. Always use a good IDE, because they catch so many dumb mistakes we all happen to write occasionally.

---

So the above was an explanation of why it makes no sense to even try, but even though integerVar instanceof Integer makes no sense, it compiles ok, but intVar instanceof Integer fails to compile, so why it that?

The reason is actually related to this mistaken statement in the question:

when Java does the Autoboxing

Java doesn't do autoboxing everywhere. Autoboxing only happens in:

• assignment contexts , eg Integer x = 5
• invocation contexts , eg foo(5) where parameter is an Integer
• casting contexts , eg (Integer) 5 or (Object) 5

It does not happen by itself in the middle of an expression, and instanceof is an expression operator.

But, more specifically, it fails to compile because JLS 15.20.2. Type Comparison Operator instanceof says so:

RelationalExpression instanceof ReferenceType

The type of the RelationalExpression operand of the instanceof operator must be a reference type or the null type, or a compile-time error occurs.

Answer 2

As the name suggests, instanceof means an instance (object) of a class. Primitive datatypes are not instances.

This is how you get the class for a primitive datatype:

int i = 1;
System.out.println(((Object)i).getClass().getName());
// prints: java.lang.Integer

So instead of instanceof, use isInstance(...) like this:

Integer.class.isInstance(1); // returns true
Integer.class.isInstance(1.2); // returns false

Hope this helps. Good luck.