Java using instanceof method on primitive type gives compiler error

Question

I am learning Java, and I do not understand why the following code does not compile without error:

public class SecondClass{

public static void main(String[] args){
    int number = 45;
    if (number instanceof String) {
        System.out.println("Not a String!");
    }
  }
}

Why do I get an error in my conditional operation? The instanceof should return true or false right? In this case there should be false since number is an int, but this code does not compile.

stackoverflow.com/questions/12361492/… Further explanation here. instanceof is only for use on Objects. — CollinD
– CollinD, Commented Sep 5, 2015 at 21:08

Jon Skeet · Accepted Answer · 2015-09-05 21:09:43Z

6

From section 15.20.2 of the JLS:

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

In your case, the case of the RelationalExpression operand is int, therefore you get a compile-time error.

Even if you had an expression of type Integer, you'd then run into:

If a cast (§15.16) of the RelationalExpression to the ReferenceType would be rejected as a compile-time error, then the instanceof relational expression likewise produces a compile-time error. In such a situation, the result of the instanceof expression could never be true.

answered Sep 5, 2015 at 21:09

Jon Skeet

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