Java在原始类型上使用instanceof方法产生编译器错误

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.

Answer 1

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.