Is null check needed before calling instanceof?

Question

Will null instanceof SomeClass return false or throw a NullPointerException?

Answer 1

No, a null check is not needed before using instanceof. The expression x instanceof SomeClass is false if x is null

public class myclass {

  public static void main(String[] args) {

    myclass mc = null;

    if(mc instanceof myclass){

      System.out.println("YES");

    } else {

      System.out.println("NO");

    }

  }

}

    Output:

     NO

Answer 2

Nope, a null check is not required before using instanceof.

You can read this here in section 15.20.

It's clearly stated that

"At run time, the result of the instanceof operator is true if the value of the RelationalExpression is not null and the reference could be cast to the ReferenceType without raising a ClassCastException. Otherwise, the result is false."

means, The expression x instanceof SomeClass will be false if x is null. 

Hence, if the operand is null, the instanceof will return false. 

Answer 3

If you are using a null reference as the first operand to instanceof then it'll return false.

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Answer 4

If you are using this (((A)null)instanceof A), it will return false.

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But, If typecasting null seems surprising, then do this:

public class Test

{

  public static void test(A a)

  {

    System.out.println("a instanceof A: " + (a instanceof A));

  }

  public static void test(B b) {

    // Overloaded version. Would cause reference ambiguity (compile error)

    // if Test.test(null) was called without casting.

    // So you need to call Test.test((A)null) or Test.test((B)null).

  }

}

So Test.test((A)null) will print a instanceof A: false.)