How come invoking a (static) method on a null reference doesn't throw NullPointerException?

Question

I wrote this program in Java

public class Why {

  public static void test() {
    System.out.println("Passed");
  }

  public static void main(String[] args) {
    Why NULL = null;
    NULL.test();
  }

}

I read that invoking a method on a null object causes NullPointerException, and yet the above program doesn't? Why is this? Am I not understanding something correctly?

Answer 1

test() is a static method. A static member belongs to the type, and do not require an instance to access.

A static member should ONLY be accessed via a type expression. That is, you should've written it as follows:

Why.test(); // always invoke static method on the type it belongs to!

Java does allow you to access a static member via an object reference expression, but this is VERY misleading, since this is NOT the actual semantics of a static member access.

Why aNull = null; 
aNull.test(); // DO NOT EVER DO THIS!
// invokes Why.test(), does NOT throw NullPointerException

When accessing a static member through an object reference expression, only the declared type of the reference matters. This means that:

• It doesn't matter if the reference is actually null, since no instance is required
• If the reference is not null, it doesn't matter what the runtime type of the object is, there is no dynamic dispatch!!!

As you can see, the exact opposites are true on both points for instance member access. This is why static members should NEVER be accessed in a "non-static" way, because it gives a very misleading appearance on what it's actually doing.

Related questions

• Why doesn't Java allow overriding of static methods ? (understanding this is crucial!)
• Why isn’t calling a static method by way of an instance an error for the Java compiler?

Answer 2

Static methods do not need a reference to the object. So you can call it even reference to the object is null. That's how main method works.

Try removing static designation from the object to see the null pointer exception.

Answer 3

It's a static method, which allows you to call methods on it without instantiating an instance.

Answer 4

You should turn on the various warnings in your IDE. Chances are you will see a warning about accessing a static member in a non-static way.

You could do something like (Why)(null).test(), it is only using the (Why)(null) to get the class.