Does Null Equal Null in Java?



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I’m a Java programmer by trade, and, for whatever reason, I tend to think about edge-cases I might (or might not) encounter while programming (see my article Four Java Limitations You May Never Encounter). One particular example I’ve been contemplating recently is if one null object equals another null object in Java.

In other words, does null equal null in Java?

Now, I could probably open up Java documentation online or read the latest Java book available. Instead, I’d rather take a more direct approach and get my hands dirty.

Well, it’s programming, so “hands dirty” is only a figure of speech. I simply wrote a short Java program to show me how Java handles null equality. Since I’m dealing with null objects, the normal Object.equals(Object other) is unavailable to me. I can only use the simple equality operator (the double-equals “==”) or some helper methods, such as Objects.equals(Object, Object).

Which is what I did. Below is the short but beautiful Java program I wrote:

import java.util.Objects;

public class Main {
   public static void main(final String[] args) {
      System.out.println("true: " + true);
      System.out.println("false: " + false);

      System.out.println();

      final String string = null;
      final Number number = null;

      System.out.println("null == null: " + (null == null));
      System.out.println("string == null: " + (string == null));
      System.out.println("number == null: " + (number == null));
      // Exception in thread "main" java.lang.Error: Unresolved compilation problem:
      // Incompatible operand types String and Number
      // System.out.println("string == number: " + (string == number));

      System.out.println();

      System.out.println("Objects.equals(null, null): " + (Objects.equals(null, null)));
      System.out.println("Objects.equals(string, null): " + (Objects.equals(string, null)));
      System.out.println("Objects.equals(number, null): " + (Objects.equals(number, null)));
      System.out.println("Objects.equals(string, number): " + (Objects.equals(string, number)));
   }
}

And below this the output:

true:    true
false:    false

null == null:    true
string == null:    true
number == null:    true

Objects.equals(null, null):    true
Objects.equals(string, null):    true
Objects.equals(number, null):    true
Objects.equals(string, number):    true

The first two outputted lines were just to get a feel as to how Java outputs the raw boolean values true and false.

Everything here pretty much is as expected; the only lines that I find interesting is the output I commented out:

// Exception in thread "main" java.lang.Error: Unresolved compilation problem:
// Incompatible operand types String and Number
// System.out.println("string == number: " + (string == number));

and the final comparison:

Objects.equals(string, number):    true

Java doesn’t allow a direct equality check between two dissimilar objects; however, I found that if I typecast one of the objects to Object, then Java states that the two objects are equal:

// Output: (Object)string == number: true
System.out.println("(Object)string == number: " + ((Object)string == number));

To conclude this post and answer the titular question Does null equal null in Java? the answer is a simple yes. Which makes sense, otherwise there’d have to be another way to check if an object is not null then the standard object != null. Furthermore, two null objects are equal even if they don’t exist in the same class hierarchy (as our Number and String objects in the above tests).

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