Why doesn't equals throw NullPointerException when one of the variables is pointing to null

Question

Let's consider following code:

String s1=null;
String s2="something";
System.out.println(s2.equals(s1));
//output: false

Why is s2.equals(s1) not throwing NullPointerException?

EDIT: Thanks for the Answers, I am sorry for the Question I wasn't aware of this

For any non-null reference value x, x.equals(null) should return false.

The confusion came because toString method was throwing exception while equals didn't and I thought to myself but equals has also to "look" inside of the object so that it can perform comparison why isn't equals also throwing excetion

That's because the `equals` method in the `String` class won't de-reference `s1` if it's null. — ernest_k, Dec 02 '19 at 12:38

score 1 · Answer 1 · answered Dec 02 '19 at 12:38

Usually, the equals implementation has something like:

if ( o == null ) return false;

So, why would you expect this to throw an NPE? If you want it to throw one, change your statement to:

System.out.println( s1.equals(s2));

Then, you are actually calling an instance method on a null without checking for null first.

score 0 · Accepted Answer · answered Dec 02 '19 at 12:44

It's because the contract for the equals method, as specified in the Javadoc for the Object.equals method , explicitly states:

For any non-null reference value x, x.equals(null) should return false.

If the method threw a NullPointerException, it would be non-compliant with the contract.

Why doesn't equals throw NullPointerException when one of the variables is pointing to null

2 Answers2