Comparing the value of two null objects without NPE

Question

This is an extremely basic question but why is the following code returning a null pointer exception?

String a = null;
String b = null;

System.out.println(a.equals(b));

According to the docs here:

http://docs.oracle.com/javase/7/docs/api/java/util/Objects.html#equals(java.lang.Object,%20java.lang.Object)

the .equals() function is first checking for null before comparing values. Shouldn't it return true since they are both null?

Answer 1

The method you linked to takes in two objects and is a static method. You need to call it like Objects.equals(a, b) . Instead you are calling .equals() on a null object which throws NPE

Answer 2

Shouldn't it return true since they are both null?

nop. since a is a null-referenced object, invoking ANY instance method on that object will throw a NPE

so what you can do:

if you are still on java 6 do

System.out.println(a == null ? b == null : a.equals(b));

and since java 7

System.out.println(Objects.equals(a, b));

Answer 3

I usually write following method,

private boolean equalsWithNull(String first, String second){
    return ((first!=null&&second!=null && first.equals(second)) || (first==null && second==null));
}

Answer 4

Objects::equals is the best option, but you can use

Optional.ofNullable(a).equals(Optional.ofNullable(b))

as well. However I don't really see any cases when you will choose Optional approach instead of Objects

Answer 5

you can check both values instead of reference,

        String a = null;
        String b = null;
        System.out.println(a == b ? "true":"false");

it returns true.