Java null check why use == instead of .equals()

Question

They're two completely different things. == compares the object reference, if any, contained by a variable. .equals() checks to see if two objects are equal according to their contract for what equality means. It's entirely possible for two distinct object instances to be "equal" according to their contract. And then there's the minor detail that since equals is a method, if you try to invoke it on a null reference, you'll get a NullPointerException.

For instance:

class Foo {
    private int data;

    Foo(int d) {
        this.data = d;
    }

    @Override
    public boolean equals(Object other) {
        if (other == null || other.getClass() != this.getClass()) {
           return false;
        }
        return ((Foo)other).data == this.data;
    }

    /* In a real class, you'd override `hashCode` here as well */
}

Foo f1 = new Foo(5);
Foo f2 = new Foo(5);
System.out.println(f1 == f2);
// outputs false, they're distinct object instances

System.out.println(f1.equals(f2));
// outputs true, they're "equal" according to their definition

Foo f3 = null;
System.out.println(f3 == null);
// outputs true, `f3` doesn't have any object reference assigned to it

System.out.println(f3.equals(null));
// Throws a NullPointerException, you can't dereference `f3`, it doesn't refer to anything

System.out.println(f1.equals(f3));
// Outputs false, since `f1` is a valid instance but `f3` is null,
// so one of the first checks inside the `Foo#equals` method will
// disallow the equality because it sees that `other` == null

if you invoke .equals() on null you will get NullPointerException

So it is always advisble to check nullity before invoking method where ever it applies

if(str!=null && str.equals("hi")){
 //str contains hi
}  

Also See

• difference-between-equals-and == in Java

In addition to the accepted answer (https://stackoverflow.com/a/4501084/6276704):

Since Java 1.7, if you want to compare two Objects which might be null, I recommend this function:

Objects.equals(onePossibleNull, twoPossibleNull)

java.util.Objects

This class consists of static utility methods for operating on objects. These utilities include null-safe or null-tolerant methods for computing the hash code of an object, returning a string for an object, and comparing two objects.

Since: 1.7

In Java 0 or null are simple types and not objects.

The method equals() is not built for simple types. Simple types can be matched with ==.

Object.equals is null safe, however be aware that if two objects are null, object.equals will return true so be sure to check that the objects you are comparing aren't null (or hold null values) before using object.equals for comparison.

String firstname = null;
String lastname = null;

if(Objects.equals(firstname, lastname)){
    System.out.println("equal!");
} else {
    System.out.println("not equal!");
}

Example snippet above will return equal!

foo.equals(null)

What happens if foo is null?

You get a NullPointerException.