Question

In Java I am told that when doing a null check one should use == instead of .equals(). What are the reasons for this?

The easiest thing is to try null checking with equals() and see. When you try it will instantly be obvious — Goran Jovic, Dec 21 '10 at 15:50
By the way, a google search with keywords "java null check" (without quotes) gave me as one of the top hitsthis thread, which has the same info as the answers here. — Mitch Schwartz, Dec 21 '10 at 15:53

T.J. Crowder · Accepted Answer · 2010-12-21 16:37:55Z

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 nullreference, 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

You should add the case for f1.equals(f3) just to make it clear what will happen. — Hiro2k, Dec 21 '10 at 16:35
@Xepoch: No, I don't generally create public fields (although it doesn't really matter for this example either way). Why? — T.J. Crowder, Dec 22 '10 at 16:11

Jigar Joshi · Answer 2 · 2010-12-21 15:54:12Z

up vote8down vote

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

edited Dec 21 '10 at 15:54

answered Dec 21 '10 at 15:49

Jigar Joshi
93.7k12120185

4

Your example is generally better written as if ("hi".equals(str)). – ColinD Dec 21 '10 at 15:51

2

@user368186: the point isn't whether the equals method includes a null check. If your object reference is null, then the call someObject.equals(null) will raise a NullPointerException without ever entering the equals method. – Dave Costa Dec 21 '10 at 15:51

1

@ColinD Agree just demonstrating here – Jigar Joshi Dec 21 '10 at 15:52

1

It's always advisable to avoid nulls at all cost, so you don't need null checks at all ;). – fwielstra Dec 28 '10 at 15:44

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user523859 · Answer 3 · 2010-12-21 15:55:43Z

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 ==.

upvote for the actual answer that is most useful as opposed to the obvious "NullPointerException will be returned" herp derp answer. — volk, Aug 20 '12 at 3:50

Kevin Orriss · Answer 4 · 2012-08-16 14:27:23Z

You could always do

if (str == null || str.equals(null))

This will first check the object reference and then check the object itself providing the reference isnt null.

i used your answer and added a check for an empty string! if im not mistaken, null and "" are not the same thing. — Louis Morda, Sep 21 '12 at 17:21

Xepoch · Answer 5 · 2010-12-21 15:49:42Z

If an Object variable is null, one cannot call an equals() method upon it, thus an object reference check of null is proper.

Nick Orton · Answer 6 · 2010-12-21 15:49:51Z

foo.equals(null)

What happens if foo is null?

You get a NullPointerException.

A_M · Answer 7 · 2010-12-21 15:50:28Z

If you try calling equals on a null object reference, then you'll get a null pointer exception thrown.

danny.lesnik · Answer 8 · 2010-12-21 15:51:40Z

Because equal is a function derived from Object class, this function compares items of the class. if you use it with null it will return false cause cause class content is not null. In addition == compares reference to an object.

DixonD · Answer 9 · 2010-12-21 15:53:24Z

According to sources it doesn't matter what to use for default method implementation:

public boolean equals(Object object) {
    return this == object;
}

But you can't be sure about equals in custom class.

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

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