CODEWITHSUNDEEP
javaobjectequals
javaAndBeyond
javaAndBeyond

Reputation: 540

equals method of Object class - how it works - Java

I was wondering how equals method in Object class works. This is the implementation of the method.

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

where its evident that equals method is using ==. So,now when I am comparing two objects a and b, if a happens to be null, it doesn't throw any exception. But a.equals(b), throws NPE, why?

Upvotes: 1

Views: 221

Answers (3)

ashisahu
ashisahu

Reputation: 397

a.equals(b) throws null pointer exception because a is null, so when you try to call the instance method on a null object it gives a null pointer exception. For more information please refer to the documentation below for null pointer exception. http://docs.oracle.com/javase/7/docs/api/java/lang/NullPointerException.html

For these types of situation you should first check whether a is null and then should use the method on this object.

if(a != null) {
    a.equals(b);
}

Upvotes: 0

Panther
Panther

Reputation: 3339

When you call method on null object, it even doesn't call the method but gives null pointer error on same movement. Same is applicable to equals.

When invoke a.equals(b);

It will give null pointer exception as you are calling equals on a which is null.

For example, if you create method blank.

public Class MyClass{
    public voidblank() {

    }
}

and now even if you call blank on null object it will give you null pointer, nothing to do with method implementation.

MyClass a = null;
a.blank();

This will also give null pointer as a is null.

Upvotes: 1

Elliott Frisch
Elliott Frisch

Reputation: 201537

But a.equals(b), why does it throw NPE?

Because you can't invoke a method on null (methods are invoked by the referene variable in Java). When a is null, there is simply nothing to call equals on (or to de-reference). All that Object (non-primitive) Java variable types can hold is the value of a reference (or null, which by definition is not a value). That's why Java (pass by value), has the functionality of pass by reference. The value of Object(s) are references.

Upvotes: 1

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