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