Collection of bad coding practices

‘equals()’ does not check for null argument

with 7 comments

If you override equals() method in your class, always check if an argument is null. If a null value is passed, equals() must unconditionally return false (no NullPointerException should be thrown!).


class Person {
    public boolean equals(Object o) {
        return (o instanceof Person) && 


class Person {
    public boolean equals(Object o) {
        return (o != null) && 
               (o instanceof Person) && 

Written by Alex

November 22, 2007 at 2:08 pm

Posted in Objects

Tagged with , ,

7 Responses

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  1. You don’t need o!=null comparison at all. In fact this is kind of premature optimization.

    According to JLS spec instanceof operator returns false when o is null. (See also Joshua Bloch, Effective Java). Also it is good to check whether o == this and skip some expensive comparisons.
    So the best form of equals may have following look:

    class Person {

    public boolean equals(Object o) {
    if (o == this)
    return true;
    if (!(o instanceof Person))
    return false;

    Person that = (Person)o;


    Grzegorz Turski

    December 10, 2007 at 2:41 pm

  2. Thanks, Grzegorz!

    You’re right. I overlooked this instanceof convention. Have you any ideas also on the ‘bad code example’ ?


    December 10, 2007 at 2:59 pm

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    May 25, 2010 at 3:43 pm

  5. If you implement equals using instanceof, you should consider making equals and hashCode final. Otherwise, any subclass that overrides it must make certain to maintain the symmetry property: that subclass must be equal to the parent class. See

    David Phillips

    June 18, 2012 at 10:44 pm

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  7. Really needed ‘dislike’ button.
    just run: boolean articleIsIdiotizm = null instanceof Object;

    Petrov Andrei

    December 6, 2015 at 7:08 pm

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