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I ran into an interesting (and very frustrating) issue with the equals() method today which caused what I thought to be a well tested class to crash and cause a bug that took me a very long time to track down.

Just for completeness, I wasn't using an IDE or debugger - just good old fashioned text editor and System.out's. Time was very limited and it was a school project.

Anyhow -

I was developing a basic shopping cart which could contain an ArrayList of Book objects. In order to implement the addBook(), removeBook(), and hasBook() methods of the Cart, I wanted to check if the Book already existed in the cart. So off I go -

public boolean equals(Book b) {
    ... // More code here - null checks
    if (b.getID() == this.getID()) return true;
    else return false;

All works fine in testing. I create 6 objects and fill them with data. Do many adds, removes, has() operations on the Cart and everything works fine. I read that you can either have equals(TYPE var) or equals(Object o) { (CAST) var } but assumed that since it was working, it didn't matter too much.

Then I ran into a problem - I needed to create a Book object with only the ID in it from within the Book class. No other data would be entered into it. Basically the following:

public boolean hasBook(int i) {
    Book b = new Book(i);
    return hasBook(b);

public boolean hasBook(Book b) {
    // .. more code here
    return this.books.contains(b);

All of a sudden, the equals(Book b) method no longer works. This took a VERY long time to track down without a good debugger and assuming the Cart class was properly tested and correct. After swaapping the equals() method to the following:

public boolean equals(Object o) {
    Book b = (Book) o;
    ... // The rest goes here   

Everything began to work again. Is there a reason the method decided not to take the Book parameter even though it clearly was a Book object? The only difference seemed to be it was instantiated from within the same class, and only filled with one data member. I'm very very confused. Please, shed some light?

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I am aware that I violated the 'Contract' regarding overriding the equals methods by being reflective - however I needed a quick way to check if the object existed in the ArrayList without using generics. –  Josh Smeaton Oct 9 '08 at 4:29
This is a good lesson to learn about Java and equals –  jjnguy Oct 9 '08 at 4:34
I'm unable to see how a shopping cart can be equal to a book. I mean how can hasBook(Book b) return the result of this.equals(b)? –  Robert Jul 14 '12 at 0:52
@Robert, wow, 4 years this question has been here - and no one picked up on that. I can't remember, but I assume the hasBook was supposed to do a this.books.contains() or something similar. –  Josh Smeaton Jul 15 '12 at 14:56

9 Answers 9

up vote 214 down vote accepted

In Java, the equals() method that is inherited from Object is:

public boolean equals(Object other);

In other words, the parameter must be of type Object.

The ArrayList uses the correct equals method, where you were always calling the one that didn't properly override Object's equals.

Not overriding the method correctly can cause problems.

I override equals the following everytime:

public boolean equals(Object other){
    if (other == null) return false;
    if (other == this) return true;
    if (!(other instanceof MyClass))return false;
    MyClass otherMyClass = (MyClass)other;
    ...test other properties here...

The use of the @Override annotation can help a ton with silly mistakes.

Use it whenever you think you are overriding a super class' or interface's method. That way, if you do it wrong you will get a compile error.

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This is a good argument in favour of the @Override annotation... if the OP had use @Override his compiler would have told him that he wasn't actually overriding a parent class method... –  Cowan Oct 9 '08 at 4:36
Was never aware of the @Override, thanks for that! I'd also like to add that overriding hashCode() really should have been done and may have spotted the error sooner. –  Josh Smeaton Oct 9 '08 at 4:52
Some IDEs (e.g. Eclipse) can even autogenerate equals() and hashcode() methods for you based on the class member variables. –  sk. Oct 9 '08 at 5:48
if (!(other instanceof MyClass))return false; returns false if MyClass extends the other class. But it wouldn't return false if the other class extended MyClass. Shouldn't equal be less contradictory? –  Robert Jul 14 '12 at 1:16
When using instanceof the previous nullcheck is redundant. –  Mateusz Dymczyk Feb 2 '13 at 3:28

If you use eclipse just go to the top menu

Source --> Generate equals() and hashCode()

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I wish I could give this answer 10 upvotes. –  dansalmo Nov 10 '13 at 18:18
I agree! This one I never knew of before and generating it makes it less error prone –  Boy Jan 12 '14 at 15:38
Same here. Thanks Fred! –  Anila Mar 13 '14 at 18:21
In IntelliJ you find this under Code → Generate… or control+N. :) –  elyse May 6 '14 at 17:31

Also I'd like to add that you should always override hashCode whenever overriding equals.

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I wonder why you got a downvote. Its not an answer to the question...but it can't be unhelpful. Its always good to have a reminder. –  jjnguy Oct 9 '08 at 4:48
thanks for your support :-) –  Julie Oct 9 '08 at 4:49
yup, see… –  Adrien Be Oct 31 '12 at 16:48

Slightly off-topic to your question, but it's probably worth mentioning anyway:

Commons Lang has got some excellent methods you can use in overriding equals and hashcode. Check out EqualsBuilder.reflectionEquals(...) and HashCodeBuilder.reflectionHashCode(...). Saved me plenty of headache in the past - although of course if you just want to do "equals" on ID it may not fit your circumstances.

I also agree that you should use the @Override annotation whenever you're overriding equals (or any other method).

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If you're an eclipse user, you can also go right click -> source -> generate hashCode() and equals(), –  tunaranch Nov 6 '08 at 4:31

Another fast solution that saves boilerplate code is Lombok EqualsAndHashCode annotation. It is easy, elegant and customizable. And does not depends on the IDE. For example;

import lombok.EqualsAndHashCode;

@EqualsAndHashCode(of={"errorNumber","messageCode"}) // Will only use this fields to generate equals.
public class ErrorMessage{

    private long        errorNumber;
    private int         numberOfParameters;
    private Level       loggingLevel;
    private String      messageCode;

See the options avaliable to customize which fields to use in the equals. Lombok is avalaible in maven. Just add it with provided scope:

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the instanceOf statement is often used in implementation of equals.

This is a popular pitfall !

The problem is that using instanceOf violates the rule of symmetry:

(object1.equals(object2) == true) if and only if (object2.equals(object1))

if the first equals is true, and object2 is an instance of a subclass of the class where obj1 belongs to, then the second equals will return false!

if the regarded class where ob1 belongs to is declared as final, then this problem can not arise, but in general, you should test as follows:

this.getClass() != otherObject.getClass(); if not, return false, otherwise test the fields to compare for equality!

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See Bloch, Effective Java, Item 8, a large section discussing issues with overriding the equals() method. He recommends against using getClass(). The main reason is that doing so breaks the Liskov Substitution Principle for subclasses that don't affect equality. –  Stuart Marks Jul 29 at 15:53


Object obj = new Book();
// Oh noes! What happens now? Can't call it with a String that isn't a Book...
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in Android Studio is alt + insert ---> equals and hashCode


public boolean equals(Object o) {
    if (this == o) return true;

    Proveedor proveedor = (Proveedor) o;

    return getId() == proveedor.getId();


public int hashCode() {
    return getId();
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recordId is property of the object

    public boolean equals(Object obj) {
        if (this == obj)
            return true;
        if (obj == null)
            return false;
        if (getClass() != obj.getClass())
            return false;
        Nai_record other = (Nai_record) obj;
        if (recordId == null) {
            if (other.recordId != null)
                return false;
        } else if (!recordId.equals(other.recordId))
            return false;
        return true;
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protected by Makoto Sep 9 at 19:47


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