Overriding the java equals() method - not working?

Question

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 `equa…

Accepted Answer

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`. This is called overriding; your method `public boolean equals(Book other)` does what is called overloading to the `equals()` method.

The `ArrayList` uses overridden `equals()` methods to compare contents (e.g. for its `contains()` and `equals()` methods), not overloaded ones. In most of your code, calling the one that didn't properly override `Object`'s equals was fine, but not compatible with `ArrayList`.

So, not overriding the method correctly can cause problems.

I override equals the following everytime:

```
@Override
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 the wrong way, you will get a compile error.

Overriding the java equals() method - not working?

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