2

I'm trying to understand the equals() method better. All examples I've seen do something like:

public class City
{
    public boolean equals(Object other)
    {
        if (other instanceof City && other.getId().equals(this.id))
        {
            return true;
        }

        // ...
    }
}

Must the method take on Object and not a City?

E.g. is this below not allowed?

public class City
{
    public boolean equals(City other)
    {
        if (other == null)
        {
            return false;
        }

        return this.id.equals(other.getId());
    }
}
JasonMArcher
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Amy B
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3 Answers3

6

Yes, it must be an Object. Else you're not overriding the real Object#equals(), but rather overloading it.

If you're only overloading it, then it won't be used by the standard API's like Collection API, etc.

Related questions:

Community
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BalusC
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1

you can have both: (see poke's comment above)

public class City
{
    public boolean equals(Object other)
    {
        return (other instanceof City) && equals((City)other) ;
    }
    public boolean equals(City other)
    {
        return other!=null && this.id.equals(other.getId());
    }
}
irreputable
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0

Don't take anything else than an Object if you want to override equals()!

Writing a specialized equals() method is a common error but tends to violate the equals() contract.

b_erb
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    You can however do both, overloading it with a special `equals( City )` method, that is specialized in checking two cities, and then overriding the original `equals( Object )` method in which you will call `equals( City )` if the passed object is a City and do something else otherwise (like return false always). – poke Jul 17 '10 at 16:37
  • @poke - +1 for great point! I find myself doing this quite often, on some rare occasions it has had a positive performance impact. – mikera Jul 17 '10 at 16:43
  • Of course you two are right. What I meant is that this often leads to error prone code. As long as you know what you are doing and use the override annotation properly, there is no big deal. – b_erb Jul 17 '10 at 16:47