What's the correct way to override equal method in java?

Question

public class Person {

    private String name;

    public Person(String name) {
        this.name = name;
    }

    public boolean equals(Person p) {
        System.out.println("..............");
        return p.name.equals(this.name);
    }
}

Hi, I was going through OCJP question and answer and answer to this question was The equals method does NOT properly override the Object.equlas method.

I think it is redundant to override here with name which is String but this answer to this question is also not correct.

Could you please point out what concept i am missing here...

Note: There are no option saying override hasCode and equlas with same parameter to this question...

possible duplicate of [Overriding the java equals() method quirk](http://stackoverflow.com/questions/185937/overriding-the-java-equals-method-quirk) — durron597, Mar 05 '15 at 14:39

Eran · Accepted Answer · 2015-03-05T14:44:58.467

13

The signature of equals should be :

public boolean equals(Object p)

Your public boolean equals(Person p) method doesn't override Object's equals. It overloads it.

A possible implementation :

@Override
public boolean equals(Object other)
{
    if (!(other instanceof Person))
        return false;
    Person p = (Person) other;
    return p.name.equals(this.name);
}

edited Mar 05 '15 at 14:44

answered Mar 05 '15 at 14:32

Eran

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public boolean equals(Object p){return ((Person)p).name.equals(this.name);} This is the correct signature right? – Rajesh Kumar Mar 05 '15 at 14:37
1

@RajeshKumar: [ClassCastException](http://docs.oracle.com/javase/7/docs/api/java/lang/ClassCastException.html) – Voicu Mar 05 '15 at 14:39
@RajeshKumar Not quite. Before casing `p` to `Person`, you should validate that it's actually an instanceof Person. – Eran Mar 05 '15 at 14:39
@LocHa If `name` may be null, you have to test for that scenario and return a proper value, but I'm assuming that a Person without a name is not a possible scenario (the constructor of Person should contain a validation that would prevent that). – Eran Mar 05 '15 at 14:55

score 7 · Answer 2 · edited May 23 '17 at 10:28

7

@Override
public boolean equals(Object ob) {

    if (!(ob instanceof Person)) {
        return false;
    }

    Person p = (Person)ob;

    return p.name.equals(this.name);
}

ClassCastException proof
NullPointerException proof (Is null check needed before calling instanceof)

edited May 23 '17 at 10:28

Community

1
1

answered Mar 05 '15 at 14:34

Tommaso Bertoni

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1
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additional I would add: if (this == obj) return true; if (obj == null) return false; if (getClass() != obj.getClass()) return false; Persona other = (Person) obj; – nano_nano Mar 05 '15 at 14:41
passing null can be valid sometimes – nano_nano Mar 05 '15 at 14:43

What's the correct way to override equal method in java?

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