How to implement equals and hashcode for enum classes

Question

I have an interface name Types

public interface Types
{       
    public String getName();    
}

Two enum classes are extending this interface like

public enum AlphaTypes implements Types
{
    Alpha("alpha"),Common("common")
    private String name;

    private AlphaTypes(String name)
    {
        this.name = name;
    }

    @Override
    public String getName()
    {
        return name;
    }   
}

public enum BetaTypes implements Types
{
    Beta("beta"),Common("common")
    private String name;

    private BetaTypes(String name)
    {
        this.name = name;
    }

    @Override
    public String getName()
    {
        return name;
    }   
}

The requirement is have a map which takes Types as key like Map<Types,Object> map;

How to implement equals and hashcode, such that the map keys are unique even for common enum values?

Have you tried just using it with the default implementations? Reference equality should be fine since these are `enum`s (so you won't have different instances with the same values), and I see no reason why the default `hashCode` wouldn't suffice. — jpmc26, Apr 24 '14 at 07:32
Remember enum members are singletons, so == is adequate as a test for equality . all enum classes implicitly extend java.lang.Enum which has final implementations of both. equals uses == and hashCode uses Object.hashCode. Try overriding them in any enum and you'll get a compiler error Possible duplicate : http://stackoverflow.com/questions/2964704/why-java-does-not-allow-overriding-equalsobject-in-an-enum — Asif Bhutto, Apr 24 '14 at 07:33

score 9 · Accepted Answer · answered Apr 24 '14 at 07:48

The class java.lang.Enum declare both equals() and hashCode() as final, thus you'll get compiler errors trying to override them.

That being said, your example above works as you desire - if you add AlphaTypes.Common and BetaTypes.Common to a Map you'll get a map with two elements:

public static void main( String[] args ) throws Exception
{
    Map<Types,Object> map = new HashMap<Types,Object>();

    map.put( AlphaTypes.Common , "b" );
    map.put( BetaTypes.Common , "b" );

    System.out.println( "size=" + map.size());
}

size=2

Cheers,

How to implement equals and hashcode for enum classes

1 Answers1