Enum in Java doesn't allow to implement a method in its implementing interface

Question

The following simple code snippet uses the interface Named containing two methods namely name() and order() which is being implemented by enum named Days. The enum Days doesn't allow to implement the name() method of its implementing interface. Doing so, causes a compile-time error and although the name() method is not implemented by the enum, it doesn't issue any error.

package enumpkg;

interface Named
{
    public String name();
    public int order();
}

enum Days implements Named
{
    Monday, Tuesday, Wednesday, Thursday, Friday, Saturday, Sunday;

    public int order()
    {
        return ordinal()+1;
    }
}

final public class Main
{
    public static void main(String[] args)
    {
        System.out.println("Monday    = "+Days.Monday.order());
        System.out.println("Tuesday   = "+Days.Tuesday.order());
        System.out.println("Wednesday = "+Days.Wednesday.order());
        System.out.println("Thursday  = "+Days.Thursday.order());
        System.out.println("Friday    = "+Days.Friday.order());
        System.out.println("Saturday  = "+Days.Saturday.order());
        System.out.println("Sunday    = "+Days.Sunday.order());
    }
}

The code works without implementing the name() method in enum. How?

The output is quite obvious as shown below.

Monday    = 1
Tuesday   = 2
Wednesday = 3
Thursday  = 4
Friday    = 5
Saturday  = 6
Sunday    = 7

I agree with SLaks - it is very important to actually read compilation error messages, and try and understand what they are telling you. — Stephen C
– Stephen C, Commented Dec 23, 2011 at 1:38
It indicates that the overridden method name() is final and can not be overridden. — Lion
– Lion, Commented Dec 23, 2011 at 1:39
I start to think the gcc devs were on to something when they made their error messages unreadable - wasted effort to try to create good, meaningful error messages :( — Voo
– Voo, Commented Dec 23, 2011 at 2:34

SLaks · Accepted Answer · 2011-12-23 01:29:54Z

8

The base Enum class already has a name() method.
Since this method is final, you can't override it (as the error message clearly states).

answered Dec 23, 2011 at 1:29

SLaks

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Comments

Stephen C · Accepted Answer · 2011-12-23 01:36:18Z

3

From the Java API, name() is defined as:

public final String name()

Thus name() exists for all enums, and is not overridable.

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