Java public enum method purpose

Question

So I have my enum

public enum Sample {
   ValueA{
      @Override
   public String getValue(){ return "A"; }   
 },
 ValueB{
    @Override
   public String getValue(){ return "B"; }

   public void doSomething(){ }
 };
abstract public String getValue();
};

and I have some other code trying to use the enum.

Sample.ValueB.doSomething();

Which seems like it should be valid, but produces the error "The method doSomething() is undefined for the type Sample". As opposed to

Sample value = Sample.ValueB;
value.doSomething();

which produces the same error and seems reasonable.

I assume there is a reasonable answer as to why the first one doesn't work and it relates to the two examples being equivalent under the hood. I was hoping someone could point me towards the documentation on why it is that way.

It would really help if you'd say what error you're getting... — Jon Skeet
– Jon Skeet, Commented Aug 30, 2011 at 12:52

Pang · Accepted Answer · 2013-07-15 04:09:24Z

15

The type of the "field" ValueA is Sample. That means that you can only invoke methods on ValueA that Sample provides. From JLS §8.9.1. Enum Constants:

Instance methods declared in these class bodies may be invoked outside the enclosing enum type only if they override accessible methods in the enclosing enum type.

More importantly: From a design perspective enum values should be uniform: if some operation is possible with one specific value, then it should be possible with all values (although it might result in different code being executed).

edited Jul 15, 2013 at 4:09

Pang

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answered Aug 30, 2011 at 12:56

Joachim Sauer

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Comments

Jon Skeet · Accepted Answer · 2011-08-30 12:53:24Z

10

Basically the compile-time type of Sample.ValueB is still Sample, even though the execution-time type of the value will be ValueB. So your two snippets of code are equivalent - clients don't get to see the "extra" methods which are only present in some of your enum values.

You can effectively think of the enum as declaring a field like this:

public static final Sample ValueB = new Sample() {
    @Override
    public String getValue(){ return "B"; }

    public void doSomething(){ }
};

answered Aug 30, 2011 at 12:53

Jon Skeet

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Comments

Swaranga Sarma · Accepted Answer · 2011-08-30 13:06:50Z

4

When you write

enum Sample
{
     Value{
        public void doSomething()
        {
              //do something
        }
    };
}

you are not creating an instance of the Sample enum, rather an instance of an anonymous sub-class of the enum Sample. Thet's why the method doSomething() is undefined.

Update: this can be proved as follows:

Try this

System.out.println(Sample.ValueB.getClass().getName());

prints Sample$2

Which means that even though you have the method doSomething() in the instance of Sample$2 which you have named ValueB, you are referring it through the super-class reference of type Sample and hence only those methods defined in the class Sample will be visible at compile time.

You can call that doSomething() at runtime via reflection though.

edited Aug 30, 2011 at 13:06

answered Aug 30, 2011 at 12:57

Swaranga Sarma

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1 Comment

Joachim Sauer Over a year ago

More importantly: the type of the "field" Value will still be Sample and not Sample$1.

Michał Šrajer · Accepted Answer · 2011-08-30 13:06:01Z

3

public void doSomething(){ } will have same effect as private void doSomething(){ }.

doSomething() is not visible outside ValueB unless you add doSomething() to the Sample.

Each Sample value has type Sample, so it cannot have different set of methods for different values (from outside perspective).

answered Aug 30, 2011 at 13:06

Michał Šrajer

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2 Comments

Joachim Sauer Over a year ago

Warning: the Sample values do have more specific types if they have a class body. The Sample fields only ever have the type Sample.

Michał Šrajer Over a year ago

@Joachim: sure, but those specific types are anonymous subclasses of Sample, so "outside" you can only use interface defined by Sample. I didn't make my self clear - sorry.

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