java - an enum question

Question

I have encountered a weird problem in my app (java).

I have an enum. Something like that

public enum myEnum implement myIntrface{
   valueA(1),valueb(2),valuec(3),valued(4)
   private int i;
   // and then - a constructor 
   public MyEnum(int number){
        i = number;
   }       


   private MyObj obj = new MyObj;
   // getter and setter for obj
}

and in another class I have this

   MyEnum.valueA.setObj(new Obj(...))

in briefe - I have an enum with a private instance member that has a set and a get.

So far so good -

The only thing that amazes me is that later on I look at the value of the MyEnum.valueA().obj is null.

there is nothing that updates the value to null, I have even gave it a default value in the constructor and I still see it null later.
any suggestions?

Can you post a complete compilable example that demonstrates the issue? — Oliver Charlesworth
– Oliver Charlesworth, Commented Jun 26, 2011 at 15:35
public MyEnum(int number) is not the constructor. the enum name is myEnum. no capital m in the name — Sean
– Sean, Commented Jun 26, 2011 at 15:38

Michael J. Lee · Accepted Answer · 2011-06-26 15:56:44Z

Enums should be un-modifiable classes so you shouldn't really be doing this. If your looking to modify the state of a type based object like an enum you should use an final class approach with embedded constants. Below is an example of a class based approach with a modifiable name an a un-modifiable name...

public final class Connection {

    public static final Connection EMAIL = new Connection("email");
    public static final Connection PHONE = new Connection("phone");
    public static final Connection FAX = new Connection("fax");
    /**/
    private final String unmodifiableName; //<-- it's final
    private String modifiableName;

    /*
     * The constructor is private so no new connections can be created outside.
     */
    private Connection(String name) { 
        this.unmodifiableName = name;
    }

    public String getUnmodifiableName() {
        return unmodifiableName;
    }

    public String getModifiableName() {
        return modifiableName;
    }

    public void setModifiableName(String modifiableName) {
        this.modifiableName = modifiableName;
    }

}

Karl Knechtel · Accepted Answer · 2011-06-26 15:41:03Z

0

The purpose of enums is to represent constant values. It does not make any sense to set the fields of a constant value.

You should declare your fields as final, and use the constructor to initialize all of them.

answered Jun 26, 2011 at 15:41

Karl Knechtel

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1 Comment

Oliver Charlesworth Over a year ago

It may not make sense, but it is allowed.

Oliver Charlesworth · Accepted Answer · 2011-06-26 15:43:18Z

0

For reference, the following code works as expected:

public class Test {

    public static enum MyEnum {
        valueA(1),valueb(2),valuec(3),valued(4);
        private int i;
        private Object o;

        private MyEnum(int number) {
             i = number;
        }

        public void set(Object o) {
            this.o = o;
        }

        public Object get() {
            return o;
        }


     } 

    public static void main(String[] args) {
        System.out.println(MyEnum.valueA.get());  // prints "null"
        MyEnum.valueA.set(new Integer(42));
        System.out.println(MyEnum.valueA.get());  // prints "42"
    }
}

answered Jun 26, 2011 at 15:43

Oliver Charlesworth

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Comments

Bick · Accepted Answer · 2011-06-27 10:27:21Z

0

the cause of this problem is the db40 framework . It loads an enum from the db using reflection. This is well documented .
http://developer.db4o.com/Forums/tabid/98/aft/5439/Default.aspx

answered Jun 27, 2011 at 10:27

Bick

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