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

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.

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

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" } }

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

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

Collectives™ on Stack Overflow

java - an enum question

4 Answers 4

Comments

1 Comment

Comments

Comments

Hot Network Questions

Collectives™ on Stack Overflow

4 Answers 4

Comments

1 Comment

Comments

Comments

Related