Generics and enums

[Frans Janssen]

Is there any way to use a generic superclass for any enum type?

I want to make a combo box that allows the user to choose from the values in an enum type. I hoped I could do something like this:

public class EnumComboBox<T extends Enum> extends javax.swing.JComboBox {
        
    public EnumComboBox(T t) {
        super(T.values());
    }
...
}

But values() is not a method defined in Enum and the constructor doesn't accept an enum type as a parameter (as apparently it isn't a subclass of Enum, even though the javadoc seems to suggest that it should be).

Does anyone know how to do this or do you have a suggestion for an elegant solution to accomplish the same thing?

Thanks in advance,

Frans.

[Jesper de Jong]

Try this.

public class EnumComboBox extends javax.swing.JComboBox {
    public <E extends Enum<E>> EnumComboBox(Class<E> enumClass) {
        super(enumClass.getEnumConstants());
    }
 
    // ...
}

You call this by passing the constructor a Class object that represents the type of an enum. For example:

public enum Stuff {
  ONE, TWO, THREE
}
 
// ...
 
EnumComboBox cb = new EnumComboBox(Stuff.class);

[Frans Janssen]

Jesper,

Thanks for your reply. I had to change the class declaration a bit in order to get it to compile:

public class EnumComboBox<E extends Enum<E>> extends JComboBox {

However, it then crashes on a NullPointerException when constructed. Apparently the getEnumConstants() method returns null, as the Class class feels it is not an enum class (isEnum() returns false). I think that is pretty awkward, but the Javadoc of isEnum() says the following:

    public boolean isEnum() {
// An enum must both directly extend java.lang.Enum and have
// the ENUM bit set; classes for specialized enum constants
// don't do the former.
return (this.getModifiers() & ENUM) != 0 && 
this.getSuperclass() == java.lang.Enum.class;
    }

If I call getSuperClass() on the enum class, it tells me it derives from Object rather than enum.

[Jesper de Jong]

Strange... what are you passing to the constructor exactly? If you pass a Class object that represents the type of an enum, it should work properly. The small test program below prints:

Is it an enum? true
Value: ONE
Value: TWO
Value: THREE

public class Main {
    public static void main(String args[]) {
        Class<Stuff> c = Stuff.class;
 
        System.out.println("Is it an enum? " + c.isEnum());
 
        Stuff[] values = c.getEnumConstants();
        for (Stuff s : values) {
            System.out.println("Value: " + s);
        }
    }
}
 
enum Stuff {
    ONE, TWO, THREE
}

[Frans Janssen]

Hmmm, this looks like a "feature" of JBuilder. If I compile and run your code with the JDK tools it works perfectly, but when I compile and run it in JBuilder (2005) it returns false on isEnum() and then throws an exception because the getEnumConstants() returns null.