generics & enums

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josef - 14 Aug 2006 19:57 GMT

class A<T extends Enum<T>> {

    Class<T> e;

    public A(Class<T> type) {
        e = type;
    }

    public T get(int ordinal) {

        /*
        * Is there a better (faster) way to access the implicitly declared
        * static method 'values' (1) for enums (i.e. without reflection)?
        *
        * (I'm trying to implement a sweet ComboboxModel)
        *
        * (1)
http://java.sun.com/docs/books/jls/third_edition/html/classes.html#302265
        */

        try {
            return ((T[])e
                    .getDeclaredMethod("values", new Class[]{})
                    .invoke(null, new Class[]{}))[ordinal];

        } catch(Exception ex) {
            //method is static and defined in java language specification
        }

        throw new AssertionError();
    }
}

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Roland de Ruiter - 14 Aug 2006 21:58 GMT

[...]

>         /*
>         * Is there a better (faster) way to access the implicitly declared
[quoted text clipped - 3 lines]
>         *
>         */

[...]

Use Class#getEnumConstants()
<http://java.sun.com/j2se/1.5.0/docs/api/java/lang/Class.html#getEnumConstants()>
Looking at the source code of this method, however, it appears to use
reflection as well, though it caches it's values (so a subsequent calls
of getEnumConstants() return the cached values).
You could apply the same technique. The following example caches the
enum constants immediately in the constructor:

public class A<E extends Enum<E>> {

private final E[] enumConstants;

public A(Class<E> enumType) {
enumConstants = enumType.getEnumConstants();
}

public E get(int ordinal) {
if (ordinal >= 0 && ordinal < size()) {
return enumConstants[ordinal];
} else {
return null;
// OR:
// throw new
IndexOutOfBoundsException(String.valueOf(ordinal));
}
}

public int size() {
return enumConstants.length;
}

public static void main(String[] args) {
A<Thread.State> a = new A<Thread.State>(Thread.State.class);
for (int i = -1; i <= a.size(); i++) {
System.out.println("A.get(" + i + ")=" + a.get(i));
}
}
}

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Regards,

Roland

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