Java Enum definition

Question

I thought I understood Java generics pretty well, but then I came across the following in java.lang.Enum:

class Enum>
         


        

Answer 1

If you look at the Enum source code, it has the following:

public abstract class Enum>
        implements Comparable, Serializable {

    public final int compareTo(E o) {
        Enum other = (Enum)o;
        Enum self = this;
        if (self.getClass() != other.getClass() && // optimization
            self.getDeclaringClass() != other.getDeclaringClass())
            throw new ClassCastException();
        return self.ordinal - other.ordinal;
    }

    @SuppressWarnings("unchecked")
    public final Class getDeclaringClass() {
        Class clazz = getClass();
        Class zuper = clazz.getSuperclass();
        return (zuper == Enum.class) ? (Class)clazz : (Class)zuper;
    }

    public static > T valueOf(Class enumType,
                                                String name) {
        T result = enumType.enumConstantDirectory().get(name);
        if (result != null)
            return result;
        if (name == null)
            throw new NullPointerException("Name is null");
        throw new IllegalArgumentException(
            "No enum constant " + enumType.getCanonicalName() + "." + name);
    } 
}

First thing first, what does E extends Enum mean? It means the type parameter is something that extends from Enum, and isn't parametrized with a raw type (it's parametrized by itself).

This is relevant if you have an enum

public enum MyEnum {
    THING1,
    THING2;
}

which, if I know correctly, is translated to

public final class MyEnum extends Enum {
    public static final MyEnum THING1 = new MyEnum();
    public static final MyEnum THING2 = new MyEnum();
}

So this means that MyEnum receives the following methods:

public final int compareTo(MyEnum o) {
    Enum other = (Enum)o;
    Enum self = this;
    if (self.getClass() != other.getClass() && // optimization
        self.getDeclaringClass() != other.getDeclaringClass())
        throw new ClassCastException();
    return self.ordinal - other.ordinal;
}

And even more importantly,

    @SuppressWarnings("unchecked")
    public final Class getDeclaringClass() {
        Class clazz = getClass();
        Class zuper = clazz.getSuperclass();
        return (zuper == Enum.class) ? (Class)clazz : (Class)zuper;
    }

This makes getDeclaringClass() cast to the proper Class object.

A way clearer example is the one that I answered on this question where you cannot avoid this construct if you want to specify a generic bound.