l如何从泛型类型的实例调用values（） <E extends Enum<E> &gt;？

Question

public static <E extends Enum<E>> void example(E e){
    e. //what should I put here to get result of values()?
}

I am trying to call the method values() that returns the values of an enum using this method with generic arguments.

How can I do that?

Answer 1

Enum doesn't have values() method. It is added by compiler later to each of your Enum subclasses (when we are creating our own enum YourEnum{...} type). So we can't invoke values() from Enum type.

Possible solution is

E[] values = e.getDeclaringClass().getEnumConstants();

To be safe we should use getDeclaringClass() instead of getClass() , because enum values can be implemented as anonymous classes like in case of TimeUnit

public enum TimeUnit {
    /**
     * Time unit representing one thousandth of a microsecond
     */
    NANOSECONDS {
        public long toNanos(long d)   { return d; }
        public long toMicros(long d)  { return d/(C1/C0); }
        public long toMillis(long d)  { return d/(C2/C0); }
        public long toSeconds(long d) { return d/(C3/C0); }
        public long toMinutes(long d) { return d/(C4/C0); }
        public long toHours(long d)   { return d/(C5/C0); }
        public long toDays(long d)    { return d/(C6/C0); }
        public long convert(long d, TimeUnit u) { return u.toNanos(d); }
        int excessNanos(long d, long m) { return (int)(d - (m*C2)); }
    },

In this case

• TimeUnit.NANOSECONDS.getClass() returns java.util.concurrent.TimeUnit$3 (but this anonymous class doesn't contain any enum values)
• TimeUnit.NANOSECONDS.getDeclaringClass() returns java.util.concurrent.TimeUnit (with contains all of enum values).

It will also save us some trouble with casting since

• getDeclaringClass returns Class<E>
• getClass() returns Class<? extends E> Class<? extends E>

So result type of

• getDeclaringClass().getEnumConstants() is E[]
• while getClass().getEnumConstants() is ? extends Enum[] ? extends Enum[] .

This allows us to create loop like without need to involve casting:

for (E value : e.getDeclaringClass().getEnumConstants()){
    //handle value
}

---

As an extra, you can also get all values from Class<E> (where <E extends Enum<E>> ) by using EnumSet.allOf(enumClass) like in case of code from this question:

EnumSet<E> allOf = EnumSet.allOf(e.getDeclaringClass());

Answer 2

Well, there is a method in the clas Class<T> called getEnumConstants() which does just that. So i had this method that printed out the enum constants given any enum value here:

public <E extends Enum<E>> void printEnums(E e){
    for(E value : e.getDeclaringClass().getEnumConstants()){
        System.out.println("Value: "+value);
    }
}

To call it i would use something like
printEnums(Enum.SOME_VALUE);

although I think the following method is better because it takes as parameter the class of the enum directly:

public static <E extends Enum<E>> void printEnums(Class<E> e){
    for(E value : e.getEnumConstants()){
        System.out.println("Value: "+value);
    }
}

You can call this method like so: printEnums(EnumClass.class);

I know you are not asking how to print the enum values, but these methods above are good to give you a headstart on how to access the constants given the type or the class.

EDIT 1: Changed the code. Thanks @JBNizet

EDIT 2: Changed the code again according to @Tunaki's comment. Note: this was initially adressed on Pshemo's answer