如何实现Java Enumeration Abstract类和接口？

Question

I have the following problem. I have a package with different enumeration classes in java.

What is the correct approach to avoid repeting myself when adding new methods that are common to all enums?

My problem:

channel.java

    public enum Channel {
           channel1, channel2;
    }

color.java

    public enum color{
           color1, color2;
    }

The method which should be used in both:

    public static boolean contains(String channel) {
       for (Channel c : Channel.values()) {
          if (c.name().equals(channel)) {
              return true;
          }
      }
    return false;
    }

Note that the loop has a reference to the enum itself. So, this will require copy and paste the method in all enums that I want to use it. Any suggestion?

Thanks Victor

Answer 1

You can't do it quite the way you want to unfortunately. What you can do is to have some sort of util class that has a general mehod on it.

For example...

public class EnumUtils {

    public static boolean contains(Enum[] enumValues, String nameToCheck) {

        for(Enum each : enumValues) {
            if(each.name().equals(nameToCheck)) {
                return true;
            }
        }
        return false;
    }
}

Then you could use it like this...

System.out.println(EnumUtils.contains(Channel.values(), "channel1")); // TRUE
System.out.println(EnumUtils.contains(Color.values(), "octarine")); // FALSE

Caveat - In more complex systems, these sorts of static util classes are sometimes a bit of a "code-smell" but I think in your case it's fine.

For Java6:

     change each.name() => each.toString()

Answer 2

I suggest to use an util-method like @Phil Anderson mentioned. I would only change it to a general pattern:

public static <T extends Enum<T>> void some_method(Class<T> clazz, String name) {
    try {
        T foundEnum = Enum.valueOf(clazz, name);
        // name matches to one of enum values, do something with it
    } catch (IllegalArgumentException e) {
        // name doesn't matches to any of enum values
    }
}

which in case of contains semantic could look like this:

public static <T extends Enum<T>> boolean contains(Class<T> clazz, String name) {
    try {
        Enum.valueOf(clazz, name);
    } catch (IllegalArgumentException e) {
        return false;
    }
    return true;
}

---

Updated:

As @phil-anderson mentioned, from performance point of view this method have certain disadvantages, because generation and throwing of exception is pretty slow (see How slow are Java exceptions? ). But this is only a case, if method is invoked with an incorrect name value.

So, in this case you could use this pattern:

public static <T extends Enum<T>> void some_method(Class<T> clazz, String name) {
    for (T each : clazz.getEnumConstants()) {
        if (each.name().equals(name)) {
            // name matches to one of enum values, do something with it
        }
    }
    // name doesn't matches to any of enum values
}

Moreover, if performance plays an important role, especially if enum consists of large number of values, it is not efficient to iterate over (maybe) all of them. The solution could be using a lazy hash map for enums and get the value by a hashcode. For example:

@SuppressWarnings("unchecked")
public static <T extends Enum<T>> void some_method(Class<T> clazz, String name) {
    Map<String, Enum<?>> enumMap = enumsMap.get(clazz);
    if (enumMap == null) {
        enumMap = new HashMap<String, Enum<?>>();
        for (T t : clazz.getEnumConstants()) {
            enumMap.put(t.name(), t);
        }
        enumsMap.put(clazz, enumMap);
    }
    T t = (T) enumMap.get(name);
    if (t != null) {
        // name matches to one of enum values, do something with it
    } else {
        // name doesn't matches to any of enum values
    }
}

Answer 3

You can use an interface with a static method to do this:

public interface CanContainChannel {
    static boolean contains(String channel) {
        for (Channel c : Channel.values()) {
           if (c.name().equals(channel)) {
               return true;
           }
        }
        return false;
    }
}

And you both your enums can implement this interface to gain this method, although I'm not sure why you would want a method like this on your Color enum.

EDIT:

On clarification of the question I this question will help you: Iterate enum values using java generics

Answer 4

With reflection one can fetch the enum constants.

enum E {
    i("unu"), ii("du"), iii("tri"), iv("kvar");

    public final String name;

    E(String name) {
        this.name = name;
    }
}

public static void main(String[] args) {
    // Normally in Java 8 for a concrete enum:
    final String sought = "tri";
    boolean found = Stream.of(E.values()).anyMatch((c) -> c.name.equals(sought));

    // A generic function:
    System.out.println("okay:  "
        + find(E.class, (c) -> c.name.equals(sought)));
    System.out.println("fails: "
        + find(E.class, (c) -> c.name.equals("prtl")));
}

public static <T extends Enum<?>> boolean find(Class<T> clazz,
        Predicate<T> predicate) {
    return Stream.of(clazz.getEnumConstants()).anyMatch(predicate);
}

As you want to access a field, it is easier to pass the entire predicate.

Under java 7 one would do it generally more tight (without Stream) and need an interface for getName() etcetera.