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java,tips,enum

Enum Tricks: Customized valueOf

When I am writing enumerations I very often found myself implementing a static method similar to the standard enum’s valueOf() but based on field rather than name:

public static TestOne valueOfDescription(String description) {
    for (TestOne v : values()) {
        if (v.description.equals(description)) {
            return v;
        }
    }
    throw new IllegalArgumentException(
    "No enum const " + TestOne.class + "@description." + description);
}

 Where “description” is yet another String field in my enum. And I am not alone. See this article for example. Obviously this method is very ineffective. Every time it is invoked it iterates over all members of the enum. Here is the improved version that uses a cache:

 private static Map map = null;
 public static TestTwo valueOfDescription(String description) {
  synchronized(TestTwo.class) {
   if (map == null) {
    map = new HashMap();
    for (TestTwo v : values()) {
     map.put(v.description, v);
    }
   }
  }

  TestTwo result = map.get(description);
  if (result == null) {
         throw new IllegalArgumentException(
                 "No enum const " + TestTwo.class + "@description." + description);
  }

  return result;
 }

It is fine if we have only one enum and only one custom field that we use to find the enum value. But if we have 20 enums, and each has 3 such fields, then the code will be very verbose. As I dislike copy/paste programming I have implemented a utility that helps to create such methods. I called this utility class ValueOf. It has 2 public methods:

public static <T extends Enum<T>, V> T valueOf(Class<T> enumType, String fieldName, V value);

which finds the required field in specified enum. It is implemented utilizing reflection and uses a hash table initialized during the first call for better performance. The other overridden valueOf() looks like:

public static <T extends Enum<T>> T valueOf(Class<T> enumType, Comparable<T> comparable);

This method does not cache results, so it iterates over enum members on each invocation. But it is more universal: you can implement comparable as you want, so this method may find enum members using more complicated criteria.

Full code with examples and JUnit test case are available here.

Conclusions

Java Enums provide the ability to locate enum members by name. This article describes a utility that makes it easy to locate enum members by any other field.

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