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C and C++ offer the enum feature that allows for a set of named integer values to act as constants. This helps prevent accidentally using an illegal value where a group of predefined constant values, and nothing else, are expected.

Java has not had such a feature before Java 5.0. We've seen a related feature in BorderLayout's constants NORTH, SOUTH, CENTER, etc. Those constants are simply use final static String declaration for each constant.

The new enumerated type feature of J2SE 5.0 allows you to define at compile time a type that takes only a certain set of constant values.

Consider a class that accepts one of the four seasons and returns the average temperature of that season. In pre-J2SE5.0 code we might implement such as class as follows

public class FourSeasons
{
  public static final int SPRING = 1;
  public static final int FALL = 2;
  public static final int SUMMER = 3;
  public static final int WINTER = 4;

  public float getAverageTemp (int season) {
    switch (season) {
      case SPRING:
        return calculateSpringAverageTemp ();
      case FALL:
        return calculateFallAverageTemp ();
      ...
   } // getAverageTemp
} // class FourSeasons

A user of this class could call it with

FourSeasons s4 = new FourSeasons ();
float average_temp = s4.getAverageTemp (Seasons4.SPRING);

But there is no way to prevent a user from calling

s4.getAverageTemp (5);

resulting in unpredicatable behavior since the value 5 is not accounted for in the method getAverageTemp ()

With the new enum statement, however, we can catch such errors at compile time. For this example, we can define

public enum Season { SPRING, SUMMER, FALL, WINTER }

and then implement a method

public float getAverageTemp (Season s) {
  ...
}

Here the method parameter is the enum type Season. The compiler catches any attempt to call getAverageTemp() with any parameter than one of the constant names defined in the enum declaration. So it acts just as if you tried to use a object of a type in a method argument that doesn't match the type in the method definition.

So, essentially, enum acts like a class definition. There is no need for a semicolon at the end of the closing brace, as shown above for the Season definition, though including one is not an error.

Enumerated types can do a lot more than replace constants. One nice feature is the ability to obtain an array of all the enumerated type values within an enum:

Season [] seasons = Season.values ();

For more about the enumerated type, we refer you to the online API documentation and the references listed below.

References & Web Resources

• Chapter 1: Supplements : More J2SE Features
• Enums - Java Language Guide 1.5.0
• Calvin Austin, J2SE 5.0 in a Nutshell, May 2004, Sun Developer Network Site.
• Calvin Austin, Take the Fast Track to J2SE 1.5: JCP experts listened and developed the best developer platform. Discover how new J2SE 1.5 features can streamline your code, JavaPro, June 7, 2004.
• Java Programming Language - Enhancements for JDK 5

Latest update: Nov. 17, 2004

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