Enumerations

What is an enumeration?

Some computations can involve a variable whose value can be one of a seemingly endless array of possibilities. In other cases, a variable’s value can have only certain values. For example, a Boolean variable must be true or false. To represent a letter grade A, B, C, D, or F, we could use a char variable—grade, for example—but restrict its value to only those five letters. In reality, however, grade could contain any character, not just the letters A, B, C, D, and F. To prevent this, instead of declaring grade as a char, we can declare it as an enumerated data type, or enumeration. An enumeration itemizes the values that a variable can have.

For instance, the following statement defines LetterGrade as an enumeration:

public enum LetterGrade {A, B, C, D, F}

LetterGrade behaves as a class type. The items listed between the braces in the definition of LetterGrade are the objects that an object of LetterGrade can reference. For example, the following statement declares grade to have LetterGrade as its data type and assigns A to grade:

LetterGrade grade = LetterGrade.A;

We qualify each of these enumerated objects with the name of the enumeration, just as we qualify the named constant PI with the name of its class Math. Assigning an object other than A, B, C, D, or F to grade will cause a syntax error. Since these objects behave as constants, we name them as we would a constant by using uppercase letters and the rules for identifiers.

When the compiler encounters an enumeration, it creates a class that has several methods. Among them is toString, which displays the name of its receiving object. Thus, if we write:

System.out.println(LetterGrade.A);

For example, toString is called implicitly and the letter A is displayed. You can verify this by running the code given below: