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Computational Problem Solving with Java and Interactive Object Graphics

Mark F. Russo, PhD

Section 2.7 Enumerated Types

In Java, enumerated types, also known as enums, provide a way to define a fixed set of named constants. Enums allow you to create a custom data type with a predefined and automatically assigned set of values, making your code more readable, maintainable, and type-safe. Enums are useful when you need a set of constants with different values, where the values themselves don’t matter, as long as they differ.
To define an enum in Java, use the enum keyword followed by the name of the new enum type. After that, add a pair of curly braces ({…}} and a comma-separated list of constant names to be in the enum. Here’s an example of a simple enum representing clothing sizes:
>jshell
|  Welcome to JShell -- Version 17.0.6
|  For an introduction type: /help intro

jshell> enum Size {
   ...>     XSMALL,
   ...>     SMALL,
   ...>     MEDIUM,
   ...>     LARGE,
   ...>     XLARGE
   ...> }
|  created enum Size

jshell>
In this example, Size is the name of the enum type, and the constants SMALL, MEDIUM, LARGE, etc., are the possible values of the enum. Note that capitalization of constant name is not required, but useful for communicating the fact that these are constants.
You can use enum constants in your code by preceding the constant name with the name of the enum and a dot to connect them. For example:
jshell> Size mySize = Size.LARGE;
mySize ==> LARGE

jshell> System.out.println("Please order a " + mySize);
Please order a LARGE

jshell>
Enums also have some useful methods automatically provided by Java. For instance, you can retrieve the name of an enum constant using the name() method.
jshell> "Please order a " + mySize.name();
Please order a LARGE
You can compare enum constants using the equality operator (==) because each constant is a unique instance of the enum type.
In summary, Java enumerated types (enums) provide a convenient way to define a fixed set of distinct named constants, making your code more expressive and robust. Enums are type-safe, can have custom behavior, and provide useful methods out-of-the-box for working with the constants they define.

Activity 2.7.1.

Define a new enum named Direction with the constants LEFT, RIGHT, FORWARD, and BACKWARD.
Answer.
jshell> enum Direction {LEFT, RIGHT, FORWARD, BACKWARD};
|  created enum Direction

Activity 2.7.2.

Declare three new variables named step1, step2, and step3 of type Direction and initialize each to the first three steps to move to a location nearby.
Answer.
jshell> Direction step1 = Direction.RIGHT;
step1 ==> RIGHT
|  created variable step1 : Direction

jshell> Direction step2 = Direction.FORWARD;
step2 ==> FORWARD
|  created variable step2 : Direction

jshell> Direction step3 = Direction.LEFT;
step3 ==> LEFT
|  created variable step3 : Direction

Activity 2.7.3.

Use the Direction enum name() method and the + operator to construct a String that concatenates all steps into a single String.
Answer.
jshell> step1.name() + " " + step2.name() + " " + step3.name()
$2 ==> "RIGHT FORWARD LEFT"
|  created scratch variable $6 : String
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