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<p>These are the main arguments for <a href="http://download-llnw.oracle.com/javase/1.5.0/docs/guide/language/enums.html" rel="nofollow noreferrer"><code>enum</code></a>, <a href="http://java.sun.com/javase/6/docs/api/java/util/EnumMap.html" rel="nofollow noreferrer"><code>EnumMap</code></a>, and <a href="http://java.sun.com/javase/6/docs/api/java/util/EnumSet.html" rel="nofollow noreferrer"><code>EnumSet</code></a> by short examples.</p> <h3>The case for <code>enum</code></h3> <p>As of Java 6, <a href="http://java.sun.com/javase/6/docs/api/java/util/Calendar.html" rel="nofollow noreferrer"><code>java.util.Calendar</code></a> is an example of a messy class that could've benefited a lot from using <code>enum</code> (among other improvements).</p> <p>Currently <code>Calendar</code> defines the following constants (<a href="http://java.sun.com/javase/6/docs/api/constant-values.html#java.util.Calendar.JANUARY" rel="nofollow noreferrer">among many others</a>):</p> <pre><code>// int constant antipattern from java.util.Calendar public static final int JANUARY = 0; public static final int FEBRUARY = 1; ... public static final int SUNDAY = 1; public static final int MONDAY = 2; ... </code></pre> <p>These are all <code>int</code>, even though they obviously represent different conceptual entities.</p> <p>The following are some serious consequences:</p> <ul> <li><em>It's brittle</em>; care must be taken to assign different numbers whenever needed. <ul> <li>If by mistake we set <code>MONDAY = 0;</code>, <code>SUNDAY = 0;</code>, then we have <code>MONDAY == SUNDAY</code></li> </ul></li> <li><em>There is no namespace and no type-safety</em>, since everything is just an <code>int</code>: <ul> <li>We can <code>setMonth(JANUARY)</code>, but we can also <code>setMonth(THURSDAY)</code> or <code>setMonth(42)</code></li> <li>Who knows what <a href="http://java.sun.com/javase/6/docs/api/java/util/Calendar.html#set%28int,%20int,%20int,%20int,%20int,%20int%29" rel="nofollow noreferrer"><code>set(int,int,int,int,int,int)</code></a> (a real method!) does!</li> </ul></li> </ul> <p>By contrast, we could have something like this instead:</p> <pre><code>// Hypothetical enums for a Calendar library enum Month { JANUARY, FEBRUARY, ... } enum DayOfWeek { SUNDAY, MONDAY, ... } </code></pre> <p>Now we never have to worry about <code>MONDAY == SUNDAY</code> (it can never happen!), and since <code>Month</code> and <code>DayOfWeek</code> are different types, <code>setMonth(MONDAY)</code> does not compile.</p> <p>Additionally, here are some before-and-after codes:</p> <pre><code>// BEFORE with int constants for (int month = JANUARY; month <= DECEMBER; month++) { ... } </code></pre> <p>Here we're making all sorts of assumptions, e.g. <code>JANUARY + 1 == FEBRUARY</code>, etc. On the other hand, the <code>enum</code> counterpart is both more concise, more readable, and makes less assumptions (and therefore less chance for bugs):</p> <pre><code>// AFTER with enum for (Month month : Month.values()) { ... } </code></pre> <hr> <h3>The case for instance fields</h3> <p>In Java, <code>enum</code> is a <code>class</code> that has many special properties, but a <code>class</code> nonetheless, allowing you to define instance methods and fields if necessary.</p> <p>Consider the following example:</p> <pre><code>// BEFORE: with int constants public static final int NORTH = 0; public static final int EAST = 1; public static final int SOUTH = 2; public static final int WEST = 3; public static int degreeFor(int direction) { return direction * 90; // quite an assumption! // must be kept in-sync with the int constants! } //... for (int dir = NORTH; dir <= WEST; dir++) { ... degreeFor(dir) ... } </code></pre> <p>On the other hand, with <code>enum</code> you can write something like this:</p> <pre><code>enum Direction { NORTH(0), EAST(90), SOUTH(180), WEST(270); // so obvious! so easy to read! so easy to write! so easy to maintain! private final int degree; Direction(int degree) { this.degree = degree; } public int getDegree() { return degree; } } //... for (Direction dir : Direction.values()) { ... dir.getDegree() ... } </code></pre> <h3>The case for instance methods</h3> <p>Consider the following example:</p> <pre><code>static int apply(int op1, int op2, int operator) { switch (operator) { case PLUS : return op1 + op2; case MINUS : return op1 - op2; case ... default: throw new IllegalArgumentException("Unknown operator!"); } } </code></pre> <p>As shown in previous example, <code>enum</code> in Java can have instance methods, but not only that but each constant can have its own specific <code>@Override</code> as well. This is shown in the following code:</p> <pre><code>enum Operator { PLUS { int apply(int op1, int op2) { return op1 + op2; } }, MINUS { int apply(int op1, int op2) { return op1 - op2; } }, ... ; abstract int apply(int op1, int op2); } </code></pre> <hr> <h3>The case for <code>EnumMap</code></h3> <p>Here's a quote from <em>Effective Java 2nd Edition</em>:</p> <blockquote> <p>Never derive a value associated with an <code>enum</code> from its <a href="http://java.sun.com/j2se/1.5.0/docs/api/java/lang/Enum.html#ordinal%28%29" rel="nofollow noreferrer"><code>ordinal()</code></a>; store it in an instance field instead. (<em>Item 31: Use instance fields instead of ordinals</em>) It is rarely appropriate to use ordinals to index arrays: use <code>EnumMap</code> instead. The general principle is that application programmers should rarely, if ever, use <code>Enum.ordinal</code>. (<em>Item 33: Use <code>EnumMap</code> instead of ordinal indexing</em>)</p> </blockquote> <p>Essentially where as before you may have something like this:</p> <pre><code>// BEFORE, with int constants and array indexing Employee[] employeeOfTheMonth = ... employeeOfTheMonth[JANUARY] = jamesBond; </code></pre> <p>Now you can have:</p> <pre><code>// AFTER, with enum and EnumMap Map<Month, Employee> employeeOfTheMonth = ... employeeOfTheMonth.put(Month.JANUARY, jamesBond); </code></pre> <hr> <h3>The case for <code>EnumSet</code></h3> <p>Power of two <code>int</code> constants are often used e.g. in C++ to denote bit sets. This relies on <a href="http://en.wikipedia.org/wiki/Bitwise_operations" rel="nofollow noreferrer">bitwise operations</a>. An example may look something like this:</p> <pre><code>public static final int BUTTON_A = 1; public static final int BUTTON_B = 2; public static final int BUTTON_X = 4; public static final int BUTTON_Y = 8; int buttonState = BUTTON_A | BUTTON_X; // A & X are pressed! if ((buttonState & BUTTON_B) != 0) { // B is pressed... ... } </code></pre> <p>With <code>enum</code> and <code>EnumSet</code>, this can look something like this:</p> <pre><code>enum Button { A, B, X, Y; } Set<Button> buttonState = EnumSet.of(Button.A, Button.X); // A & X are pressed! if (buttonState.contains(Button.B)) { // B is pressed... ... } </code></pre> <h3>References</h3> <ul> <li><a href="http://docs.oracle.com/javase/1.5.0/docs/guide/language/enums.html" rel="nofollow noreferrer">Java Language Guide/Enums</a> -- quite complete treatment with many examples</li> </ul> <h3>See also</h3> <ul> <li><em>Effective Java 2nd Edition</em> <ul> <li>Item 30: Use <code>enum</code> instead of <code>int</code> constants</li> <li>Item 31: Use instance fields instead of ordinals</li> <li>Item 32: Use <a href="http://java.sun.com/javase/6/docs/api/java/lang/EnumSet.html" rel="nofollow noreferrer"><code>EnumSet</code></a> instead of bit fields</li> <li>Item 33: Use <a href="http://java.sun.com/javase/6/docs/api/java/util/EnumMap.html" rel="nofollow noreferrer"><code>EnumMap</code></a> instead of ordinal indexing</li> </ul></li> </ul> <h3>Related questions</h3> <ul> <li><a href="https://stackoverflow.com/questions/2077634/enum-in-java-advantages">Enum in Java. Advantages?</a></li> <li><a href="https://stackoverflow.com/questions/2765687/is-there-a-a-c-like-way-to-get-item-number-from-enum-in-java">Is there a a C-like way to get item number from enum in java ?</a> -- yes, <code>ordinal()</code>, however...</li> <li><a href="https://stackoverflow.com/questions/1574580/is-it-bad-practice-to-use-an-enums-ordinal-value-to-index-an-array-in-java">Is it bad practice to use an Enum’s ordinal value to index an array in Java?</a> -- most probably, yes!</li> <li><a href="https://stackoverflow.com/questions/3360283/java-number-for-each-enum-item">[java] Number for each enum item?</a> -- a prime candidate for <code>EnumSet</code>!</li> </ul>
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