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copied!<p>You can use a <code>static Map<Integer,TestEnum></code> with a <code>static</code> initializer that populates it with the <code>TestEnum</code> values keyed by their <code>number</code> fields.</p> <p>Note that <code>findByKey</code> has been made <code>static</code>, and <code>number</code> has also been made <code>final</code>.</p> <pre><code>import java.util.*; public enum TestEnum { ONE(1), TWO(2), SIXTY_NINE(69); private final int number; TestEnum(int number) { this.number = number; } private static final Map<Integer,TestEnum> map; static { map = new HashMap<Integer,TestEnum>(); for (TestEnum v : TestEnum.values()) { map.put(v.number, v); } } public static TestEnum findByKey(int i) { return map.get(i); } public static void main(String[] args) { System.out.println(TestEnum.findByKey(69)); // prints "SIXTY_NINE" System.out.println( TestEnum.values() == TestEnum.values() ); // prints "false" } } </code></pre> <p>You can now expect <code>findByKey</code> to be a <code>O(1)</code> operation.</p> <h3>References</h3> <ul> <li><a href="http://java.sun.com/docs/books/jls/third_edition/html/classes.html#8.7" rel="noreferrer">JLS 8.7 Static initializers</a></li> <li><a href="http://java.sun.com/docs/books/jls/third_edition/html/classes.html#8.9" rel="noreferrer">JLS 8.9 Enums</a></li> </ul> <h3>Related questions</h3> <ul> <li><a href="https://stackoverflow.com/questions/335311/static-initalizer-in-java">Static initalizer in Java</a></li> <li><a href="https://stackoverflow.com/questions/507602/how-to-initialise-a-static-map-in-java">How to Initialise a static Map in Java</a></li> </ul> <hr> <h3>Note on <code>values()</code></h3> <p>The second <code>println</code> statement in the <code>main</code> method is revealing: <code>values()</code> returns a newly allocated array with every invokation! The original <code>O(N)</code> solution could do a little better by only calling <code>values()</code> once and caching the array, but that solution would still be <code>O(N)</code> on average.</p>

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