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    <p>The linear search patterns discussed are ideal for the posed problem. However, in the situation were the enum class grows (or if you're using pre-Java 1.5 type-safe enums using EnumSyntax to create run-time configured enums) you might want something a little faster. In that case, you could define a static initialization block that populates a Map with the values so you can look up by key-value pairs. In this case you would define Map> which is keyed by the mass then the radius. You would then provide a static method that returns the lookup from the map.</p> <p>This is overkill is most circumstances since the linear search is more than sufficient for performance. But if you're perform these look-ups multiple times, then this solution provides a one-time hit at initialization.</p> <p>Example Code:</p> <pre><code>public enum Planet { MERCURY (3.303e+23, 2.4397e6), VENUS (4.869e+24, 6.0518e6), EARTH (5.976e+24, 6.37814e6), MARS (6.421e+23, 3.3972e6), JUPITER (1.9e+27, 7.1492e7), SATURN (5.688e+26, 6.0268e7), URANUS (8.686e+25, 2.5559e7), NEPTUNE (1.024e+26, 2.4746e7), PLUTO (1.27e+22, 1.137e6); static { map = new HashMap&lt;Double, Map&lt;Double, Planet&gt;&gt;(); for (Planet p : Planet.values()) { if (!map.containsKey(p.getMass())) { p.put(p.getMass(), new HashMap&lt;Double, Planet&gt;()); } p.get(p.getMass()).put(p.getRadius(), p)); } } private final double mass; // in kilograms private final double radius; // in meters private static final Map&lt;Double, Map&lt;Double, Planet&gt;&gt; map; Planet(double mass, double radius) { this.mass = mass; this.radius = radius; } public double mass() { return mass; } public double radius() { return radius; } // universal gravitational constant (m3 kg-1 s-2) public static final double G = 6.67300E-11; public double surfaceGravity() { return G * mass / (radius * radius); } public double surfaceWeight(double otherMass) { return otherMass * surfaceGravity(); } public static Planet getPlanet(double mass, double radius) { if (map.contains(mass)) { return map.get(mass).get(radius); } return null; } </code></pre> <p>}</p>
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    1. USJohn Engelman
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