Java enums find enumeration

Question

I just read the enumeration tutorial and asked one question. I studied an example:

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);

    private final double mass;   // in kilograms
    private final double radius; // in meters
    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();
    }
}

and the question is: how can I find the type of enumeration, for example, MERCURY, if I know the mass and radius? Thank.

Answer 1

O (n) - repeat all the enumeration values ​​and compare:

for (Planet planet : Planet.values()) {
   if (..) {..}
}

The best place for this is the method staticin the enumeration class itself.

Answer 2

Give Planetenum a static method searchthat takes these two facts and looks for it. For something of this size, a simple linear probe strategy should be fast enough.

Answer 3

enum values() , enum , . , , Planet, .

for (Planet p : Planet.values()) {
    if (p.mass() == searchMass && p.radius == searchRadius) {
      //do something with p
    }
}

An enum , .

Answer 4

. , enum ( Java 1.5 EnumSyntax ), - . , Map , -. >, , . , .

In most cases, this is redundant, as linear searching is more than sufficient for performance. But if you perform these searches several times, then this solution provides a one-time hit during initialization.

Code example:

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<Double, Map<Double, Planet>>();
       for (Planet p : Planet.values()) {
          if (!map.containsKey(p.getMass())) {
             p.put(p.getMass(), new HashMap<Double, Planet>());
          }
          p.get(p.getMass()).put(p.getRadius(), p));
       }
    }

    private final double mass;   // in kilograms
    private final double radius; // in meters

    private static final Map<Double, Map<Double, Planet>> 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;
    }
}

Answer 5

You can get an array from all Planetwith Planet.values()and iterate over them, looking for the one with the indicated mass and radius.