Enums as Objects

Key points

• An enum is a class whose instances are fixed and named. Each constant is one object, created once.
• Constants can take constructor arguments, so an enum carries data as well as a name.
• A constant can also override a method in its own body, which is polymorphism with no subclass to write.
• That kills the switch: op.apply(a, b) instead of a switch that someone will forget to update.
• Enums are comparable, serializable and safe with ==. Use EnumMap and EnumSet over HashMap.
• Do not lean on ordinal(). Reordering constants would silently change its meaning; store a field instead.

Example

import java.util.EnumMap;
import java.util.Map;

public class Main {
    enum Op {
        PLUS("+")  { @Override int apply(int a, int b) { return a + b; } },
        MINUS("-") { @Override int apply(int a, int b) { return a - b; } },
        TIMES("*") { @Override int apply(int a, int b) { return a * b; } };

        private final String symbol;

        Op(String symbol) { this.symbol = symbol; }    // constructors are always private

        abstract int apply(int a, int b);              // each constant supplies a body

        String symbol() { return symbol; }
    }

    public static void main(String[] args) {
        for (Op op : Op.values()) {
            System.out.println("6 " + op.symbol() + " 3 = " + op.apply(6, 3));
        }

        Op picked = Op.valueOf("TIMES");
        System.out.println("picked " + picked + " at ordinal " + picked.ordinal());
        System.out.println("one instance per constant? " + (picked == Op.TIMES));

        Map<Op, Integer> uses = new EnumMap<>(Op.class);   // array-backed, very fast
        uses.put(Op.PLUS, 3);
        uses.put(Op.TIMES, 1);
        System.out.println("usage " + uses);
    }
}

Give the enum the behaviour and the switch statements disappear.