Enum

 abstract 

public enum Operation {
PLUS("+") {
double apply(double x, double y) { return x + y; }
},
MINUS("-") {
double apply(double x, double y) { return x - y; }
},
TIMES("*") {
double apply(double x, double y) { return x * y; }
},
DIVIDE("/") {
double apply(double x, double y) { return x / y; }
};
private final String symbol;
Operation(String symbol) { this.symbol = symbol; }
@Override public String toString() { return symbol; }
abstract double apply(double x, double y);
}

strategy with enum

// The strategy enum pattern
enum PayrollDay {
MONDAY(PayType.WEEKDAY), TUESDAY(PayType.WEEKDAY),
WEDNESDAY(PayType.WEEKDAY), THURSDAY(PayType.WEEKDAY),
FRIDAY(PayType.WEEKDAY),
SATURDAY(PayType.WEEKEND), SUNDAY(PayType.WEEKEND);
private final PayType payType;
PayrollDay(PayType payType) { this.payType = payType; }
double pay(double hoursWorked, double payRate) {
return payType.pay(hoursWorked, payRate);
}
// The strategy enum type
private enum PayType {
WEEKDAY {
double overtimePay(double hours, double payRate) {
return hours <= HOURS_PER_SHIFT ? 0 :
(hours - HOURS_PER_SHIFT) * payRate / 2;
}
},
WEEKEND {
double overtimePay(double hours, double payRate) {
return hours * payRate / 2;
}
};
private static final int HOURS_PER_SHIFT = 8;
abstract double overtimePay(double hrs, double payRate);
double pay(double hoursWorked, double payRate) {
double basePay = hoursWorked * payRate;
return basePay + overtimePay(hoursWorked, payRate);
}
}
}

 EnumMap

// Using a nested EnumMap to associate data with enum pairs
public enum Phase {
SOLID, LIQUID, GAS;

public enum Transition {
MELT(SOLID, LIQUID), FREEZE(LIQUID, SOLID),
BOIL(LIQUID, GAS), CONDENSE(GAS, LIQUID),
SUBLIME(SOLID, GAS), DEPOSIT(GAS, SOLID);
final Phase src;
final Phase dst;
Transition(Phase src, Phase dst) {
this.src = src;
this.dst = dst;
}
// Initialize the phase transition map
private static final Map<Phase, Map<Phase,Transition>> m = new EnumMap<Phase, Map<Phase,Transition>>(Phase.class);
static {
    for (Phase p : Phase.values())
        m.put(p,new EnumMap<Phase,Transition>(Phase.class));
    for (Transition trans : Transition.values())
        m.get(trans.src).put(trans.dst, trans);
}

public static Transition from(Phase src, Phase dst) {
    return m.get(src).get(dst);
}
}
}