- You cannot write an extensible enum type
- You can emulate it by writing an interface to go with a basic enum type that implements the interface
- This allows clients to write their own enums that implement the interface
// Emulated extensible enum using an interface
public interface Operation {
double apply(double x, double y);
}
public enum BasicOperation implements Operation {
PLUS("+") {
public double apply(double x, double y) { return x + y; }
},
MINUS("-") {
public double apply(double x, double y) { return x - y; }
},
TIMES("*") {
public double apply(double x, double y) { return x * y; }
},
DIVIDE("/") {
public double apply(double x, double y) { return x / y; }
};
private final String symbol;
BasicOperation(String symbol) {
this.symbol = symbol;
}
@Override public String toString() {
return symbol;
}
}
// Emulated extension enum
public enum ExtendedOperation implements Operation {
EXP("^") {
public double apply(double x, double y) { return Math.pow(x, y); }
},
REMAINDER("%") {
public double apply(double x, double y) { return x % y; }
};
private final String symbol;
ExtendedOperation(String symbol) {
this.symbol = symbol;
}
@Override public String toString() {
return symbol;
}
}
Use case
public static void main(String[] args) {
double x = Double.parseDouble(args[0]);
double y = Double.parseDouble(args[1]);
test(ExtendedOperation.class, x, y);
}
private static <T extends Enum<T> & Operation> void test(Class<T> opSet, double x, double y) {
for (Operation op : opSet.getEnumConstants())
System.out.printf("%f %s %f = %f%n", x, op, y, op.apply(x, y));
}
public static void main(String[] args) {
double x = Double.parseDouble(args[0]);
double y = Double.parseDouble(args[1]);
test(Arrays.asList(ExtendedOperation.values()), x, y);
}
private static void test(Collection<? extends Operation> opSet, double x, double y) {
for (Operation op : opSet)
System.out.printf("%f %s %f = %f%n", x, op, y, op.apply(x, y));
}