Effective Java! Emulate Extensible Enums With Interfaces.

We continue digging into different considerations while working with enums. Earlier in this series we considered the alternatives to the use of enums. Throughout the preceding chapters we have shown that enums are the preferred method. There is one place where the alternatives of using regular classes are superior to enums, that is with extending existing functionality. For good reason the enum type does not allow extension.

That being said, there are times when we could want this type of functionality, particularly when working with enums that represent different operations. In a previous chapter we saw this use case with our Calculator class and taking the Operation enum. Luckily there is a way to mimic the extension behavior via interfaces. Let's look at what this would look like with our Operation enum.

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;
  }
}

While the actual enum BasicOperation is not extensible, via the interface, we can mimic somewhat the behavior of extensibility. For example we can define a new enum that implements the Operation interface and can be used in the place of BasicOperation. That might look something like:

public enum ExtendedOperation implements Operation {
  EXPONENT("^") {
    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;
  }
}

So as you can see we again implement the Operation interface, this allows us to use this enum in the same place as our other enum that implemented Operation. Now let's look at a use of this.

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> opEnumType, double x, double y) {
  for (Operation operation : opEnumType.getEnumConstants()) {
    System.out.printf("%f %s %f = %f%n", x, operation, y, operation.apply(x, y));
  }
}

Now I'll be the first to admit that that is a fairly intense type declaration, basically it just means that we want an enum that implements the Operation interface. That being said you should be able to see how we could pass either type of enum to this method and have it work.

So what are the downsides here? Well as you could see I had to repeat myself some in the two enums. Now these two examples are pretty straightforward therefore there wasn't a ton of duplication but there was still some. If the shared code was larger you would likely want to look into creating a helper class.

To sum it all up, even though we cannot have our enums extend from each other we can mimic some of the behavior of inheritance by using interfaces.