How to return an instance from Enum?

Question

Consider that I have Algorithm enum as

public enum Algorithm {
  SHA1("sha1"),
  HMAC("hmac"),;

  Algorithm(final String algorithm) {
    this.algorithm = algorithm;
  }
  private final String algorithm;

  public String getAlgorithm() {
    return algorithm;
  }
}

and I have different algorithms as

public class Sha1 {
   public static String hash(final String text, final byte[] sb) {...}
}

and

public class Hmac {
   public static String hash(final String text, final byte[] sb) {...}
}

I want to return their instances when someone calls for example

Algorithm.SHA1.getInstance()

Question

• How can I return the instance since my method is static? (It is static so that multiple threads can not play around with each other data)

Answer 1

You can't return an instance when your method is static, but you can make your enum implement an interface , and make an instance method that calls the static method perform the virtual dispatch for you:

public interface EncryptionAlgo {
    String hash(final String text, final byte[] sb);
}

public enum Algorithm implements EncryptionAlgo {
    SHA1("sha1") {
        public String hash(final String text, final byte[] sb) {
            return Sha1.hash(text, sb);
        }
    },
    HMAC("hmac") {
        public String hash(final String text, final byte[] sb) {
            return Hmac.hash(text, sb);
        }
    };

    Algorithm(final String algorithm) {
        this.algorithm = algorithm;
    }
    private final String algorithm;

    public String getAlgorithm() {
        return algorithm;
    }
}

Now you can call hash on the SHA1 or HMAC instance, like this:

Algorithm.HMAC.hash(someText, sb);

or pass around EncryptionAlgo instances, like this:

EncryptionAlgo algo = Algorithm.SHA1;