clean way to implement similar enum types in Java

Question

I have multiple enum types, a typical implementation is as follows.

public enum SomeType {
    TYPE_A("aaa"),
    TYPE_B("bbb"),

    private final String type;

    private SomeType(String type) { this.type = type; }

    public boolean equals(String otherType) {
        return (otherType == null) ? false : type.equals(otherType.toUpperCase());
    }
}

Other than specific enum types, the constructor and methods (i.e., equals and a few more) are the same for these enums. I cant create a super "class" and extends from it. Is there a clean solution so that I only need 1 copy of these methods and make all enum types inherit them? Thanks.

VGR · Accepted Answer · 2015-10-28 19:44:43Z

2

Enum types are permitted to implement interfaces, like any other class. An example in Java SE is StandardCopyOption and LinkOption, which both implement CopyOption.

Although CopyOption doesn't have any methods, you certainly can define methods in such an inherited interface if you wish.

public interface TypedValue {
    String getType();

    default boolean equals(String otherType) {
        return otherType != null && getType().equals(otherType.toUpperCase());
    }
}

public enum SomeType
implements TypedValue {
    TYPE_A("aaa"),
    TYPE_B("bbb");

    private final String type;

    private SomeType(String type) { this.type = type; }

    @Override
    public String getType() {
        return type;
    }
}

answered Oct 28, 2015 at 19:44

VGR

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2 Comments

Holger Over a year ago

We should not forget to emphasize how dangerous method signatures, that can easily mixed up with the inherited Object.equals but have a different meaning, are…

VGR Over a year ago

@Holger Agreed. equals(TypeOtherThanObject) is a practice that the designers of Java SE learned early on was a bad idea.

Paul Boddington · Accepted Answer · 2015-10-28 19:53:48Z

0

With Java 8 you can do something like this:

interface TypeChecker {

    default boolean equals(String otherType) {
        return otherType != null && otherType.toUpperCase().equals(toString());
    }
}

Then you just need

enum SomeType implements TypeChecker {
    AAA, BBB
}

The drawback of this approach is that enum constants cannot have names that are different from their type strings. One workaround is to override toString for individual constants (or use a method other than toString as in VGR's solution). The problem with doing that is that it doesn't really cut down on the boilerplate.

edited Oct 28, 2015 at 19:53

answered Oct 28, 2015 at 19:48

Paul Boddington

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