Are interfaces a valid substitute for utility classes in Java 8?

Question

For the past decade or so, I've been using the pattern below for my Java utility classes. The class contains only static methods and fields, is declared final so it can't be extended, and has a private constructor so it can't be instantiated.

public final class SomeUtilityClass {
    public static final String SOME_CONSTANT = "Some constant";

    private SomeUtilityClass() {}

    public static Object someUtilityMethod(Object someParameter) {
        /* ... */

        return null;
    }
}

Now, with the introduction of static methods in interfaces in Java 8, I lately find myself using a utility interface pattern:

public interface SomeUtilityInterface {
    String SOME_CONSTANT = "Some constant";

    static Object someUtilityMethod(Object someParameter) {
        /* ... */

        return null;
    }
}

This allows me to get rid of the constructor, and a lot of keywords (public, static, final) that are implicit in interfaces.

Are there any downsides to this approach? Are there any benefits to using a utility class over a utility interface?

Answer 1

Going based on the person who coined the Constant Interface pattern an anti-pattern, I would say although you don't intend the client(s) to implement the interface, it's still possible, possibly easier, and shouldn't be allowed:

APIs should be easy to use and hard to misuse. It should be easy to do simple things; possible to do complex things; and impossible, or at least difficult, to do wrong things.

Although as mentioned below, it really depends on the target audience

---

A lot of easy-to-use designs patterns get a lot of criticism (Context pattern, Singleton pattern, Constant Interface pattern). Heck, even design principles such as the law of demeter gets criticised for being too verbose.

I'd hate to say it, but these kinds of decisions are opinion based. Although the context pattern is seen as an anti-pattern, it's apparent in mainstream frameworks such as Spring and the Android SDK. It boils down to the environment, as well as target audience.

The main downside that I can find is listed as the third listing under "downsides" in the Constant Interface wiki:

If binary code compatibility is required in future releases, the constants interface must remain forever an interface (it cannot be converted into a class), even though it has not been used as an interface in the conventional sense.

If you ever figure "Hey, this actually isn't a contract and I want to enforce stronger design", you will not be able to change it. But as I've said, it's up to you; maybe you won't care to change it in the future.

On top of that, code clarity as mentioned by @TagirValeev. Interfaces have the intent of being implemented; if you don't want someone to implement the API you're supplying, don't make it implementable. But I believe this revolves around the "target audience" statement. Not gonna lie, I'm with you on the less-verbose foundation, but it depends on who my code is for; wouldn't wanna use a constant interface for code that may get reviewed.

Answer 2

You should use interface only if you expect that somebody would implement it. For example, java.util.stream.Stream interface has a bunch of static methods which could be located in some Streams or StreamUtils class prior to Java 8. However it's a valid interface which has non-static methods as well and can be implemented. The java.util.Comparable is another example: all static methods there just support the interface. You cannot forbid users from implementing your public interface, but for utility class you can forbid them to instantiate it. Thus for the code clarity I suggest not to use interfaces unless they are intended to be implemented.

A note regarding @saka1029 answer. While it's true that you cannot define helper private methods and constants in the same interface, it's not a problem to create a package-private class in the same package like MyInterfaceHelper which will have all the necessary implementation-related stuff. In general package-private classes are good to hide your implementation details from the outer world.

Answer 3

I think it would work. I think the variable SOME_CONSTANT defaults to being static final in your SomeUtilityInterface, even though you didn't explicitly say so. So, it would work as a Utility but wouldn't you have some mutability problems that you wouldn't have with a regular class with all member variables being required to be final? As long as thats not an issue with your particular implementation of the default methods, I can't think of a problem.

Answer 4

You should not use interface. Interfaces cannot have private constants and static initializers.

public class Utility {

    private Utility() {}

    public static final Map<String, Integer> MAP_CONSTANT;
    static {
        Map<String, Integer> map = new HashMap<>();
        map.put("zero", 0);
        map.put("one", 1);
        map.put("three", 3);
        MAP_CONSTANT = Collections.unmodifiableMap(map);
    }

    private static String PRIVATE_CONSTANT = "Hello, ";

    public static String hello(String name) {
        return PRIVATE_CONSTANT + name;
    }
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