Alternatives to static methods on interfaces for enforcing consistency

Question

In Java, I\'d like to be able to define marker interfaces, that forced implementations to provide static methods. For example, for simple text-serialization/deserialization

Answer 1

It seems like you need to separate the factory out from the created object.

public interface TextTransformer<T> {
    public T fromText(String text);
    public String toText(T source);
}

You can register TextTransformer classes however you like:

public class FooTextTransformer implements TextTransformer<Foo> {
    public Foo fromText(String text) { return ...; }
    public String toText(Foo source) { return ...; }
}

If you want FooTextTransformer to be a singleton, you can use a container like Spring to enforce that. Google has initiated a project to remove all manually enforced singletons from their codebase, but if you want to do an old-fashioned singleton, you could use a utility class:

public class TextTransformers {
    public static final FooTextTransformer FOO = new FooTextTransformer();
    ...
    public static final BarTextTransformer BAR = new BarTextTransformer();
}

In your client code:

Foo foo = TextTransformers.FOO.fromText(...);
...
foo.setSomething(...);
...
String text = TextTransformers.FOO.toText(foo);

This is just one approach off the top of my head.