PROPOSAL: Static Methods in Interfaces (original) (raw)

Stephen Colebourne scolebourne at joda.org
Wed Mar 4 13:32:51 PST 2009

Reinier Zwitserloot wrote:

MAJOR DISADVANTAGE:

No opportunity to use the static keyword in interfaces for some sort of factory interface concept (an interface for constructors and static methods).

I think this is a key one, as it limits future Java changes. I'd like to see Java support 'static implements':

public class Foo implements Bar, static Baz { ... }

where any methods defined by Baz have to be implemented by Foo as static methods.

ALTERNATIVES:

The usual solution to this problem right now is to offer a separate utility class (a class that is not instantiable and contains only static methods) that contain the utility methods, along with a reference in the javadoc of the interface to this utility class. For example, java.util.Collections is the utility class that goes with Map, List, Set and other Java Collections API interfaces.

I don't consider the utils class to be a particularly bad alternative.

java.util.List/Map/Set: All methods in java.util.Collections should also be made available on the appropriate java collections API interface. java.io.Closeable: should contain a utility method 'closeAndIgnoreException' (arguably better suited on InputStream instead). java.util.List/Set: Should contain an 'of' method that makes unmodifiable lists and sets via varargs. java.io.FileFilter: Should contain an 'ofExtension(String)' method that creates a FileFilter for the provided extension.

Making these changes would appear to be backwards incompatible if the implementing class already defines the method added to the interface.

Stephen

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