Why can't I define a static method in a Java interface?

Question

EDIT: As of Java 8, static methods are now allowed in interfaces.

Here\'s the example:

public interface IXMLizable         


        

Answer 1

First, all language decisions are decisions made by the language creators. There is nothing in the world of software engineering or language defining or compiler / interpreter writing which says that a static method cannot be part of an interface. I've created a couple of languages and written compilers for them -- it's all just sitting down and defining meaningful semantics. I'd argue that the semantics of a static method in an interface are remarkably clear -- even if the compiler has to defer resolution of the method to run-time.

Secondly, that we use static methods at all means there is a valid reason for having an interface pattern which includes static methods -- I can't speak for any of you, but I use static methods on a regular basis.

The most likely correct answer is that there was no perceived need, at the time the language was defined, for static methods in interfaces. Java has grown a lot over the years and this is an item that has apparently gained some interest. That it was looked at for Java 7 indicates that its risen to a level of interest that might result in a language change. I, for one, will be happy when I no longer have to instantiate an object just so I can call my non-static getter method to access a static variable in a subclass instance ...

Answer 2

Static methods aren't virtual like instance methods so I suppose the Java designers decided they didn't want them in interfaces.

But you can put classes containing static methods inside interfaces. You could try that!

public interface Test {
    static class Inner {
        public static Object get() {
            return 0;
        }
    }
}

Answer 3

Commenting EDIT: As of Java 8, static methods are now allowed in interfaces.

It is right, static methods since Java 8 are allowed in interfaces, but your example still won't work. You cannot just define a static method: you have to implement it or you will obtain a compilation error.

Answer 4

While I realize that Java 8 resolves this issue, I thought I'd chime in with a scenario I am currently working on (locked into using Java 7) where being able to specify static methods in an interface would be helpful.

I have several enum definitions where I've defined "id" and "displayName" fields along with helper methods evaluating the values for various reasons. Implementing an interface allows me to ensure that the getter methods are in place but not the static helper methods. Being an enum, there really isn't a clean way to offload the helper methods into an inherited abstract class or something of the like so the methods have to be defined in the enum itself. Also because it is an enum, you wouldn't ever be able to actually pass it as an instanced object and treat it as the interface type, but being able to require the existence of the static helper methods through an interface is what I like about it being supported in Java 8.

Here's code illustrating my point.

Interface definition:

public interface IGenericEnum <T extends Enum<T>> {
    String getId();
    String getDisplayName();
    //If I was using Java 8 static helper methods would go here
}

Example of one enum definition:

public enum ExecutionModeType implements IGenericEnum<ExecutionModeType> {
    STANDARD ("Standard", "Standard Mode"),
    DEBUG ("Debug", "Debug Mode");

    String id;
    String displayName;

    //Getter methods
    public String getId() {
        return id;
    }

    public String getDisplayName() {
        return displayName;
    }

    //Constructor
    private ExecutionModeType(String id, String displayName) {
        this.id = id;
        this.displayName = displayName;
    }

    //Helper methods - not enforced by Interface
    public static boolean isValidId(String id) {
        return GenericEnumUtility.isValidId(ExecutionModeType.class, id);
    }

    public static String printIdOptions(String delimiter){
        return GenericEnumUtility.printIdOptions(ExecutionModeType.class, delimiter);
    }

    public static String[] getIdArray(){
        return GenericEnumUtility.getIdArray(ExecutionModeType.class);
    }

    public static ExecutionModeType getById(String id) throws NoSuchObjectException {
        return GenericEnumUtility.getById(ExecutionModeType.class, id);
    }
}

Generic enum utility definition:

public class GenericEnumUtility {
    public static <T extends Enum<T> & IGenericEnum<T>> boolean isValidId(Class<T> enumType, String id) {       
        for(IGenericEnum<T> enumOption : enumType.getEnumConstants()) {
            if(enumOption.getId().equals(id)) {
                return true;
            }
        }

        return false;
    }

    public static <T extends Enum<T> & IGenericEnum<T>> String printIdOptions(Class<T> enumType, String delimiter){
        String ret = "";
        delimiter = delimiter == null ? " " : delimiter;

        int i = 0;
        for(IGenericEnum<T> enumOption : enumType.getEnumConstants()) {
            if(i == 0) {
                ret = enumOption.getId();
            } else {
                ret += delimiter + enumOption.getId();
            }           
            i++;
        }

        return ret;
    }

    public static <T extends Enum<T> & IGenericEnum<T>> String[] getIdArray(Class<T> enumType){
        List<String> idValues = new ArrayList<String>();

        for(IGenericEnum<T> enumOption : enumType.getEnumConstants()) {
            idValues.add(enumOption.getId());
        }

        return idValues.toArray(new String[idValues.size()]);
    }

    @SuppressWarnings("unchecked")
    public static <T extends Enum<T> & IGenericEnum<T>> T getById(Class<T> enumType, String id) throws NoSuchObjectException {
        id = id == null ? "" : id;
        for(IGenericEnum<T> enumOption : enumType.getEnumConstants()) {
            if(id.equals(enumOption.getId())) {
                return (T)enumOption;
            }
        }

        throw new NoSuchObjectException(String.format("ERROR: \"%s\" is not a valid ID. Valid IDs are: %s.", id, printIdOptions(enumType, " , ")));
    }
}

Answer 5

This was already asked and answered, here

To duplicate my answer:

There is never a point to declaring a static method in an interface. They cannot be executed by the normal call MyInterface.staticMethod(). If you call them by specifying the implementing class MyImplementor.staticMethod() then you must know the actual class, so it is irrelevant whether the interface contains it or not.

More importantly, static methods are never overridden, and if you try to do:

MyInterface var = new MyImplementingClass();
var.staticMethod();

the rules for static say that the method defined in the declared type of var must be executed. Since this is an interface, this is impossible.

The reason you can't execute "result=MyInterface.staticMethod()" is that it would have to execute the version of the method defined in MyInterface. But there can't be a version defined in MyInterface, because it's an interface. It doesn't have code by definition.

While you can say that this amounts to "because Java does it that way", in reality the decision is a logical consequence of other design decisions, also made for very good reason.

Answer 6

Why can't I define a static method in a Java interface?

All methods in an interface are explicitly abstract and hence you cannot define them as static because static methods cannot be abstract.