How to make a Java Generic method static?

问题:

The following is a snippet on how to make a java generic class to append a single item to an array. How can I make appendToArray a static method. Adding static to the method signature results in compile errors.

public class ArrayUtils<E> {

        public E[] appendToArray(E[] array, E item) {
            E[] result = (E[])new Object[array.length+1];
            result[array.length] = item;
            return result;
        }
}

回答1:

the only thing you can do is to change your signature to

public static <E> E[] appendToArray(E[] array, E item)

Important details:

Generic expressions preceding the return value always introduce (declare) a new generic type variable.

Additionally, type variables between types (ArrayUtils) and static methods (appendToArray) never interfere with each other.

So, what does this mean: In my answer <E> would hide the E from ArrayUtils<E> if the method wouldn't be static. AND <E> has nothing to do with the E from ArrayUtils<E>.

To reflect this fact better, a more correct answer would be:

public static <I> I[] appendToArray(I[] array, I item)

回答2:

public static <E> E[] appendToArray(E[] array, E item) { ...

Note the <E>.

Static generic methods need their own generic declaration (public static <E>) separate from the class's generic declaration (public class ArrayUtils<E>).

If the compiler complains about a type ambiguity in invoking a static generic method (again not likely in your case, but, generally speaking, just in case), here's how to explicitly invoke a static generic method using a specific type (_class_.<_generictypeparams_>_methodname_):

String[] newStrings = ArrayUtils.<String>appendToArray(strings, "another string");

This would only happen if the compiler can't determine the generic type because, e.g. the generic type isn't related to the method arguments.

回答3:

You need to move type parameter to the method level to indicate that you have a generic method rather than generic class:

public class ArrayUtils {
    public static <T> E[] appendToArray(E[] array, E item) {
        E[] result = (E[])new Object[array.length+1];
        result[array.length] = item;
        return result;
    }
}