Can I use nested generics (aka higher kinded types) in Java?

Question

I was trying to do something like:

public class MyClass <A, B, C <A, B> > {
  ...
}

But Eclipse highlights "B," and says "unexpected , expected extends". What gives? Are nested generics not allowed?

Answer 1

It's because you haven't defined C as a type that is itself typed with 2 type parameters.
Try something like this:

public class MyClass <A, B, C extends Map<A, B>> {
    // This compiles
}

Answer 2

If your template parameters don't share share a class hierarchy, you can use an interface.

For example:

interface IConverter<TFrom, TTo>
{
    TTo convert(TFrom from);
}

class IntToStringConverter implements IConverter<Integer, String>
{
    public String convert(Integer from)
    {
        return "This is a string: " + from.toString();
    }
}

class ConverterUser<TConverter extends IConverter<TFrom, TTo>, TFrom, TTo>
{
    public ConverterUser()
    {
    }

    private List<TConverter> _converter2;

    private TConverter _converter;

    public void replaceConverter(TConverter converter)
    {
        _converter = converter;
    }

    public TTo convert(TFrom from)
    {
        return _converter.convert(from);
    }
}

class Test
{
    public static void main(String[] args)
    {
        ConverterUser<IntToStringConverter, Integer, String> converterUser =
            new ConverterUser<IntToStringConverter, Integer, String>();

        converterUser.replaceConverter(new IntToStringConverter());

        System.out.println(converterUser.convert(328));
    }
}

Answer 3

This is not possible in Java. See the Type Variables section of the language definition along with Generic Classes and Type Parameters . I recently saw (somewhere) a mention that Java is incapable of this but Scala can do it. This is confirmed by S4.4 of the Scala Language Specification .

This is also somewhat confirmed by the following code compiling successfully.

class MyClass [A, B, C [A, B]] {
}

Compiling in java yielded the follwing answers.

MyClass.java:1: > expected
class MyClass <A, B, C<A, B>> {
                      ^
MyClass.java:1: <identifier> expected
class MyClass <A, B, C<A, B>> {
                        ^
MyClass.java:1: ';' expected
class MyClass <A, B, C<A, B>> {
                           ^
MyClass.java:2: reached end of file while parsing
}
 ^
4 errors

I would guess that there is an easier solution to your problem however, as this is somewhat unusual.

Answer 4

You don't have to declare nested types like that. Simply

class MyClass<A, B, C> {}

And when you create a MyClass, you could do something like

MyClass<List<String>, Set<Date>, Map<Integer, Long>> instance;

Answer 5

I am guessing you want MyClass to be a generic class with type parameters A, B, and C. Furthermore you want C to be a generic class with type parameters A and B.

So that I could write

1. MyClass < String , Date , Map < String , Date > >
2. MyClass < String , Date , Hashtable < String , Date > > but not
3. MyClass < String , Date , ElementVsitor < Date , String > >

Then I don't think you can do that.

Answer 6

This is effectively asking for higher kinded types in Java. Java does not support this directly, but it can be simulated in a somewhat roundabout way like so

interface H<K, T> { }

Here H encodes a higher kinded type that takes a type parameter K which itself takes parameter T .

You can use this to eg implement a generic functor. Note how fmap is effectively a function from H<K<T>> to H<K<R>> although we cannot directly declare it that way.

public interface Functor<K> {
    <T, R> Function<H<K, T>, H<K, R>> lift(Function<T, R> f);

    default <T, R> H<K, R> fmap(Function<T, R> f, H<K, T> h) {
        return lift(f).apply(h);
    }
}

See also my Github repository for full working examples. Further more have a look at this question , which takes the concept much further still.