Can I use nested generics 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));
    }
}