型クラス？

Hoge.java

package acceptance;

import java.net.URL;
import java.util.ArrayList;
import java.util.List;

public class Hoge {

    static interface ToString<T> {
        String str(T t);
    }
    static class IntToString implements ToString<Integer> {
        @Override
        public String str(Integer integer) {
            return integer.toString();
        }
    }
    static class ListToStr<T> implements ToString<List<T>>{
        private final ToString<T> toStr;
        ListToStr(ToString<T> toStr){
            this.toStr = toStr;
        }
        @Override
        public String str(List<T> ts) {
            StringBuilder sb = new StringBuilder();
            for(T t : ts) {
                sb.append(toStr.str(t)).append(", ");
            }
            return sb.toString();
        }
    }
    static class UrlToString implements ToString<URL> {
        @Override
        public String str(URL url) {
            return url.toString();
        }
    }
    public static <T> String toStr(T t, ToString<T> toStr) {
        return toStr.str(t);
    }


    static interface Calc<T>{
        public T calc(T a, T b);
    }
    static class IntAdd implements Calc<Integer>{
        @Override
        public Integer calc(Integer a, Integer b) {
            return a + b;
        }
    }
    static class IntMul implements Calc<Integer>{
        @Override
        public Integer calc(Integer a, Integer b) {
            return a * b;
        }
    }
    static class StringAdd implements Calc<String>{
        @Override
        public String calc(String a, String b) {
            return a + b;
        }
    }
    public static <T> T calc(T a, T b, Calc<T> calc){
        return calc.calc(a, b);
    }

    public static void main(String[] args) throws Exception {

        toStr(10         , new IntToString());
        toStr(new URL(""), new UrlToString());
        toStr(new ArrayList<Integer>(), new ListToStr<>(new IntToString()));

        calc(1, 2, new IntAdd());
        calc(1, 2, new IntMul());
        calc("a", "b", new StringAdd());
    }
}