Really simple functional extensions for Java

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

public class Util {

    public interface Predicate<T> {

        boolean check(T instance);

    }

   

    public interface Attr<T, A> {

        A get(T instance);

    }

    public static <T> List<T> truncate(List<T> list, int maxLength) {

        if (list.size() > maxLength) {

            return new ArrayList<T>(list.subList(0, maxLength));

        }

        return list;

    }

    public static <T> List<T> filter(List<T> list, Predicate<T> checker) {

        List<T> filtered = new ArrayList<T>();

        for (T item: list) {

            if (checker.check(item)) {

                filtered.add(item);

            }

        }

        return filtered;

    }

   

    public static <T> int count(List<T> list, Predicate<T> checker) {

        int count = 0;

        for (T item: list) {

            if (checker.check(item)) {

                count++;

            }

        }

        return count;

    }

   

    public static <T, A> Map<A, List<T>> rollup(List<T> list, Attr<T, A> attr) {

        Map<A, List<T>> result = new HashMap<A, List<T>>();

        for (T item: list) {

            A key = attr.get(item);

            List<T> itemList = result.get(key);

            if (itemList == null) {

                itemList = new ArrayList<T>();

                itemList.add(item);

                result.put(key, itemList);

            } else {

                itemList.add(item);

            }

        }

        return result;

    }

}