Тип возврата метода

Question

Чтобы скомпилировать мою программу, мне нужно знать тип возвращаемого значения метода find. Я не знаю, как получить тип Value с помощью следующего интерфейса IBinarySearch и класса Unit:

Итак, это интерфейс:

package u8a1;

import java.util.List;

/**
 * Binary search on things with comparable keys.
 * 
 * @param <Key>
 *            The type of the key of the things. Must implement the {@link Comparable} interface.
 * @param <Value>
 *            The type of the things themselves
 */
public interface IBinarySearch<Key extends Comparable<Key>, Value> {
    /**
     * Performs a binary search on the vector of {@link Unit}s.
     * 
     * @param needle
     *            We are looking for something whose key equals this needle with
     *            respect to the {@link Comparable} interface.
     * @param haystack
     *            The vector of {@link Unit}s where we are searching for the
     *            needle. This vector must be sorted ascending with respect to
     *            the keys and the {@link Comparable} interface.
     * @return A thing from the haystack whose key equals the needle or null
     *         if no such thing is in the haystack.
     */
    public Value find(List<Unit<Key, Value>> haystack, Key needle);
}

А это класс:

package u8a1;

/**
 * A key-value pair
 * 
 * @param <Key>
 *            the type of the key. Must be {@link Comparable}
 * @param <Value>
 *            the type of the value
 */
public class Unit<Key extends Comparable<Key>, Value> {
    public Key key;
    public Value value;

    public Unit(Key key, Value value) {
        this.key = key;
        this.value = value;
    }
}

Есть еще один интерфейс IMeasure:

package u8a1;

/**
 * Support statistics of binary search algorithms
 */
public interface IMeasure {
    /** 
     * Set the factor for the binary search.
     * 
     * A factor of two means that the search space is split into half-half in each recursion step.
     * A factor of three means that the search space is split into one third and two thirds in each recursion step.
     * In each case integer division is assumed, which means fractions are rounded down.
     * 
     * This method is called first after instantiation.
     * 
     * @param factor
     *            an integer value
     */
    public void setFactor(int factor);

    /**
     * Retrieve the statistics.
     * 
     * @return the number of recursive calls of the binary search algorithm
     *         performed since instantiation.
     */
    public int getNumberofCalls();
}

А вот класс BinarySearch, который реализует оба интерфейса:

package u8a1;

import java.util.List;

public class BinarySearch<Key extends Comparable<Key>, Value> implements IBinarySearch<Key, Value>, IMeasure {
    public Value find(List<Unit<Key, Value>> haystack, Key needle)
    {
        return haystack;
    }
    public void setFactor(int factor)
    {
        return;
    }
    public int getNumberofCalls()
    {
        return 0;
    }
}

Я хочу, чтобы программа была скомпилирована с правильным типом перед началом реализации методов.