Quiz 04 sample questions and answers

The quiz will have three questions, and cover material through the end of the Set topics we’ve gotten to so far.

In the first question, you will write a comparator. For example, a question might state:

Suppose you want to sort a list of integers into descending order. Write a class DescendingIntegerComparator that implements Comparator. It should allow you to use List.sort to perform this task. For example, if the unsorted list is [2, 1, 4, 3], then the list should be [4, 3, 2, 1] after sorting using this comparator.

public class DescendingIntegerComparator implements Comparator<Integer> {
    public int compare(Integer o1, Integer o2) {
        return -Integer.compare(o1, o2);
    }
}

In the second and third questions, you will write a short class or method according to a textual description. (Yes, these are the same as last week!) For example, a question might state:

Write a method public static <E> List<E> notIn(List<E> list, Set<E> set). notIn should return a list that contains the elements present in list but not present in set. The original list and set must not be modified. For full credit, your method must correctly use generics, and must not contain unnecessary method calls or loops, even if they do not otherwise impact correctness. You may assume List, Set, ArrayList, and HashSet are correctly imported from java.util. You may not import other classes.

public static <E> List<E> notIn(List<E> list, Set<E> set) {
    List<E> l = new ArrayList<>();
    for (E e: list) {
        if (!set.contains(e)) {
            l.add(e);
        }
    }
    return l;
}

or, more succinctly:

public static <E> List<E> notIn(List<E> list, Set<E> set) {
    List<E> l = new ArrayList<>(list);
    l.removeAll(set);
    return l;
}

Another question might state:

Write a generic class MyList<E> that extends an existing implementation of List. MyList should include a method public List<E> reversed(). reversed returns a new list consisting of the elements of the current list in reverse order. reversed must not modify the list. For full credit, your method must correctly use generic types, and must not contain unnecessary method calls or loops, even if they do not otherwise impact correctness. You may assume List and ArrayList are correctly imported from java.util. You may not import other classes.

public List<E> reversed() {
    List<E> l = new ArrayList<>();
    for (int i = this.size() - 1; i >= 0; i--) {
        l.add(get(i));
    }
    return l;
}