Quiz 02 sample questions and answers

In the first question, you will read some code and write a concise, high-level, English description of the code’s action. For example:

public static List<Integer> mystery(List<Integer> l, int i) {
  List<Integer> r = new ArrayList<>();
  for (int j = i; j < l.size(); j++) {
    r.add(l.get(j));
  }
  return r;
}

This method returns a new list composed of the last but i elements of l in the order they appear in l.

In the second question, you’ll identify a logic error in a short snippet of code. That is, you will find a conceptual error, not an error that the compiler would catch. Here is an example:

/** 
  * Swaps the `i`th and `j`th elements of the List `l`. 
  * Assume 0 <= i <= j <= l.size().   
  */
public static void swap(List<Integer> l, int i, int j) {
  l.set(i, l.get(j));
  l.set(j, l.get(i));
}

This method overwrite the element at i with the element at j, but does not overwrite the element at j with the element at i. (A temp variable is needed.)

In the final question, you will write a short class or method according to a textual description. For example, a question might state:

Write a method static List<Integer> concatenate(List<Integer> l1, List<Integer> l2). concatenate should return, as a new list, all the elements of l1 followed by all the elements of l2, in the order they appeared. Your code must not modify l1 or l2.

For example, with an input of l1 = [1, 2, 5], l2 = [2, 4, 1], it should return the list [1, 2, 5, 2, 4, 1].

Assume List and ArrayList are correctly imported.

One acceptable answer:

public static List<Integer> concatenate(List<Integer> l1, List<Integer> l2) {
  List<Integer> r = new ArrayList<>();
  for (Integer e: l1) {
    r.add(e);
  }
  for (Integer e: l2) {
    r.add(e);
  }
  return r;
}

Another possible answer, that uses the copy constructor of ArrayList and the addAll method:

public static List<Integer> concatenate(List<Integer> l1, List<Integer> l2) {
  List<Integer> r = new ArrayList<>(l1);
  r.addAll(l2);
  return r;
}