Solutions

Problem 1. (10 points) A miscellany.

a. We'll accept any of the following.

• To reference the object the currently running method was called on.
• To call a constructor from another constructor of the same class (operating on the same object).
• To refer to an instance variable that is overshadowed by a local variable.

b.

  for (int i = 1; i < n; i = i + 2) {
    process(n);
  }

c.

• a: float or double.
• b: Java will only compile this code if b has type Object; however, we will also accept “X or any superclass of X.”
• c: boolean.
• d: Any reference (Object or any subclass of Object).
• e: Any superclass or subclass of Y.

d.

Problem 2. (8 points) Inheritance.

public interface X {
  public void whatever();
}

public abstract class Profile implements X {
  protected double weight;              // Maximize my privacy, please.

  public Profile(double weight) {
    this.weight = weight;
  }

  public abstract double date(double food);

  public int date(int food) {
    return (int) date((double) food);
  }

  public String introduce() {
    return "I'm neurotic and vindictive and I weigh " + weight;
  }

  public static void main(String[] args) {
    Profile p = new DatingProfile(260.0);
    ((DatingProfile) p).diet(p.weight, 25.0);
    System.out.println(p.introduce());
  }
}

public class DatingProfile extends Profile {
  public DatingProfile(double weight) {
    super(weight - 70.0);
  }

  public double date(double food) {
    weight = weight + (double) food;
    return weight;
  }

  public void diet(double weight, double loss) {
    weight = weight - loss;
  }

  public String introduce() {
    return "I'm feisty and spontaneous and I weigh " + (weight - 60);
  }

  public void whatever() { }
}

Problem 3. (7 points) Lengthening runs.

Here's an example of a recursive solution.

  public int lengthenRuns() {
    if (next == null) {
      next = new SListNode(item, null);
      return 1;
    }

    if (item == next.item) {
      return next.lengthenRuns();
    }

    next = new SListNode(item, next);
    return 1 + next.next.lengthenRuns();
  }

  public int lengthenRuns() {
    int count = 1;
    SListNode n = this;

    while (n.next != null) {
      if (n.item != n.next.item) {
        n.next = new SListNode(n.item, n.next);
        n = n.next;
        count++;
      }
      n = n.next;
    }

    n.next = new SListNode(n.item, null);
    return count;
  }