Solutions

Problem 1. (6 points) Quickies.

a. A final field is set by an initializer and cannot change. Since it usually has the same value for every object in the class, there's no point in having a separate copy of that value for every object.

b. Errors that might be produced by the code line Object o = s.subString(0);

• A run-time error (a NullPointerException to be precise) if s is null.
• A compile-time error if the static type of s does not support a subString method.
• A compile-time error if subString does not return an object.
• A compile-time error if s is a local variable that is not initialized before this line of code executes (and the Java compiler manages to figure this out, which depends on compiler flow analysis techniques that you guys don't need to know anything about).
• A compile-time error if s is a primitive type, or simply hasn't been declared at all.
• A compile-time error if o has already been declared as a local field in this method.

c. The switch statement is buggy because it is missing break statements at the end of each case (or at least the first two). For instance, it will always report that i/3 has a remainder of 2, regardless of the value of i.

d.

  public void moveToEnd() {
    // Moves the first node of the list to the end of the list.
    if (head != null) {
      tail.next = head;
      head.prev = tail;
      tail = head;
      head = head.next;
      head.prev = null;
      tail.next = null;
    }
  }
}

Problem 2. (5 points) Inheritance.

a. No changes are necessary. No code in the DateAndPlace class attempts to access any fields in the Date class.

b. The last line causes a compile-time error, because the isSamePlace method isn't part of the Date class, and there is no cast to ``reassure'' the compiler that d references a DateAndPlace object.

c. Adding this constructor will cause a compile-time error because there is already a constructor with the same signature.

Problem 3. (5 points) Fields, objects, and the stack.

Problem 4. (6 points) Counting runs.

  public int countRuns() {
    int count = 1;
    int i;

    if (myArray.length == 0) {
      return 0;
    }
    for (i = 1; i < myArray.length; i++) {
      if (myArray[i] != myArray[i - 1]) {
        count++;
      }
    }
    return count;
  }

  public abstract int countRuns();