CS2102   Exam #3 Solutions

Q1. [20 points] This question tests your knowledge of Java. Circle your answers True or False below.

• An object has two types - its actual type and its apparent type. False. An object is instantiated from a single type. When you create a variable and reference an object via one of its supertypes, then from the perspective of the variable, the apparent type of the object is the variable's type.
  
• The default visibility of a method restricts access to only the class within which it is defined. False. The default visibility is package-private; thus visibility is restricted to the package within which the method is defined.
  
• An array of int is an object instantiated on the heap. True. The array is instantiated on the heap. In fact, (new int[] {1,2} instanceof Object) is a true statement.
  
• A class can have any number of subclasses. True. The only exception is when you assign the modifier 'final' to a class, in which case it is not allowed to have any subclasses.
  
• (10 AM class) A class can have any number of methods. True. There is some physical limit, according to the way in which java byte code is compiled, but there is no theoretic limit.
  
• (1 PM class) An interface can have any number of methods. True. There is some physical limit, according to the way in which java byte code is compiled, but there is no theoretic limit.

Q2. [15 points] Describe at least three differences between an abstract class and a Java interface.

1. An abstract class can have a constructor, but an interface cannot
2. An abstract class can define instance variables, an interface cannot
3. An abstract class can extend another class; an interface can (optionally) only extend another interface
4. An abstract class can be defined with some methods with implementations; an interface is restricted to have no method implementations, just specification

Q3. [20 points] There are at least five logical errors in the following code. The intent of the programmer is to design a concept called Pen that can paint into a Graphics device using arbitrary pixel width & height. Each Pen instance uses a point to know its location within the Graphics device. The PenDriver class is intended to test the class, but when run it only prints out “pen[030] @ (10,30)”. Circle the errors and suggest fixes.

/** Represents a point in the graphical plane. */
public class Point {
  int x;
  int y;
  /** Default constructor. */	
  Point (int x, int y) {
    this.x = x;
    this.y = y;
  }

  public String toString () {
    return "(" + x + "," + y + ")";
  }
}		

/** Represents a pen that has width and height in pixels. */
public class Pen extends Point {
  static int width;     /** Each pen knows its width. */
  static int height;    /** Each pen knows its height. */
  Point loc;            /** Each pen knows its location. */
  /** Constructor is given initial (w,h) values. */
  public Pen (int w, int h) {
    super (w,h);
  }
  /* Return string for object, like "pen[3x7] @ point (10,30)" */
  public String toString () {
    return "pen[" + width + x + height + "] @ " + loc;
  }
}

public class PenDriver {
    public static void main(String[] args) {
        /** Construct Pen to be 3x7 @ point (10,30) */
        Pen p = new Pen (3,7);
        p.loc = new Point (10,30);
        System.out.println (p);
    }
}

1. [8 points] In Pen class, the use of 'static' with width and height is inappropriate. This would enforce all Pen objects to have the same width and height, and the intent of the programmer is clearly to have this information associated with each Pen object.
2. [4 points] If Pen extends Point, then it shouldn't have a loc field, since this information would be inherited as (x,y) from Point
3. [4 points] If Pen extends Point, then the constructor taking (w,h) should not simply call super; rather, it should set width and height as appropriate.
4. [4 points] In the Pen class, the toString() implementation has variable x referenced when it should be the string "x" or the character 'x'. The only reason this code works is because (miraculously) the Pen class inherits instance variable 'x' from Point.

Q4. [15 points] This question tests more advanced Java concepts

• For objects of a class to be used as a key for a Hashtable, what are the two methods that must be implemented by that class?
  
  hashCode and equals
  
• You are given the following proposal “Change the class java.lang.Object so that it implements the java.lang.Comparable interface. By doing so, java.lang.Object will provide an int compareTo(Object o) method for all classes.” Explain whether you support this proposal and give your reasons.
  
  The problem with this approach is that the compareTo() method is not automatically reasonable for each Class one would define. For example, how would one compare two (x,y) Points? Or two rectangles? If this method were implemented by the java.lang.Object class, there would be no information with which to make the decision. Note that java.lang.Object can implement 'equals (Object o)' simply because one can use the '==' operator in Java to determine if two variables refer to the same object instance -- a useful default behavior that can be provided. There is no similar ability for compareTo.

Q5. [10 points] We recommended that GUI applications be divided into three sets of classes. Describe their responsibilities.

• Model -- Stores information of use by the application.
• View -- Provides a Graphical User Interface for the user to interact with.
• Controller -- Contains the actual logic for how to react to events that occur within the View.

The responsibility of knowing when to refresh the view could be done in one of two ways (1) the Controller is responsible for updating the model and then alerting the view that it must refresh itself (at which point the View pulls the latest information from the Model); or (2) the Controller updates the model, who is then responsible for alerting potential views that it has changed. The view could be pushed the new changes or could, once again, pull the appropriate data from the Model.

Q6. Extra points if milk comes out of my nose because I can’t stop laughing

Q7 (10AM). [20 points] You have been asked to write a method for the BinaryTree class that returns the height of a binary tree. The height is determined by the longest path from the root node to any node in the tree. The following example trees are provided to help explain the concept.

		public class BinaryTree { /** Value as Integer. */ Integer value; /** Left & Right Children */ BinaryTree left; BinaryTree right; … }
#leaves = 1 height = 1	#leaves = 2 height  = 3

Q7 (1 PM). [20 points] You have been asked to write a method for the BinaryTree class that returns the number of leaves in the tree. A leaf node is one that has no left or right children are null. The following example trees are provided to help explain the concept.