EECS1030M Test 2

Wednesday January 28, 2015, 13:00-13:30
Lab 02
Version B

import java.util.Map;
import java.util.Set;
import java.util.TreeMap;

/**
 * This utility class contains some methods for sets of integers.
 *  
 * @author Franck van Breugel
 */
public class Test2B 
{
    private Test2B() {}

    /**
     * Tests whether the given set is empty.
     * 
     * @param set a set of integers.
     * @pre. set != null
     * @return true if the set is empty, false otherwise.
     */
    public static boolean isEmpty(Set set)
    {
	return set.isEmpty();
    }

    /**
     * If the given set contains 0, then it is removed from the
     * set, otherwise it is added.
     * 
     * @param set a set of integers.
     * @pre. set != null
     */
    public static void swapZero(Set set)
    {
	if (set.contains(0))
	{
	    set.remove(0);
	}
	else
	{
	    set.add(0);
	}
    }

    /**
     * Returns the sum of the integers in the given set.
     * 
     * @param set a set of integers.
     * @pre. set != null
     * @return the sum of the integers in the given set.
     */
    public static int sum(Set set)
    {
	int sum = 0;
	for (Integer element : set)
	{
	    sum += element;
	}
	return sum;
    }

    /**
     * Returns the difference between any pair of different
     * integers in the set that occurs most frequently.  
     * In case of ties, the smallest such difference is returned.
     * The difference is of integers i and j is | i - j |.
     * Consider for example the set

     * { 2, 6, 10, 14, 15, 20, 25, 27 }

     * The differences 4 and 5 both occur six times.  Since 
     * 4 is smaller than 5, 4 would be returned by this method.
     * 
     * @param set a set of integers.
     * @pre. set != null && set.size() > 1
     * @return the difference between any pair of different
     * integers in the set that occurs most frequently.
     */
    public static int maximalDifference(Set set)
    {
	Map differenceCount = new TreeMap();
	for (Integer first : set)
	{
	    for (Integer second : set)
	    {
		if (first != second)
		{
		    int difference = Math.abs(first - second);
		    int count;
		    if (!differenceCount.containsKey(difference))
		    {
			count = 0;
		    }
		    else
		    {
			count = differenceCount.get(difference);
		    }
		    differenceCount.put(difference, count + 1);
		}
	    }
	}

	int maximalCount = 0;
	int maximalDifference = 0;
	for (Integer element : differenceCount.keySet())
	{
	    if (maximalCount < differenceCount.get(element))
	    {
		maximalCount = differenceCount.get(element);
		maximalDifference = element;
	    }
	}

	return maximalDifference;
    }
}

Other questions

Question 1

Non-static attributes should be initialized in the constructors



Question 2

The class MyInteger has a single attribute named value of type int. Consider the following implementation of the equals method. 

public boolean equals(Object object)
{
   boolean equal;
   if (object != null && this.getClass() == object.getClass())
   {
      MyInteger other = (MyInteger) object;
      equal = this.value <= other.value;
   }
   else
   {
      equal = false;
   }
   return equal;
}
Which of the following properties does the above equals not satisfy?
For all x, y and z different from null,
A. x.equals(x) returns true.
B. x.equals(y) returns true if and only if y.equals(x) returns true.
C. if x.equals(y) returns true and y.equals(z) returns true then x.equals(z) returns true.
D. x.equals(null) returns false.


Question 3

Modify the above equals method so that it satisfies the above four properties. 

public boolean equals(Object object)
{
   boolean equal;
   if (object != null && this.getClass() == object.getClass())
   {
      MyInteger other = (MyInteger) object;
      equal = this.value == other.value;
   }
   else
   {
      equal = false;
   }
   return equal;
}