Implementing an equals() method to compare contents of two 'bag' objects

Question

I am working on a school assignment. The objective is to practice GUI's, clone() methods, and using/ modifying existing code. I am trying to write an equals method in the way the instructor desires-- by using a clone of the object, removing items from the bag (returns boolean based on success or failure to remove).

The bag is represented in an array, and should return true in cases such as {1,2,3} and {3,2,1}, ie order does not matter, only the number of each number present in the arrays.

Here is the issue

It works in most cases, however there is a bug in cases where the bags contain numbers as such: {1,1,2} and {1,2,2} and other similar iterations. It is returning true instead of false.

I believe it has something to do with the remove() method we are supposed to use. If i understand it correctly, it is supposed to put the value at the 'end' of the array and decrease the manyItems counter (this is a variable for number of items in the array, because array.length is by default in the constructor 10.)

The code is largely written by another person. We had to import the existing files and write new methods to complete the task we were given. I have all the GUI part done so i will not include that class, only the used methods in the IntArrayBag class.

A second pair of eyes would be helpful. Thanks.

public class IntArrayBag implements Cloneable
{
// Invariant of the IntArrayBag class:
//   1. The number of elements in the bag is in the instance variable 
//      manyItems, which is no more than data.length.
//   2. For an empty bag, we do not care what is stored in any of data;
//      for a non-empty bag, the elements in the bag are stored in data[0]
//      through data[manyItems-1], and we don�t care what�s in the
//      rest of data.
private int[ ] data;
private int manyItems;

public IntArrayBag( )
{
  final int INITIAL_CAPACITY = 10;
  manyItems = 0;
  data = new int[INITIAL_CAPACITY];
}

public IntArrayBag clone( )
{  // Clone an IntArrayBag object.
  IntArrayBag answer;

  try
  {
     answer = (IntArrayBag) super.clone( );
  }
  catch (CloneNotSupportedException e)
  {  // This exception should not occur. But if it does, it would probably
     // indicate a programming error that made super.clone unavailable.
     // The most common error would be forgetting the "Implements Cloneable"
     // clause at the start of this class.
     throw new RuntimeException
     ("This class does not implement Cloneable");
  }

  answer.data = data.clone( );

  return answer;
}

public int size( )
{
  return manyItems;
}

public boolean remove(int target)
{
  int index; // The location of target in the data array.

  // First, set index to the location of target in the data array,
  // which could be as small as 0 or as large as manyItems-1; If target
  // is not in the array, then index will be set equal to manyItems;
  for (index = 0; (index < manyItems) && (target != data[index]); index++)
     // No work is needed in the body of this for-loop.
     ;

  if (index == manyItems)
     // The target was not found, so nothing is removed.
     return false;
  else
  {  // The target was found at data[index].
     // So reduce manyItems by 1 and copy the last element onto data[index].
     manyItems--;
     data[index] = data[manyItems];
     return true;
  }
}
//I added extra variables that are not needed to try to increase readability,
//as well as when i was trying to debug the code originally
public boolean equals(Object obj){

   if (obj instanceof IntArrayBag){

       IntArrayBag canidate = (IntArrayBag) obj; // i know this can be changed, this was required
       IntArrayBag canidateTest = (IntArrayBag) canidate.clone(); //this was created
       //as a clone because it was otherwise referring to the same memory address
       //this caused items to be removed from bags when testing for equality
       IntArrayBag test = (IntArrayBag) this.clone();

       //fast check to see if the two objects have the same number of items, 
       //if they dont will return false and skip the item by item checking
       if (test.size() != canidateTest.size())
           return false;

       //the loop will go through every element in the test bag it will 
       //then remove the value that is present at the first index of the test bag
       for (int i = 0; (i < (test.size()) || i < (canidateTest.size())); i++){
           int check = test.data[i];
           //remove() returns a boolean so if the value is not present in each bag
           //then the conditional will be met and the method will return false
           boolean test1 = test.remove(check);
           boolean test2 = canidateTest.remove(check);
           if (test1 != test2)
               return false;

       }//end for loop

       // if the loop goes through every element
       //and finds every value was true it will return true
       return true;  
   }//end if 
   else
       return false;
}//end equals
}

Answer 1

Is it explicitly stated to use clone? You can achieve it easily by overriding the hashCode() for this Object.

You can override the hashCode() for this object as follows:

@Override
public int hashCode() {
    final int prime = 5;
    int result = 1;
    /* Sort Array */
    Arrays.sort(this.data);
    /* Calculate Hash */
    for(int d : this.data) {
        result = prime * result + d;
    }
    /* Return Result */
    return result;
}

@Override
public boolean equals(Object obj) {
    if (this == obj) return true;
    if (obj == null || this.getClass() != obj.getClass()){
            return false;
    }
    return false;
}

If you want to continue using your implementation for equals to compare test and CandidateTest then also you can compute unique hashes and make decision based on the results.

Here is the code snippet:

/* Assuming that you have put size comparison logic on top 
   and the two objects are of same size */

final int prime = 31;
int testResult = 1;
int candidateTestResult = 1;
for(int i = 0; i < test.size(); i++) {
    testResult = prime * testResult + test.data[i];
    candidateTestResult = prime * candidateTestResult + candidateTest.data[i];
}
/* Return Result */
return testResult == candidateTestResult;

Answer 2

I cannot see the big picture, as I havent coded GUIs in Java before, however, as far as comparing 2 int[] arrays, I would sort the arrays before the comparison. This will allow you to eliminate problem cases like the one you stated ( if sorting is possible), then apply something like:

while(array_1[index]==array_2[index] && index<array_1.length)
{index++;}

and find where did the loop break by checking the final value of index

Answer 3

I believe the problem is in this line:

for (int i = 0; (i < (test.size()) || i < (canidateTest.size())); i++){

The problem here is that test and canidateTest are the clones that you made, and you are removing elements from those bags. And any time you remove an element from the bag, the size will decrease (because you decrease manyItems, and size() returns manyItems). This means you're only going to go through half the array. Suppose the original size is 4. Then, the first time through the loop, i==0 and test.size()==4; the second time, i==0 and test.size()==3; the third time, i==2 and test.size()==2, and you exit the loop. So you don't look at all 4 elements--you only look at 2.

You'll need to decide: do you want to go through the elements of the original array, or the elements of the clone? If you go through the elements of the clone, you actually never need to increment i. You can always look at test.data[0], since once you look at it, you remove it, so you know test.data[0] will be replaced with something else. In fact, you don't need i at all. Just loop until the bag size is 0, or until you determine that the bags aren't equal. On the other hand, if you go through the elements of this.data (i.e. look at this.data[i] or just data[i]), then make sure i goes all the way up to this.size().

(One more small point: the correct spelling is "candidate".)

Answer 4

Maybe you should try SET interface view this in detail ：http://www.tutorialspoint.com/java/java_set_interface.htm A set object cannot contains duplicate elements, so it's suitable for your assignment than build your own class.

For example：[1,1,2] and [1,2,2] you can use this to test whether they are equal

arr1 = {1,1,2}
arr2 = {1,2,2}
Set<Integer> set = new HashSet<Integer>();
for(int i : arr1){//build set of arr1
    if(set.contains(i)==false){
        set.add(i)
    }
}
for(int i:arr2){
    if(set.contains(i)==false){
        System.out.println('not equal');
        break;
    }
}

Hope this is helpful.