使用equals方法检查两个整数子集是否相等

Question

I have this java method in class called IntArray. The class has methods to add integers to a set or remove integers from a set,check size of a set, and check if 2 sets are equal. the 2 sets are created using 2 different objects of type IntArray in main, lets say object A and B. equals method supposed to check if two sets of integers are equal. for example set A = {1,2,3} and B = {1,2,3,4}. The method still return true even though one set is a subset of the other set. What exactly I am doing wrong? Thanks.

//part of the code in main
IntArray A = new IntArray();
IntArray B = new IntArray();
if(A.equals(B))
System.out.println("A and B are equal");



 //equals method in IntArray class
 public boolean equals(Object b)
 {
  if (b instanceof IntArray)
    {
      IntArray A = (IntArray) b;
      for (int i = 0; i < data.length; i++)
      if (countOccurrences(data[i]) != A.countOccurrences(data[i]))
      return false;
      return true;
    }
 else return false;  
}

Answer 1

 if (countOccurrences(data[i]) != A.countOccurrences(data[i]))

It may be

 if (countOccurrences(data[i]) != A.countOccurrences(A.data[i]))

EDIT:

If by equals set you mean each element in the subset are in the same order (A = {1,2,3} and B = {1,2,3}):

Then, you want to check if two subsets of integers are equal using equals method:

if (data[i].equals(A.data[i]));

Make sure to only compare the two sets when both have the same length. Otherwise, return false.

If your definition of equals set means two sets with the same elements, without mattering their position :

You should check if you countOccurrences is doing something like this:

public int countOccurrences(int element) 
{
     int count = 0;
     for(int i = this.data.length - 1; i >= 0; i--) 
        if(this.data[i] == element) 
          count ++;
    return count;
}

In this latter case you should keep if (countOccurrences(data[i]) != A.countOccurrences(data[i])) .

Answer 2

Pre-check that the two lists are the same length. If they are not the same length, return false. If they are the same length, do the element-by-element comparison.

Answer 3

Here is a solution to the problem as stated. Note that it assumes that the IntArray object represents a true set, not a bag / multi-set. It does not assume that the values in the data arrays are ordered.

public boolean equals(Object otherObject) {
    if (otherObject == this) {  
        // This is an optimization for the case where an object
        // is compared with itself
        return true;
    } else if (otherObject instanceof IntArray) {
        IntArray other = (IntArray) other;
        if (this.data.length != other.data.length) {
            // If the sets have different nos of elements they cannot be equal
            return false;
        }
        for (int i = 0; i < this.data.length; i++) {
            boolean found = false;
            for (int j = 0; j < this.data.length; j++) {
                if (this.data[i] == other.data[j]) {
                    found = true;
                    break;
                }
            }
            if (!found) {
                return false;
            }
        }
        return true;
    } else {
        return false;
    }
}

If the data arrays are guaranteed to be ordered, then you can do a simple elememt-by-element comparison. Replace the for loops in the code above with this:

        for (int i = 0; i < this.data.length; i++) {
            if (this.data[i] != other.data[i]) {
                return false;
            }
        }

Finally, here is the solution for the multi-set case; ie where the elements of this.data are not necessarily unique in the array:

public boolean equals(Object otherObject) {
    if (otherObject == this) {  
        return true;
    } else if (otherObject instanceof IntArray) {
        IntArray other = (IntArray) other;
        if (this.data.length != other.data.length) {
            return false;
        }
        for (int i = 0; i < this.data.length; i++) {
            if (this.count(this.data[i]) != other.count(this.data[i]) {
                return false;
            }
        }
        return true;
    } else {
        return false;
    }
}

public int count(int x) {
    int count = 0;
    for (int y : this.data) {
        if (x == y) {
            count++;
        }
    }
    return count;
}

Note that it is

  if (this.count(this.data[i]) != other.count(this.data[i]) {

rather than

  if (this.count(this.data[i]) != other.count(other.data[i]) {

because we want to count the occurrences of the same value ... not the occurrences of the values at corresponding positions (which are probably different value!)