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I'm trying to implement the equals method in my class.. notice: the '_data' property is an double[][] array im trying to compare between the two objects.

anyway it compiles and everything but i always get a false answer and this can't be because both arrays are same :o

am i doing something wrong? is there any other easy way? (only with using equals from object class!!)

My code (JAVA):

public boolean equals(Object obj) {
    if (!(obj instanceof MyClass)) {
        return false;
    }

    MyClass myObj = (MyClass) obj;
    return this._data.equals(myObj._data);
}
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9
  • you have to paste a bigger code snippet. right now it's hard to judge what's causing problems. Commented Dec 7, 2011 at 23:00
  • 5
    Have you tried java.util.Arrays.deepEquals()? Commented Dec 7, 2011 at 23:01
  • For debugging purposes, would you please do something like MyClass mc1 = new MyClass(); MyClass mc2 = mc1; boolean toCheck = mc1.equals(mc2); Does toCheck == true? Commented Dec 7, 2011 at 23:01
  • Tomasz Nurkiewicz - I can't use the method u suggested, i only asked for equals method implementation.. Commented Dec 7, 2011 at 23:18
  • 1
    Im sure that if you use equals it is the same as using ==, that is, only comparing if they are the same instance (the same reference), not if they have the same contents. Test it: int[] a = { 1 }; int[] b = { 1 }; System.out.println(a.equals(b)); Commented Dec 8, 2011 at 8:47

5 Answers 5

Reset to default
8

Use 

Arrays.deepEquals(this._data, myObj._data)

to make the test on the arrays.

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3 Comments

Thanks but i asked only by using equals method.. (Not array.equals())
Seriously, when three people tell you to do it a certain way that's built right in to the standard library, and you're objecting to it based on "I can't use that", you need to justify why not.
If you don't want to use the standard library, do a loop iterating on both dimensions. But that's horrible.
5

Use Arrays.deepEquals(data, myObj._data)

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1 Comment

Thanks but i asked only by using equals method.. (Not array.equals())
2

You can take a look at this post which describes the deepEquals method: Java Arrays.equals() returns false for two dimensional arrays

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1 Comment

Thanks but i asked only by using equals method.. (Not array.equals())
0

I would override the equals method in your MyClass class.

@Override
public boolean equals(Object obj) {//if you insist in using an Object as argument instead of double[][]
   if (!(obj instanceof MyClass)) {
      return false;
   }

   MyClass myObj = (MyClass) obj;
   if(_data.length == myObj._data.length){
      for(int i=0; i<_data.length; i++){
         if(_data[i].length == myObj._data[i].length){
            for(int j=0; j<_data[i].length; j++){
               if(_data[i][j] != myObj._data[i][j]){
                  return false;
               }
            }
         }else{
            return false;
         }
      }
   }else{
      return false;
   }
   return true;
}

This code considers the case you will have a two dimensional array but not a square matrix. i.e. first row with three elements, second row with 27 elements, third row with N elements... For example, a test case:

    double[][] first= new double[5][3];
    double[][] second= new double[5][3];
    for(int i=0; i<first.length; i++){
        for(int j=0; j<first[i].length; j++){
            first[i][j] = 5;
        }
    }
    for(int i=0; i<second.length; i++){
        for(int j=0; j<second[i].length; j++){
            second[i][j] = 5;
        }
    }
    second[4][2] = 2;

    MyClass c1 = new MyClass(first);
    MyClass c2 = new MyClass(second);

    System.out.println("Equals: "+c1.equals(c2));
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Comments

0

Joshua Bloch tells you how to implement equals and hashCode (they must be done in tandem) in chapter 3 of "Effective Java". That should tell you all you need to know in order to override equals for your class.

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4 Comments

I believe the question is not about implementing equals, but how to compare arrays,, mostly why the equals method does not work with arrays!
I thought it was about how to override equals in the OP's class so it could compare using array data members. Is that not correct? "I'm trying to implement the equals method in my class..." - as for the why, it's obvious: equals only performs "shallow equals" when it's not overridden.
not so obvious for the OP, considering his comment "...im pretty positive that i can compare these arrays with 'equals'...", but I can be wrong at that. Actually I believe it is less than a "shallow equals" - he's just comparing references.
That's what I'm calling "shallow equals". Deep equals would ensure that the arrays are the same length and all the elements are identical. It'll be harder with doubles; the best you can do is say that they're all within an epsilon error of each other.

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