Can I compare the equality of an array with an ArrayList of arrays?

Question

I have arrays of integers:

int[] arrA = {1,2,3,4,5,6,7,8,9};
int[] arrB1= {7,3,4,1,5,8,9,2,6};
int[] arrB2= {9,5,1,4,7,6,8,2,3};
int[] arrB3= {4,6,1,3,8,9,5,7,2};
int[] arrB4= {2,3,1,5,8,9,4,6,7};
int[] arrB5= {1,2,3,4,5,6,7,8,9};

and all the arrBs are added to the list, as follows:

List<int[]> list= new ArrayList<int[]>();

list.add(arrB1);
list.add(arrB2);
list.add(arrB3);
list.add(arrB4);
list.add(arrB5);

and I want to compare the equality of the array arrA with the list of arrays and I did it using this code:

boolean isEqual=false;
for(int index=0; index<list.size(); index++){
    if(list.get(index).equals(arrA)){
          isEqual=true;
    }
    System.out.println(isEqual);
}

I know that arrB5 is equal to arrA but why is it that I always get false? Am I doing the comparison right?

Jon Skeet · Accepted Answer · 2013-08-11 07:55:50Z

7

You're calling equals on arrays - which will just compare by reference, as arrays don't override equals. You want Arrays.equals(int[], int[]) to compare the values within the arrays.

(You don't need deepEquals in this case as you're dealing with int[] - and indeed calling deepEquals wouldn't compile, as an int[] isn't an Object[].)

edited Aug 11, 2013 at 7:55

answered Aug 11, 2013 at 7:47

Jon Skeet

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3 Comments

ApproachingDarknessFish Over a year ago

At first this was confusing, as I thought the whole point of equals was to test for value equality, but since arrays aren't classes in the same way most other objects are they can't really override Object.equals (which checks for reference equality), so in the end it made sense. Very counter-intuitive, however, for equals to work like that.

Rohit Jain Over a year ago

@ValekHalfHeart. Array is indeed a class. Just that it doesn't override equals method.

Jon Skeet Over a year ago

@ValekHalfHeart: Well arrays could have been implemented to override equals (and hashCode and toString) - it's just not the way the language was designed. Possibly a mistake; possibly not... but too late to change now.

Khinsu · Accepted Answer · 2013-08-11 07:47:03Z

2

ArrayUtils.isEquals() from Apache Commons does exactly that. It also handles multi-dimensional arrays.

You also should try Arrays.deepEquals(a, b)

answered Aug 11, 2013 at 7:47

Khinsu

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Comments

hanlj · Accepted Answer · 2013-08-11 07:59:01Z

0

according to java.sun.com The equals method for class Object implements the most discriminating possible equivalence relation on objects; that is, for any non-null reference values x and y, this method returns true if and only if x and y refer to the same object (x == y has the value true). It doesn't perform an intelligent comparison for most classes unless the class overrides it. It has been defined in a meaningful way for most Java core classes. If it's not defined for a (user) class, it behaves the same as ==. so try Arrays.equals(arrB5,arrA).

answered Aug 11, 2013 at 7:59

hanlj

1

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