Question

I have two ArrayLists of type Answer (self-made class).

I’d like to compare the two lists to see if they contain the same contents, but without order mattering.

Example:

//These should be equal.
ArrayList<String> listA = {"a", "b", "c"}
ArrayList<String> listB = {"b", "c", "a"}

List.equals states that two lists are equal if they contain the same size, contents, and order of elements. I want the same thing, but without order mattering.

Is there a simple way to do this? Or will I need to do a nested for loop, and manually check each index of both lists?

Note: I can’t change them from ArrayList to another type of list, they need to remain that.

Answer 1

You could sort both lists using Collections.sort() and then use the equals method. A slighly better solution is to first check if they are the same length before ordering, if they are not, then they are not equal, then sort, then use equals. For example if you had two lists of Strings it would be something like:

public  boolean equalLists(List<String> one, List<String> two){     
    if (one == null && two == null){
        return true;
    }

    if((one == null && two != null) 
      || one != null && two == null
      || one.size() != two.size()){
        return false;
    }

    //to avoid messing the order of the lists we will use a copy
    //as noted in comments by A. R. S.
    one = new ArrayList<String>(one); 
    two = new ArrayList<String>(two);   

    Collections.sort(one);
    Collections.sort(two);      
    return one.equals(two);
}