Java compare unordered ArrayLists

java arraylist unordered
Alex Tape picture Alex Tape · Nov 18, 2013 · Viewed 15.2k times · Source

anybody know an efficient way to decide if two arraylists contain the same values?

Code:

ArrayList<String> dummy1= new ArrayList<String>();
list1.put("foo");
list1.put("baa");

ArrayList<String> dummy2= new ArrayList<String>();
list1.put("baa");
list1.put("foo");

dummy1 == dummy2

the challenge is that the arraylists has not the same value order..

(foo, baa) == (foo, baa) // per definition :)

i need to get this

(foo, baa) == (baa, foo) // true

so what would be your approach?

Answer

frankie liuzzi picture frankie liuzzi · Nov 18, 2013

Just sort it first. 

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);
}

Honestly, you should check your data structure decision. This seems more like a set problem. Sorting then comparing will take O(nlog n) while a HashSet comparison will only be O(n).

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