Most efficient way to see if an ArrayList contains an object in Java

I have an ArrayList of objects in Java. The objects have four fields, two of which I'd use to consider the object equal to another. I'm looking for the most efficient way, given those two fields, to see if the array contains that object.

The wrench is that these classes are generated based on XSD objects, so I can't modify the classes themselves to overwrite the .equals.

Is there any better way than just looping through and manually comparing the two fields for each object and then breaking when found? That just seems so messy, looking for a better way.

Edit: the ArrayList comes from a SOAP response that is unmarshalled into objects.

Solution 1:

It depends on how efficient you need things to be. Simply iterating over the list looking for the element which satisfies a certain condition is O(n), but so is ArrayList.Contains if you could implement the Equals method. If you're not doing this in loops or inner loops this approach is probably just fine.

If you really need very efficient look-up speeds at all cost, you'll need to do two things:

1. Work around the fact that the class is generated: Write an adapter class which can wrap the generated class and which implement equals() based on those two fields (assuming they are public). Don't forget to also implement hashCode() (*)
2. Wrap each object with that adapter and put it in a HashSet. HashSet.contains() has constant access time, i.e. O(1) instead of O(n).

Of course, building this HashSet still has a O(n) cost. You are only going to gain anything if the cost of building the HashSet is negligible compared to the total cost of all the contains() checks that you need to do. Trying to build a list without duplicates is such a case.

Solution 2:

You could use a Comparator with Java's built-in methods for sorting and binary search. Suppose you have a class like this, where a and b are the fields you want to use for sorting:

class Thing { String a, b, c, d; }

You would define your Comparator:

Comparator<Thing> comparator = new Comparator<Thing>() {
  public int compare(Thing o1, Thing o2) {
    if (o1.a.equals(o2.a)) {
      return o1.b.compareTo(o2.b);
    }
    return o1.a.compareTo(o2.a);
  }
};

Then sort your list:

Collections.sort(list, comparator);

And finally do the binary search:

int i = Collections.binarySearch(list, thingToFind, comparator);

Solution 3:

Given your constraints, you're stuck with brute force search (or creating an index if the search will be repeated). Can you elaborate any on how the ArrayList is generated--perhaps there is some wiggle room there.

If all you're looking for is prettier code, consider using the Apache Commons Collections classes, in particular CollectionUtils.find(), for ready-made syntactic sugar:

ArrayList haystack = // ...
final Object needleField1 = // ...
final Object needleField2 = // ...

Object found = CollectionUtils.find(haystack, new Predicate() {
   public boolean evaluate(Object input) {
      return needleField1.equals(input.field1) && 
             needleField2.equals(input.field2);
   }
});

Solution 4:

If the list is sorted, you can use a binary search. If not, then there is no better way.

If you're doing this a lot, it would almost certainly be worth your while to sort the list the first time. Since you can't modify the classes, you would have to use a Comparator to do the sorting and searching.

Solution 5:

Even if the equals method were comparing those two fields, then logically, it would be just the same code as you doing it manually. OK, it might be "messy", but it's still the correct answer