Find intersection between two ArrayLists

Question

Find intersection of two ArrayLists of Strings.

Here is the code:

public ArrayList<String> intersection( ArrayList<String> AL1, ArrayList<String> AL2){   
    ArrayList<String> empty = new ArrayList<String>();
    ArrayList<String> empty1 = new ArrayList<String>();
    if (AL1.isEmpty()){
        return AL1;
    }
    else{
        String s = AL1.get(0);
        if(AL2.contains(s))
            empty.add(s);


            empty1.addAll(AL1.subList(1, AL1.size()));
            empty.addAll(intersection(empty1, AL2));
            return empty;
    }
}

I want the output to look like this: For example,

 [a, b, c] intersect [b, c, d, e] = [b, c]

The above code give me this output, but I want to know how to make this code more easier to understand.

Answer 1

You could make it easier to understand by writing it like this:

/**
 * Computes the intersection of two Lists of Strings, returning it as a new ArrayList of Strings
 *
 * @param list1 one of the Lists from which to compute an intersection
 * @param list2 one of the Lists from which to compute an intersection
 *
 * @return a new ArrayList of Strings containing the intersection of list1 and list2
 */
public ArrayList<String> intersection( List<String> list1, List<String> list2) {   
    ArrayList<String> result = new ArrayList<String>(list1);

    result.retainAll(list2);

    return result;
}

Answer 2

Java collections already have support for this with the retainAll call. Rather than return a new set, the intersection happens in place, which is why you must create a new ArrayList if you want to retain the original list1. retainAll returns a boolean if the calling object is modified

ArrayList<String> list1 = new ArrayList<String>();
list1.add("A");
list1.add("B");
list1.add("C");
ArrayList<String> list2 = new ArrayList<String>();
list2.add("D");
list2.add("B");
list2.add("C");
ArrayList<String> intersection = new ArrayList<String>(list1);
intersection.retainAll(list2);
for(String s: intersection){
    System.out.println(s);
}

Output:

B
C

Answer 3

public ArrayList<String> intersection( ArrayList<String> AL1, ArrayList<String> AL2){   
    ArrayList<String> returnArrayList = new ArrayList<String>();
    for(String test : AL1)
    {
        if(!returnArrayList.contains(test))
        {
            if(AL2.contains(test))
            {
                returnArrayList.add(test);
            }
        }
    }
    return returnArrayList;
}

You could use loops instead of recursion.

Answer 4

If you are ok with a dependency, i would recommend you to take a look at apache commons collections ( http://commons.apache.org/proper/commons-collections/release_4_0.html ).

For your specific use it would be the method intersection from CollectionUtils ( https://commons.apache.org/proper/commons-collections/apidocs/org/apache/commons/collections4/CollectionUtils.html )

Answer 5

You can use Apache Commons Collections ListUtils.intersection

http://commons.apache.org/proper/commons-collections/javadocs/api-2.1.1/org/apache/commons/collections/ListUtils.html#intersection(java.util.List , java.util.List)