Efficient intersection of two List<String> in Java?

Question

Question is simple:

I have two List

List<String> columnsOld = DBUtils.GetColumns(db, TableName);
List<String> columnsNew = DBUtils.GetColumns(db, TableName);

And I need to get the intersection of these. Is there a quick way to achieve this?

Answer 1

You can use retainAll method:

columnsOld.retainAll (columnsNew);

Answer 2

Using Google's Guava library:

Sets.intersection(Sets.newHashSet(setA), Sets.newHashSet(setB))

Note: This is much more efficient than naively doing the intersection with two lists: it's O(n+m), versus O(n×m) for the list version. With two million-item lists it's the difference between millions of operations and trillions of operations.

Answer 3

Since retainAll won't touch the argument collection, this would be faster:

List<String> columnsOld = DBUtils.GetColumns(db, TableName); 
List<String> columnsNew = DBUtils.GetColumns(db, TableName); 

for(int i = columnsNew.size() - 1; i > -1; --i){
    String str = columnsNew.get(i);
    if(!columnsOld.remove(str))
        columnsNew.remove(str);
}

The intersection will be the values left in columnsNew. Removing already compared values fom columnsOld will reduce the number of comparisons needed.

Answer 4

How about

private List<String> intersect(List<String> A, List<String> B) {
    List<String> rtnList = new LinkedList<>();
    for(String dto : A) {
        if(B.contains(dto)) {
            rtnList.add(dto);
        }
    }
    return rtnList;
}