Efficient intersection of two List<String> in Java?

You can use retainAll method:

columnsOld.retainAll (columnsNew);

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.

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.