Question

I would like to implement a fast search on an ArrayList of objects. These objects consist of an int oldId, int newId and int inList, among with other things.

Now, I tried implementing a binary search using the Collections.binarySearch on my list, but the problem is I need to search using the oldId and inList, to get the newId from the corresponding object. Example: I have oldId = 9 and inList = 1, and try to get the newId that has been assigned somewhere else. Essentially mapped the oldId-newId’s and grouped them. There may be duplicate oldId’s but they must be in different lists, and have unique newIds.

Do you think it would be better to use a hashmap for these map objects? Or else, is there a solution (maybe a comparator for the objects) to get the newId’s from the oldId and inList info. I am also looking for a fast search algorithm.

Thanks a lot for helps, I appreciate ideas.

Here is a comparator I have written for the binary search, however I couldn’t figure out how to add the inList info here.

public class CompareTermId implements Comparator <MObj> {

public CompareTermId(){}

public int compare(MObj a, MObj b){

    if(a.oldTermId < b.oldTermId) return 1;
    else if(a.oldTermId > b.oldTermId) return -1;
    else return 0;
}

}

Answer 1

As you have invited I am going to recommend HashMap 🙂

The reason is because HashMap provides effectively constant time lookups for a single key.

If you construct a hash map as follows:

Map<Id, Map<InList, Id>> oldIdToNewIdMap = new HashMap<>();

And then populate it:

Map<InList, Id> inListMap = new HashMap<>();
inListMap.put(oldId, newId);
oldIdToNewIdMap.put(inList, inListMap);

Then you can lookup as follows:

Id newId = oldIdToNewIdMap.get(inList).get(oldId);