Ytukyg

I have two classes:

public class AClass{



    String name;

    int id;

    int total;

}



public class BClass{

    String batchName;

    int id;

}

Now I have two lists:

List<AClass> aLst;

List<BClass> bLst;

Between these two list i need to check if AClass.id==BClass.id.
One way to achieve this is to have two for loops and compare. But this is not the efficient way.

Other way is by using HashMap. Traverse the BClass list and use the BClass.id as the key and the respective object as the value:

Map<int,BClass> map = new HashMap<int,BClass>();

List requiredLst <BClass> = new ArrayList<BClass>();

foreach(BClass b : bLst){

    map.put(BClass.id, b);

}



foreach(AClass a : aLst){

    BClass b = map.get(a.id);

    requiredLst.add(b);

}

Is this efficient way to compare?

It would be easier with a Set<Integer> to store the identifiers from AClass. Since Set.contains() is O(1) the total complexity will be O(n).

List<AClass> aLst = ...;

List<BClass> bLst = ...;



Set<Integer> ids = aLst.stream()

    .map(AClass::getId)

    .collect(Collectors.toSet());

List<BClass> required = bLst.stream()

    .filter(b -> ids.contains(b.getId()))

    .collect(Collectors.toList());

However for small lists the cost of creating additional Set and boxing identifiers into Integer might outweigh the benefit of reduced number of iterations.