Comparing two ArrayLists of Objects

Question

I have class named Student. Also I have two array lists for Students A and Students B.

Students A could be: Maria, Alex, Lora, Vlad, Lauren, Catherine.

Students B is some students from A: Maria, Alex

Students A has the proper id and names stored. But Students B has only prepared names and hardcoded id stored.

I separated these two lists and now want do the following:

if Maria form Array A has student id : 2427

Maria from Array B should receive the same student id.

Can you please help me to do that.

 //array lists of students a and b
public void common(){

     ArrayList<ArrayList<Student>> lists = new ArrayList<ArrayList<Student>>();
     lists.add(a);
     lists.add(b);

     System.out.println(getCommonElements(lists));
     for (int i = 0; i < lists.size(); i++) {
         aa = lists.get(0);
         bb = lists.get(1);

     }
     System.out.println(aa);
     System.out.println(bb);
}


public static <T> Set<T> getCommonElements(Collection<? extends Collection<T>> collections) {

    Set<T> common = new LinkedHashSet<T>();
    if (!collections.isEmpty()) {
       Iterator<? extends Collection<T>> iterator = collections.iterator();
       common.addAll(iterator.next());
       while (iterator.hasNext()) {
          common.retainAll(iterator.next());
       }
    }
    return common;
}

Thank you in advance! the question is rarely why this do not work?

for (int i = 0; i < a.size(); i++) {
  for (int j = 0; j < b.size(); j++) {

      if(a.get(i).getName().equals(b.get(j).getName())){
                //does not go there at all
   }
  }
 }

Answer 1

Your iteration is not doing what you meant it to do. You are comparing elements at index 'i' in both arrays over and over, instead of elements in A at index 'i' and elements in B at index 'j'.

If you print out the name of each student that you comparing it should be clear to you.