Comparing two arraylist in Java

Question

I have two array lists listA and listB which may or may not contain same elements. For Example:

listA= [1,2,3] &
listB=[2,3,4]

I need to compare the two array lists and add the element which is present in listA and not in listB to listB. Also if any other element which is present in listB is not there in listA, I should delete it from listB.

Please help me how to code for this in Java.

I prefer using Iterator method if needed.

this sounds a bit like homework? if so it should be tagged as such — mfsiega, Oct 16 '12 at 13:56
this might help you: http://stackoverflow.com/questions/2762093/java-compare-two-lists — zaffargachal, Oct 16 '12 at 13:57
What's the point? Wouldn't you end up with `listA` as your output? — Sergey Kalinichenko, Oct 16 '12 at 13:57
is the list index relevant in the comparison? e.g. if position 0 in list A is 1 and position 2 in list B is 1, does that count as a match or not because of the different position (index)? — amphibient, Oct 16 '12 at 13:58
This looks like a merge. Have a look [at this post](http://stackoverflow.com/questions/189559/how-do-i-join-two-lists-in-java). — Maxbester, Oct 16 '12 at 13:58
@Deepthi I recommend you post your code or what have you done so far and where exactly you are stuck. You may want to read the faq (top right link) — Ravi Gupta, Oct 16 '12 at 14:57

score 1 · Answer 1 · answered Oct 16 '12 at 14:00

Your description suggests you want to be working with sets. For example, say an element is in A twice but only in B once. Should it be added to B once?

The simplest answer to your question is to

List listA = ...
List listB = ...

listB.clear();
listB.addAll(listA);

however assuming you want to preserve some order you can do.

List listA = ...
List listB = ...
Set setA = new HashSet(listA);
setA.removeAll(listB);

Set setB = new HashSet(listB);
setB.removeAll(listA);

listB.addAll(setA);
listB.removeAll(setB);

Note: this will not add entries if A of an element has more than B and it will not remove entries if B has more of an element than A.

Somehow I think this homework is supposed to demonstrate the use of iterators. I think Sets are taking it a bit too far :) — Shark, Oct 16 '12 at 14:06
@Shark If its homework, I leave it as an exercise for the OP to translate. ;) — Peter Lawrey, Oct 16 '12 at 14:09

Shark · Answer 2 · 2012-10-16T14:04:52.083

0

Try this

for(int i : listA)
    if(!listB.contains(i)) listB.add(i);
for(int j : listB)
    if(!listA.contains(j)) listB.remove(j);

Otherwise

Arrays.sort(listA); Arrays.sort(listB);
for(int i = 0; i < listA.length(); i++)
{
   if(!listB.contains(listA.get(i))) listB.add(listA.get(i));
   if(!listA.contains(listB.get(i))) listB.remove(listA.get(i));
}

edited Oct 16 '12 at 14:04

answered Oct 16 '12 at 13:59

Shark

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What if listB is shorter than listA ? – Peter Lawrey Oct 16 '12 at 14:10
I wanted to do "i < Math.min(listA.length, listB.length)" but somehow saw it as overkill as it depends whether the OP thinks we should assume it or the teacher will ask for it. – Shark Oct 16 '12 at 14:16
I would use `i < listA.length() && i < listB.length()` rather than use `min()` – Peter Lawrey Oct 16 '12 at 14:19

Yogendra Singh · Answer 3 · 2012-10-16T14:05:52.833

Simple steps for writing the code

First perform the deletion

   Iterate ListB
    For each element in ListB, check if it is present in List A
      If Not, delete from ListB
      else continue
 Close your loops

Then Addition

Iterate ListA
  For each element in ListA, check if it is present in List B
    If Not, add to ListB
    else continue
Close your loops

Sample Code:

    List<Integer> listA = ..
    List<Integer> listB = ..

    for(Integer intB: listB)
      if(!listA.contains(intB)){
         listB.remove(intB);
      }
    }   

   for(Integer intA: listA)
      if(!listB.contains(intA)){
         listB.add(intA);
      }
    }

Comparing two arraylist in Java

3 Answers3