In java How to merge two different size arraylist and make a new Arraylist?

Question

I have two arraylist name preBusinessList, businessList. In business List I have data from server, and in preBusinessList is the local one. In lists I have id, count value Betterly demonstrate as below

enter image description here

Now I wanted to make a newBusinessList like this

How can I do it in java, please help me to solve this

Question is unclear. Do you want to take the count from the server? Or the sum of both? — Thilo
– Thilo, Commented May 4, 2013 at 8:31
I want to only merge these two lists, based on when I not have data in businesslist than take data from preBusinessList. — Ujjwal Bansal
– Ujjwal Bansal, Commented May 4, 2013 at 8:32
on what criteria do you want to do the merge? On list index? on value? from your example it seems to be none of these. so could you clarify the merge criteria. — Sebastien
– Sebastien, Commented May 4, 2013 at 8:36
Actually, I want to put value from preBusinesslIst when I am not getting value from businessList on a particular index. — Ujjwal Bansal
– Ujjwal Bansal, Commented May 4, 2013 at 8:41

Sebastien · Accepted Answer · 2013-05-04 08:40:57Z

4

Then I would use a map to do the merge using id as the key and convert it back to your list of (id,value) pairs

answered May 4, 2013 at 8:40

Sebastien

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3 Comments

Michal Borek Over a year ago

I agree with you, mine solution below wouldn't remove duplicates.

Ujjwal Bansal Over a year ago

can you please give me some example code because I am really stuck on my end.

radai Over a year ago

@user900779 - something like my answer i think

Michal Borek · Accepted Answer · 2013-05-04 08:33:47Z

1

You can use:

Collections.sort(new ArrayList<...>(preBusinessList).addAll(businessList), comparator)

Where comparator is a class that implements Comparator interface (will be responsible for sorting as you wish)

answered May 4, 2013 at 8:33

Michal Borek

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4 Comments

Ujjwal Bansal Over a year ago

can you please give me the implementation of comparator.

radai Over a year ago

but he wants items from businessList to "overwrite" those from preBusinessList

Michal Borek Over a year ago

@user900779 Please tell me what the type of ArrayList is (is it a String ?)

Ujjwal Bansal Over a year ago

it's a class model <placeItems> public class PlaceItems { public final static String BUSINESS_ID = "id"; public final static String BUSINESS_RESULT = "count"; public String businessId, businessResult; }

radai · Accepted Answer · 2013-05-04 08:43:00Z

0

assumming i understood your problem correctly (big if...):

also, i assume each element in the lists is a Pair - as it looks from your data (just a dumb wrapper class that holds 2 integers). if its some other class you'll need to adjust this code.

private Map<Integer,Integer> finalValues = new HashMap<Integer,Integer>();

for (Pair<Integer,Integer> entry : preBusinessList) {
    finalValues.put(entry.getFirst(), entry.getSecond());
}
//2nd list overwrites values from 1st (anything not overwritten remains)
for (Pair<Integer,Integer> entry : businessList) {
    finalValues.put(entry.getFirst(), entry.getSecond());
}
ArrayList<Pair<Integer,Integer>> finalList = new ArrayList<>();
for (Map.Entry<Integer,Integer> entry : finalValues) {
   finalList.add(new Pair(entry.getKey(), entry.getValue());
}
//and now sort the list
Collections.sort(finalList, new Comparator<Pair<Integer,Integer>> {
    int compare(Pair<Integer,Integer> a, Pair<Integer,Integer>b) {
       return a.getFirst.compareTo(b.getFirst()); //compare by 1st number in pair only
    }
});

answered May 4, 2013 at 8:43

radai

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5 Comments

Ujjwal Bansal Over a year ago

what is entry.getFirst() and getSecond?

radai Over a year ago

@user900779 getter methods for the class that holds 2 integers. im assumming you use something like it in your lists since each entry in your list has 2 values - so its definitely not a lit of simple objects

Ujjwal Bansal Over a year ago

I am using this class and have only two variables in it. public class PlaceItems { public final static String BUSINESS_ID = "id"; public final static String BUSINESS_RESULT = "count"; public String businessId, businessResult; }

Ujjwal Bansal Over a year ago

I am getting error in eclipse at these two lines I change your code according to my class implementation: ArrayList<PlaceItems> finalList = new ArrayList<PlaceItems>(); for (Map.Entry<String,String> entry : finalValues) { finalList.add(new PlaceItems(entry.businessId, entry.businessResult); } //and now sort the list Collections.sort(finalList, new Comparator<PlaceItems> { int compare(Pair<Integer,Integer> a, Pair<Integer,Integer>b) { return a.getFirst.compareTo(b.getFirst()); //compare by 1st number in pair only } });

radai Over a year ago

@user900779 - the compare() method for a Comparator<PlaceItems> would be public int compare (PlaceItems a, PlaceItems b). you forgot to update it

Neil Townsend · Accepted Answer · 2013-05-04 08:48:34Z

Assuming something like:

public class Info {
    public int id;
    public int info;
}

You could merge them on the basis of wanting the keep the one with higher info field as follows:

// Assumes:
// - that the ArrayLists are sorted to have id in order going up
// - no repeated ids in a or in b (but same id can be in both a and b)
ArrayList<Info> merge(ArrayList<Info> a, ArrayList<Info> b) {
    int aLength = a.size();
    int bLength = b.size();

    int ai = 0;
    int bi = 0;

    ArrayList<Info> result = new ArrayList<Info>();

    while ((ai < aLength) && (bi < bLength))
        Info aInfo = a.get(ai);
        Info bInfo = b.get(bi);
        if (aInfo.id == bInfo.id) {
            if (aInfo.info >= bInfo.info) result.add(aInfo);
            else                          result.add(bInfo);
            ai++;
            bi++;
        }
        else if (aInfo.id < bInfo.id) {
            result.add(aInfo);
            ai++;
        }
        else  {
            result.add(bInfo);
            bi++;
        }
    }

    // Add the remaining terms - only one of the loops will actually do anything
    for (; ai<aiLength; ai++) {
        result.add(a.get(ai));
    }
    for (; bi<biLength; bi++) {
        result.add(b.get(bi));
    }
}

Anirudh · Accepted Answer · 2013-05-04 09:44:40Z

0

Pseudocode :

Iterate over preBusinessList.
Fetch key and see if this key(1,2,3,4,5,6) exists in businesslist
If yes conitnue

Else If no, then add it to businesslist

for(Map.Entry<Integer, Integer> keyValue : preBusinessList.entrySet()) {
    if(!businesslist.containsKey(keyValue.getKey())) {
        businesslist.put(keyValue.getKey(), keyValue.getValue());
    }
}

Updated Answer as per new requirements

boolean ifExists = false;
for(PlaceItems itemPreBusinessList : preBusinessList) {
   ifExists = false;
   for(PlaceItems itemBusinessList : businessList) {
    if(itemBusinessList.businessId == itemPreBusinessList.businessId) {
        // Already exists
        ifExists = true;
        break;
    }
   }

   if(!isExists) {
    businessList.add(itemPreBusinessList);
   }
 }

edited May 4, 2013 at 9:44

answered May 4, 2013 at 8:41

Anirudh

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6 Comments

Ujjwal Bansal Over a year ago

Hi Anirudh, I have a Class for Arraylist Items name <placeItems>. Which have two String variables

Ujjwal Bansal Over a year ago

public class PlaceItems { public final static String BUSINESS_ID = "id"; public final static String BUSINESS_RESULT = "count"; public String businessId, businessResult; }

Ujjwal Bansal Over a year ago

Can you suggest me what should I write instead of Mao.Entry<Integer, Integer>

Anirudh Over a year ago

Any specific reason for doing this? This will degrade performance of your app since for every item in list you will need to create a separate PlaceItems object.

Ujjwal Bansal Over a year ago

actually this class holds many variables in future that's why I am using this.

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