Adding values of repeated elements in ArrayList

Question

I have an Arraylist of Records.

package com.demo.myproject;

public class Records 
{
    String countryName;
    long numberOfDays;

    public String getCountryName() {
        return countryName;
    }
    public void setCountryName(String countryName) {
        this.countryName = countryName;
    }
    public long getNumberOfDays() {
        return numberOfDays;
    }
    public void setNumberOfDays(long numberOfDays) {
        this.numberOfDays = numberOfDays;
    }

    Records(long days,String cName)
    {
        numberOfDays=days;
        countryName=cName;
    }
}

My Arraylist<Records> is containing the values

 Singapore     12 
 Canada         3
 United Sates  12
 Singapore     21

I need to modify it such that my output is

Canada         3
Singapore     33
United States 12

Please help me with solution,approach.

assylias · Accepted Answer · 2012-10-22T07:42:44.783

1

You could store your Records in a Map, where the key would be the country.

When you receive a new Record, check if the country already is in the map, if it is, add the number of days, if not create it.

Map<String, Record> map = new HashMap<String, Record> ();

addRecord(map, someRecord);

private void addRecord(Map<String, Record> map, Record record) {
    Record inMap = map.get(record.getCountryName());
    if (inMap == null) {
        inMap = record;
    } else {
        inMap.setNumberOfDays(inMap.getNumberOfDays() + record.getNumberOfDays());
    }
    map.put(record.getCountryName(), inMap);
}

Notes:

I have assumed that it is fine to modify the records - if not just create a new one using the sum of the days.
you can still get the collection of records by calling map.values(); and iterate over them
ArrayList is not very well suited for your use case. If you really need to stick to ArrayList, for evey new record, you would need to loop over the list, check if one of the records in the list has the same country as the new record, update that record if you find it, or add a new record if not.

edited Oct 22 '12 at 07:42

answered Oct 22 '12 at 07:36

assylias

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I have done everything as per ArrayList in the app. Any suggestions that would help in modifying the ArrayList would be appreciated – Oct 22 '12 at 07:38
Collection c = map.values(); Iterator itr = c.iterator(); while(itr.hasNext()){ System.out.println("SORTED RECORDS ARE "+itr.next()); } I am getting 10-22 13:25:45.132: I/System.out(7129): SORTED RECORDS ARE com.demo.myproject.Records@40525cb8 Please help – Oct 22 '12 at 07:57
1

What would be a better approach/alternate for ArrayList. – Oct 22 '12 at 07:58
If you really want to use ArrayList you have 2 simple solutions: 1. If it doesn't need to be really optimized (you don't have many objects there) before adding an object just make a for loop and see if there is one already with this name. It's the same solution as with HashMap but instead of simple HashMap.get() you have a for loop which you can break when you find the element. Solution 2. When you need a super speed you can use normal arraylist and hashmap to hold String to index only. And before adding to ArrayList you check if there is one already in the HashMap first. – Mark Oct 22 '12 at 08:31
@onkar If you want records sorted by country, replace `HashMap` in my example with `TreeMap` (nothing else changes). For the Records to print nicely, you need to override the `toString` method - see for example: http://stackoverflow.com/questions/10503964/how-to-print-object-content-in-correct-way – assylias Oct 22 '12 at 08:44

score 1 · Answer 2 · edited Oct 22 '12 at 15:12

public class RecordsMain {

    static ArrayList<Records> al = new ArrayList<Records>();
    static boolean flag = false;

    public static void main(String[] args) {
        Records rec1 = new Records(12,"Singapore");
        Records rec2 = new Records(3,"Canada");
        Records rec3 = new Records(12,"United States");
        Records rec4 = new Records(21,"Singapore");

        addToList(rec1);
        addToList(rec2);
        addToList(rec3);
        addToList(rec4);

        for (int i = 0; i < al.size(); i++) {     
            System.out.println(al.get(i).getCountryName() + " :: " + al.get(i).getNumberOfDays());
        }
    }

    public static void addToList(Records records) {
        for (int i = 0; i < al.size(); i++) {     
            if(al.get(i).getCountryName().equals(records.getCountryName())) {
                al.get(i).setNumberOfDays(al.get(i).getNumberOfDays()+records.getNumberOfDays());
                flag=true;
            }
        }
        if (flag == false)
            al.add(records);
    }
}

Note:

The function addToList adds records and while adding itself checks whether the CountryNames are duplicate, if they are it adds the No of days and does not marks any new entry to the ArrayList.
I was not sure if you were looking for sorting of the List too, thus did not try that.

Mark · Answer 3 · 2012-10-22T08:29:28.363

0

I suppose you create these records on your own. If you don't need any specific order of the elements you should use the HashMap and as assylias said - create country elements only when they doesn't exist. When you need to keep the order of elements (or sort them later by name etc) you can still use the ArrayList and "indexOf()" method to easily find them.

edited Oct 22 '12 at 08:29

answered Oct 22 '12 at 07:50

Mark

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score 0 · Answer 4 · answered Oct 22 '12 at 10:55

0

I dont know what exactly you want to do there but if you want to sort it with specific criteria then You could use comparable or comparator interfaces to sort your records using your criteria in ArrayList And use collections.sort() method to sort it.

answered Oct 22 '12 at 10:55

neel.1708

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Adding values of repeated elements in ArrayList

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