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.

Answer 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.