Most efficient way to split one arraylist data into 4 seperate arraylists?

Issue

I am currently writing code which contains an arraylist. This arraylist includes data which is name, lastname, job and id. I need to seperate the data into different arraylists. Currently i am using the method which is shown below.

for (int i = 0; i < details.size(); i = i + 4) {
    names.add(details.get(i));
    lastname.add(details.get(i + 1));
    job.add(details.get(i + 2));
    id.add(details.get(i+3));
}

I want to know if there is a better way of doing this. The initial arraylist can be very long, and i dont know if there are any issues with this method.

Solution

You asked: "I want to know if there is a better way of doing this". There is a better way.

You should consider creating a class called Record that contains the data (name, last name, job, and ID), and create an ArrayList. Then, instead of using index locations (and potentially grab the wrong data item), you could use the Record getter methods to get the data item you need (and perhaps store it in a different list).

Step 1: Create a Record class:

public class Record
{
    private String firstName;
    private String lastName;
    private String job;
    private String id;

    // TODO add constructor(s), getters and setters
}

Step 2: Create a list of Records (this is an better alternative that create a list having the information in different index locations. That way, each set of name, last name, job, and ID will be self-contained which is way better than disjointed in different index locations in a list.

ArrayList<Record> records = new ArrayList<Record>(); 

Step 3: Instead of using index locations (and potentially grab the wrong data item), you could use the Record getter methods to get the data item you need (and perhaps store it in a different list).

ArrayList<String> names = new ArrayList<String>();
ArrayList<String> jobs = new ArrayList<String>();
...
names.add(records.getLastName() + ", " + records.getFirstName());
jobs.add(records.getJob());

Alternatively, and maybe a better solution, you could use a Map to store this information. For example, a job ID could be the key in a Map that returns a job description and who has been assigned to perform it. Job IDs have to be unique. Adding IDs to a list can be duplicated, because the List interface doesn't restrict entering duplicate data. If you use a Map, they keys are guaranteed to be unique. The value being returned from the Map could be a Record object (or some other kind) that contains the name of the person and the job the person is responsible for. Since values can be duplicates, you can have a person performing multiple jobs, which is probably what you want to do. To use a Map:

Map<String, Record> jobs =  new HashMap<String, Record>(); //This record class doesn't have ID in it.
jobs.put("ABC123", new Record("John", "Doe", "Fix Drywall");
jobs.put("321CBA", new Record("Bill", "Smith", "Install Light Fixtures");

A few things to consider if using a Map. If you try to make a new entry with an existing key, the old one will be overwritten.

jobs.put("ABC123", new Record("John", "Doe", "Fix Drywall");
jobs.put("ABC123", new Record("Bill", "Smith", "Install Light Fixtures"); //Overwrote the previous entry because key is the same

If you want to change the key for an existing value, you must obtain the value, store temporarily, remove the old record, and make a new entry with the old temp value:

jobs.put("ABC123", new Record("John", "Doe", "Fix Drywall");
Record rec = jobs.remove("ABC123"); // gets the record and removes old entry
jobs.put("321CBA", rec); // new job ID for old record

Answered By - hfontanez