use an array or arrayList ofan objects so that all of its records from the file can be stored in memory

I'm a Java beginner and I'm having trouble with my exercises.

This is my Client.txt file:
<!-- language: lang-none -->
1,Jay, Walker,91 Boland Drive,BAGOTVILLE,NSW,2477
2,Mel, Lowe,45 Ocean Drive,MILLERS POINT,NSW,2000
3,Hugh, Manatee,32 Edgecliff Road,REDFERN,NSW,2016
4,Elizabeth, Turner,93 Webb Road,MOUNT HUTTON,NSW,2290

This is my Client class (have constructor):

public class Client {
	private int clientID;
	private String firstName;
	private String surName;
	private String street;
	private String suburb;
	private String state;
	private int postcode;
	
	// constructor
	public Client (int ID, String fName, String sName, String str, String sb, String sta, int pCode) {
		
		clientID = ID;
		firstName = fName;
		surName = sName;
		street = str;
		suburb = sb;
		state = sta;
		postcode = pCode;
	}

This is my code creating object and read the records that are read from the txt file:

File inFile = new File("clients.txt");
Scanner inputFile = new Scanner(inFile);
String str;
String[] tokens;
while (inputFile.hasNext()) {
	
	str = inputFile.nextLine();         // read a line of text from the file 
	tokens = str.split(",");            // split the line using commas as delimiter
	
	// map each token that is already printed to corresponding field in Client class
	// Because tokens[0] is of type String but clientID is of type int,
	// we need to parse it and get the integer representation.
	// we also do the same thing with postcode 
	int clientID = Integer.parseInt(tokens[0]);
	String firstName = tokens[1];
	String surName = tokens[2];
	String street = tokens[3];
	String suburb = tokens[4];
	String state = tokens[5];
	int postcode = Integer.parseInt(tokens[6]);	
	
	// create a new object of `Client` type
	// and pass all the gathered information.
	Client client = new Client(clientID, firstName, surName, street, suburb, state, postcode);
    System.out.println(client + "\n");
		
} // end while

The requirement of the exercise is now I have to modify the program to utilise an array or arrayList of this Client object, so that all of the Client records from the file can be easily stored in memory, and each objects is created from the file place it into the array/arrayList of client objects

I have just begin to study array and arrayList few week, so I have no idea of doing this exercises. Can anyone help me?

Before your while loop create a ArrayList

ArrayList<Client> cList = new ArrayList<> ();

In your loop,

Client client = new Client(clientID, firstName, surName, street, suburb, state, postcode);

// add to list
cList.add (client);

Above the while loop you can simply create a new list

List<Client> clientList = new ArrayList<>();

and then add to the client list after you make each new Client(..)

with clientList.add(client)

You could override the constructor of Client so that it could also just take in a line from the file:

import java.util.stream.Stream;

public class Client {
    private int clientId;
    private String firstName;
    private String lastName;
    private String street;
    private String suburb;
    private String state;
    private int postcode;

    public Client(int clientId, String firstName, String lastName, String street, String suburb, String state,
            int postcode) {
        this.clientId = clientId;
        this.firstName = firstName;
        this.lastName = lastName;
        this.street = street;
        this.suburb = suburb;
        this.state = state;
        this.postcode = postcode;
    }

    public Client(String line) {
        String[] split = line.split(",");
        this.clientId = Integer.parseInt(split[0]);
        this.firstName = split[1];
        this.lastName = split[2].trim();
        this.street = split[3];
        this.suburb = titleCase(split[4]);
        this.state = split[5];
        this.postcode = Integer.parseInt(split[6]);
    }

    @Override
    public String toString() {
        return String.format("%s %s - %s, %s - %s, %d", this.firstName, this.lastName, this.street, this.suburb,
                this.state, this.postcode);
    }

    private String titleCase(String suburb) {
        return Stream.of(suburb.split(" ")).map(w -> w.toUpperCase().charAt(0) + w.toLowerCase().substring(1))
                .reduce((s, s2) -> s + " " + s2).orElse("");
    }
}

And also use try-with-resources for your Scanner:

import java.io.File;
import java.io.IOException;
import java.util.ArrayList;
import java.util.List;
import java.util.Scanner;

class Main {
    public static void main(String[] args) {
        List<Client> clientList = new ArrayList<>();

        try (Scanner scanner = new Scanner(new File("Clients.txt"))) {
            while (scanner.hasNext()) {
                String line = scanner.nextLine();
                clientList.add(new Client(line));
            }
        } catch (IOException e) {
            e.printStackTrace();
        }

        for (Client client : clientList) {
            System.out.println(client);
        }
    }
}

Output:

Jay Walker - 91 Boland Drive, Bagotville - NSW, 2477
Mel Lowe - 45 Ocean Drive, Millers Point - NSW, 2000
Hugh Manatee - 32 Edgecliff Road, Redfern - NSW, 2016
Elizabeth Turner - 93 Webb Road, Mount Hutton - NSW, 2290

Try it out here.