Accessing a pojo object using arraylist

Question

i am trying to solve the following project where I need to choose out the best customer's based on their purchases. All the necessary details like name,and amount purchased is included in my POJO object(Customer). I made an ArrayList of Cusomers and trying to access getName()/getAmount method in a for Loop.

              import java.util.ArrayList;
               import java.util.Scanner;

    public class Store {
     ArrayList<Customer> Customers = new ArrayList<>();
    Customer[] csa = new Customer[1000];

     public void addSale(String customerName, double amount) {
    String cn = customerName;
    double am = amount;
    Customer cs = new Customer(cn, am);
    Customers.add(cs);
}

public String nameOfBestCustomer() {
    String name = null;
    //Customer csa=new Customer();
    double largest = csa[0].getAmount();

    // gives me:java.lang.NullPointerException

    for (int i = 1; i < Customers.size(); i++) {

        if (largest < csa[i].getAmount()) {
            largest = csa[i].getAmount();
            name = csa[i].getName();
        }
    }

    // return name+""+largest;
    return name;
}


public static void main(String[] args) {

    Store s = new Store();
    double am;

    Scanner scanner = new Scanner(System.in);

    while (true) {

        System.out.println("Enter Customer name:");
        String cn = scanner.next();

        if (cn.equals("done")) {
            am = 0;
            scanner.close();

            break;
        } else {
            System.out.println("Enter Amount:");
            am = scanner.nextDouble();
            s.addSale(cn, am);

        }
    }

    System.out.println("Best customer " + s.nameOfBestCustomer());

}

            }

Is it okay to use Customer[] csa = new Customer[1000]? when I already have an Arraylist for Customers?

Can anybody tell me how to access customer-methods using Array or ArrayList? And also why is largest = csa[0].getAmount() giving me a NullPointerException?

Answer 1

there are many mistakes in your source code like

1. Customer class constructor was wrong assigned
2. You were not stored any objects on array so objects are null.

Refer below code i have updated source code, just copy paste it will work...

package demo;

import java.util.ArrayList;
import java.util.Scanner;

public class Store {
    ArrayList<Customer> Customers = new ArrayList<Customer>();
   Customer[] csa = new Customer[1000];

    public void addSale(String customerName, double amount) {
   String cn = customerName;
   double am = amount;
   Customer cs = new Customer(cn, am);
   Customers.add(cs);
}

public String nameOfBestCustomer() {
   String name = null;
   //Customer csa=new Customer();
   double largest = Customers.get(0).getAmount();

   // gives me:java.lang.NullPointerException

   for (int i = 1; i < Customers.size(); i++) {

       if (largest < Customers.get(i).getAmount()) {
           largest = Customers.get(i).getAmount();
           name = Customers.get(i).getName();
       }
   }

   // return name+""+largest;
   return name;
}


public static void main(String[] args) {

   Store s = new Store();
   double am;

   Scanner scanner = new Scanner(System.in);

   while (true) {

       System.out.println("Enter Customer name:");
       String cn = scanner.next();

       if (cn.equals("done")) {
           am = 0;
           scanner.close();

           break;
       } else {
           System.out.println("Enter Amount:");
           am = scanner.nextDouble();
           s.addSale(cn, am);

       }
   }

   System.out.println("Best customer " + s.nameOfBestCustomer());

}

           }

class Customer {
private String name;
private double amount;


public Customer(){

}
//@SuppressWarnings("null")
public  Customer(String name,double price) {
    this.name=name;
    this.amount= price;

}





public String getName() {
    return name;
}
public void setName(String name) {
    this.name = name;
}
public double getAmount() {
    return amount;
}
public void setAmount(double amount) {
    this.amount = amount;
}

                 }




Output :
Enter Customer name:
ramesh
Enter Amount:
100
Enter Customer name:
pramod
Enter Amount:
200
Enter Customer name:
done
Best customer pramod