Exception in thread "main" java.lang.NullPointerException

[yotamoo]

So, i am trying to build a simple database (array of Car type).
the code is quite simple but for some reason i keep getting the same error after running the program (compiling went through). so i have 3 classes:

PHP Code:

class DataBase {
    
    private Car[] database = new Car[3];

    public DataBase() {
    for (int i=0; i<database.length; i++) {
        database[i].formatCar();
    }
    
    } 
    
    public void addCar(Car car1) {
    for (int i=0; i<database.length; i++) {
        if (database[i].getModel()=="empty") {
        database[i]=car1;
        }
    }
    } 

    public void printDataBase() {
    int i=0;
    while (i<database.length) {
        database[0].printCar();
        i=i+1;
    }
    
    }

} 


PHP Code:

class mainpage {

    public static void main (String[] args) {
    Car car1 = new Car();
    DataBase database1 = new DataBase();
    car1.setCar();
     database1.addCar(car1);
    database1.printDataBase();
    }

} 


PHP Code:

import java.util.*;

class Car {

    private String _model;
    private String _color;
    private int _year;
    private Scanner _input = new Scanner(System.in);

    public Car() {

    _model = "empty";
    _color = "empty";
    _year = 0;

    }

    public void setCar() {

    System.out.print("Enter model: ");
    _model = _input.next();
    System.out.print("Enter color: ");
    _color = _input.next();
    System.out.print("Enter year: ");
    _year = _input.nextInt();

    }

    public void formatCar() {
    _model = "empty";
    _color = "empty";
    _year = 0;
    }

    public String getModel() {
    String model=_model;
    return model;
    }

    public String getColor() {
    String color=_color;
    return color;
    }

    public int getYear() {
    int year=_year;
    return year;
    }

    public void printCar() {
    System.out.println("Model :"+_model);
    System.out.println("Color :"+_color);
    System.out.println("Year :"+_year);
    }


} 


And the error i get:

PHP Code:

yotam@main computer:~/Documents/java/newcar$ java mainpage 
Exception in thread "main" java.lang.NullPointerException
    at DataBase.<init>(DataBase.java:7)
    at mainpage.main(mainpage.java:5)
yotam@main computer:~/Documents/java/newcar$ 


i searched for possible reasons and i came across the possibility that the array wasn't initialized, but then i changed to constructor to set model and color to "empty" and year to 0.
so now i have no clue, need your help guys.
Thanks!

[TheShaner]

It's because in this line:

private Car[] database = new Car[3];

You are creating a new array of Car, but when you do this:

for (int i=0; i<database.length; i++) {
    database[i].formatCar();
}

The code in red returns a null Car object because the Car instance has not been created yet for that position in the array. Then when it attempts to run the formatCar method, it returns a NullPointerException because you're trying to do the formatCar method on a null object. Before doing the line in red, you should do:

database[i] = new Car();

However, looking at your code, you should replace the array and use an ArrayList object instead like so:

List<Car> database = new ArrayList<Car>(3);

Then you won't have to create your own addCar method since an ArrayList already has that method implemented.

-Shane

[yotamoo]

i can't thank you enough, plus the dynamic array is just what i needed.
thanx!

also - can you pls link me to a tutorial explaining more about the list you told me about? i wasn't really sure what to look for.

p.s. can i mark this thread solved?

[paranrat]

import java.util.*;

class CustomerInfo{
String name, type;
int no;
double balance;
}

public class Customer_Account {
int N = 2;
CustomerInfo[] student = new CustomerInfo[N];

public static void main (String[] args)
{
int N = 2;
int i;
Customer_Account arr = new Customer_Account();
CustomerInfo[] Std = new CustomerInfo[N];
Std = arr.InputData();
arr.SelectOperation();
arr.PrintInfo(Std);

}
public CustomerInfo[] InputData() {
int i, num;
double amount, total;
Scanner input = new Scanner (System.in);
CustomerInfo[] customer = new CustomerInfo[N];
System.out.println("\nEnter Customer Information ");
System.out.println("___________________________ \n");
for (i = 0; i< N; i++)
{
customer[i] = new CustomerInfo();
System.out.print("Input customer name : ");
customer[i].name = input.next();
System.out.print("Input acct No : ");
customer[i].no = input.nextInt();
System.out.print("Input acct Type : ");
customer[i].type = input.next();
System.out.print("Input acct Balance : ");
customer[i].balance = input.nextDouble();
System.out.println();
}
return customer;
}

public void SelectOperation (){
int num, i=N;
double amount, total;
Scanner input = new Scanner (System.in);
CustomerInfo[] customer = new CustomerInfo[N];
Scanner choose = new Scanner (System.in);
System.out.println("Please choose your operation (input either 1 or 2): ");
System.out.println("1. Withdrawal");
System.out.println("2. Deposit");
System.out.println();
System.out.print("You choose: ");
num = choose.nextInt();
System.out.println();

switch (num) {
case 1:

System.out.println("Withdrawal Operation:");
System.out.println("___________________________ \n");
System.out.print("Input acct munber:");
customer[i].no = input.nextInt();
System.out.print("Input amount to withdraw:");
amount = input.nextInt();
total= (customer[i].balance-amount);
customer[i].balance = total;
System.out.println();
break;

case 2:
Scanner output = new Scanner (System.in);
System.out.println("Deposit Operation:");
System.out.println("___________________________ \n");
System.out.print("Input acct munber:");
customer[i].no = output.nextInt();
System.out.print("Input amount to deposit:");
amount = output.nextInt();
total= (amount+customer[i].balance);
customer[i].balance=total;
System.out.println();
break;
}
}

public void PrintInfo(CustomerInfo[] Std)
{
int i;
System.out.println();
System.out.println("List of customers :\n");
for (i=0;i<N;i++)
{
System.out.println("Customer "+(i+1) + "\n" +
"Customer Name : " + Std[i].name + "\n" +
"Customer Account Number : " + Std[i].no + "\n" +
"Customer Account Type :" + Std[i].type + "\n" +
"Customer Account Balnce : " + Std[i].balance + "\n");
}
}
}


The output:
Enter Customer Information
___________________________

Input customer name : Ali
Input acct No : 1111
Input acct Type : saving
Input acct Balance : 2222

Input customer name : Abu
Input acct No : 2222
Input acct Type : saving
Input acct Balance : 3333

Please choose your operation (input either 1 or 2):
1. Withdrawal
2. Deposit

You choose: 1

Withdrawal Operation:
___________________________

Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: 2
at customer_account.Customer_Account.SelectOperation(Customer_Account.java:77)
at customer_account.Customer_Account.main(Customer_Account.java:30)
Input acct munber:Java Result: 1