Expert: Brian Ngure - 11/28/2006

Question
Hi,
I'm teaching myself Java and I'm trying to use an array of objects I created but I'm getting a null pointer exception the first time I try to assign one of the fields. When I instantiate an array of objects doesn't that instantiate each object of the array? The code is below and the line I get the error on is indicated. Thanks!

//This program calculates the gross pay for three employees
import javax.swing.*;

public class TestDriver {
  //Employee class
  public class Employee{
     double hours;
     double wage;
     double totalPay;
  }
  public static void main(String[] args){
     //Array of employees
     Employee employeeArray[] = new Employee[3];
     //Temp string
     String temp;
     
     //Get and process employee data
     for( int i = 0; i < 3; i++ ){

        temp = JOptionPane.showInputDialog("Enter wage for employee " + (i+1));
        employeeArray[i].wage = Double.parseDouble(temp);  //I get a null pointer exception here
        
        temp = JOptionPane.showInputDialog("Enter hours for employee " + (i+1));
        employeeArray[i].hours = Double.parseDouble(temp);
        
        if( employeeArray[i].hours <= 40.00){
           employeeArray[i].totalPay = employeeArray[i].wage * employeeArray[i].hours;
        }
        else{
           employeeArray[i].totalPay =
              (employeeArray[i].wage * 40) + (1.5 * employeeArray[i].wage *
                    (employeeArray[i].hours - 40));
        }
     }
     temp = "Employee pay results:\n";
     //Create output string
     for( int i = 0; i < 3; i++ ){
        temp += "Employee " + (i+1) + " worked " + employeeArray[i].hours +
        "\nat $" + employeeArray[i].wage + " per hour for a gross pay of $" +
        employeeArray[i].totalPay + ".\n";
     }
     
     //Display output
     JOptionPane.showMessageDialog(null, temp);
     
     System.exit(0);
        
  }
}

Answer
Hi,

When I instantiate an array of objects doesn't that instantiate each object of the array? -> No it does not. The three elements in the array are still null.

See the comments I have added to your modified code below for more details.



import javax.swing.JOptionPane;

public class TestDriver {
   //Employee class
   public class Employee {
       double hours;
       double wage;
       double totalPay;
   }


   public static void main(String[] args) {
       // Create an instance of this class to reference the Employee class
       TestDriver td = new TestDriver();
       //Array of employees
       Employee[] employeeArray = new Employee[3];
       
       // Initially, each object in the array is null (not auto-initialized)
       // Use td reference to create a new Employee object
       employeeArray[0] = td.new Employee();
       employeeArray[1] = td.new Employee();
       employeeArray[2] = td.new Employee();
       
       // Proceed with your unchanged code
       
       //Temp string
       String temp;

       //Get and process employee data
       for (int i = 0; i < 3; i++) {
           temp = JOptionPane.showInputDialog("Enter wage for employee " + (i + 1));
           employeeArray[i].wage = Double.parseDouble(temp); // null pointer exception does not occur

           temp = JOptionPane.showInputDialog("Enter hours for employee " + (i + 1));
           employeeArray[i].hours = Double.parseDouble(temp);

           if(employeeArray[i].hours <= 40.00) {
               employeeArray[i].totalPay = employeeArray[i].wage * employeeArray[i].hours;
           } else {
               employeeArray[i].totalPay = (employeeArray[i].wage * 40) + (1.5 * employeeArray[i].wage * (employeeArray[i].hours - 40));
           }
       }
       temp = "Employee pay results:\n";
       //Create output string
       for (int i = 0; i < 3; i++) {
           temp += "Employee " + (i + 1) + " worked " + employeeArray[i].hours + "\nat $" + employeeArray[i].wage + " per hour for a gross pay of $" + employeeArray[i].totalPay + ".\n";
       }

       //Display output
       JOptionPane.showMessageDialog(null, temp);

       System.exit(0);
   }
}