Exception in thread "main" java.lang.NullPointerException

[Christian Mucyo]

This is the parent class

import java.util.*;
import javax.swing.*;
public class Person
{
	protected String firstName;
	protected String lastName;
	protected String address;
	protected int zipCode;
	protected String zipString;
	protected int phoneNum;
	protected String phoneString;
	
	
	public Person()
	{
		firstName = "";
		lastName = "";
		address = "";
		zipCode = 0;
		phoneNum = 0;
	}
	
	public void setFirstName()
	{
		firstName = JOptionPane.showInputDialog(null,"Enter your first name");
	}
	
	public void setLastName()
	{
		lastName = JOptionPane.showInputDialog(null,"Enter your last name");
	}
	
	public void setAddress()
	{
		address = JOptionPane.showInputDialog(null,"Enter your street address");
	}
	
	public void setZipCode()
	{
		zipString = JOptionPane.showInputDialog(null,"Enter your zip code");
		zipCode = Integer.parseInt(zipString);
	}
	
	public void setPhoneNum()
	{
		phoneString = JOptionPane.showInputDialog(null,"Enter your phone number");
		phoneNum = Integer.parseInt(phoneString);
	}
	
	public String getFirstName()
	{	return firstName;	}
	
	public String getLastName()
	{	return lastName;	}
	
	public String getAddress()
	{	return address;	}
	
	public int getZipCode()
	{	return zipCode;	}
	
	public int getPhoneNum()
	{	return phoneNum;	}
	
	public void display()
	{
		System.out.println("Person's information:\nFirst name: "
                       + getFirstName() + "\nLast name: " + getLastName()
         		+ "\nStreet address: " + getAddress() + "\nZip Code: "
                        + getZipCode() + "\nPhone Number: " + getPhoneNum());
	}
}

This is one Subclass

import javax.swing.*;
public class CollegeEmployee extends Person
{
	protected int ssn;
	protected String ssnString;
	protected double annualSal;
	protected String annualSalString;
	protected String deptName;
	
	public CollegeEmployee()
	{
		super();
		ssn = 0;
		annualSal = 0.0;
		deptName = "";
	}
	
	public void setSsn()
	{
		ssnString = JOptionPane.showInputDialog(null,"Enter your Social security number");
		ssn = Integer.parseInt(ssnString);
	}
	
	public void setAnnualSal()
	{
		annualSalString = JOptionPane.showInputDialog(null,"Enter your annual salary");
		annualSal = Double.parseDouble(annualSalString);
	}
	
	public void setDeptName()
	{
		deptName = JOptionPane.showInputDialog(null,"Enter your department name");
	}
	
	public int getSsn()
	{	return ssn;	}
	
	public double getAnnualSal()
	{	return annualSal;	}

	public String getDeptName()
	{	return deptName;	}
	
	public void display()
	{
		System.out.println("College employee information:");
		super.display();
		System.out.println("SSN: " + getSsn() + "\nAnnual Salary: "
                    + getAnnualSal() + "\nDepartment name: " + getDeptName());
	}

}

And I have two other Subclass that I wont because they are basically the same as this one above. here is the class that contains Main():

import javax.swing.*;
public class CollegeList
{
	public static void main(String[] args)
	{
		//declare the arrays of persons
		CollegeEmployee[] emp = new CollegeEmployee[4];
		Faculty[] fac = new Faculty[3];
		Student[] stu = new Student[7];
		
		int x = 0;
		int y = 0;
		int z = 0;
		String answer = "";
		
		//prompt user to chose the type of person
		answer = JOptionPane.showInputDialog(null,
                      "Enter the type of person you wish to enter the data for:\n"
                      + "C - College Employee\nF - Faculty\nS - Student \n\nQ - Quit\n ");
		
		while(!(answer.equals("Q")))
		{
			//Prompt user to enter College Employee data if he choses "C"
			if(answer.equals("C") && x < emp.length)
			{
				emp[x].setFirstName();
				emp[x].setLastName();
				emp[x].setAddress();
				emp[x].setZipCode();
				emp[x].setPhoneNum();
				emp[x].setSsn();
				emp[x].setAnnualSal();
				emp[x].setDeptName();
				
				emp[x].display();
				
				x++;
			}
			
			//Prompt user to enter College Employee data and states that the employee is a faculty if he choses "F"
			else if(answer.equals("F") && y < fac.length)
			{
				fac[y].setFirstName();
				fac[y].setLastName();
				fac[y].setAddress();
				fac[y].setZipCode();
				fac[y].setPhoneNum();
				fac[y].setSsn();
				fac[y].setAnnualSal();
				fac[y].setDeptName();
				
				fac[y].display();
				
				y++;
			}
			
			//Prompt user to enter Student data if he choses "S"
			else if(answer.equals("S") && y < fac.length)
			{
				stu[z].setFirstName();
				stu[z].setLastName();
				stu[z].setAddress();
				stu[z].setZipCode();
				stu[z].setPhoneNum();
				stu[z].setMajor();
				stu[z].setGpa();
				
				fac[y].display();
				
				z++;
			}
			
			//Display an error if the user enters anything else than C, F, S,or Q
			else
				JOptionPane.showMessageDialog(null,
                                     "Wrong entry, or you have reach the limit for that type of person");
				
			answer = JOptionPane.showInputDialog(null,
                                 "Enter the type of person you wish to enter the data for:\n"
                                 + "C - College Employee\nF - Faculty\nS - Student \n\nQ - Quit\n ");	
		}
	}
}

I have not finish the coding yet, but at this point I am getting the same error over and over, can't seem to fix it:


Exception in thread "main" java.lang.NullPointerException
at CollegeList.main(CollegeList.java:41)


It is the first time I see this error and I don't understand it, can anyone please help me.

[Paulo DeLimma]

The code is attempting to call methods on null references. In Java, we work with arrays of references to objects, not arrays of objects. So before any method call, the reference must be pointing to some object. If you call the constructor for each one of the elements you want to insert before calling setFirstName(), it will work fine. For example: emp[x] = new CollegeEmployee(), before emp[x].setFirstName().

[Christian Mucyo]

I appreciate your comment, it was very helpful.

But what about the display() method ? I am getting the same error each time I try to call it.
I called it like this:

emp[x].display();

[Ralph Cook]

The same principle as the previous answer gave you applies to each element of an array. If you execute "Object o[] = new Object()[4];", you only allocate space for 4 objects in an array. You have not yet allocated the objects IN the array, only the space to hold references TO those objects. So you would need to, at some point somehow, either execute something like "o[0] = new Object();", or "Object singleObject = new Object(); o[0] = singleObject;"

When you attempt emp[4].setFirstName();, if you haven't executed "new Employee()" in some fashion and assigned it to that element of the array, then that element holds null, and calling setFirstName() on it results in a null pointer exception.

[Christian Mucyo]

Thank you, I worked on it this morning and I got what you said.
Also one Last question, This program has to display at the end a report off all data entered for each type of person(CollegeEmployee, Faculty and Student)

And if there were no data entered for one type of person I have to display a message,
how can I search in the array if there is data or not?? I have never done that before and I have no clue.

[Campbell Ritchie]

Welcome to the Ranch

You would search for existence of data the same way you can check anything else. For example

for (Foo f : foos)
{
   if (f != null)
   {
      System.out.println(f.getBaa());
   }
}

Have a look in the API documentation and find what the println() method does, and how it copes with nulls and similar.

I think one object-oriented way to do this is to put such information into the toString() method in each class.

[Jeff Verdegan]

Campbell Ritchie wrote:Welcome to the Ranch

You would search for existence of data the same way you can check anything else. For example

for (Foo f : foos)
{
   if (f != null)
   {
      System.out.println(f.getBaa());
   }
}

Of course, one has to wonder why there's a null in that list in the first place. Is it really valid for that to be the case? It seems unlikely. And if there's no valid reason for there to be a null there, then the presence of one is an error, and we should not be coding around it--we should let it throw the exception, and if it does, then fix the buggy code that put the null there in the first place.

[Campbell Ritchie]

Agree, Jeff, you are right there.