Need help with Null Pointer Exception (program using inheritance)

**kali1980** · 06-26-2011, 06:19 PM

I am writing a program for class. The specifications for this program was that the given
TestSmallRoom.java coding MUST NOT CHANGE. The Person class was written by myself a couple of weeks ago. We were instructed to make 3 new classes, Tutor ( extends Person ), Student ( extends Person ), and SmallRoom, which represents a small room in which only 1 person can occupy at a time. The output of TestSmallRoom.java should output various statistics of objects that are changed throughout the program's execution.

I am not understanding why i am getting a null pointer exception, yet alone having the error point to coding that works.

Can anyone help me with this?

Java Code:

public class Person
{
	
	private	String	personName;
	private	int	dollars;
	private	boolean	happy;
	
	//constructor initializes the Person's money
	public Person( String name, int money, boolean bool )
	{
		setPersonName(name);
		
		if( money >= 0 )
		{
			setDollars(money);
		}
		else
		{
			setDollars(50);
		}
		
		setHappy(bool);
		
	}//end constructor
	
	//method to set personName
	public void setPersonName( String name )
	{
		personName = name;
	}
	//method to get personName
	public String getPersonName()
	{
		return personName;
	}
	
	//method to set personMoney
	public void setDollars( int money )
	{
		if( money >= 0 )
		{
			dollars = money;
		}
		else
		{
			dollars = 50;
		}
	}
	//method to get personMoney
	public int getDollars()
	{
		return dollars;
	}
	
	//method to set happy
	public void setHappy( boolean bool )
	{
		happy = bool;
	}
	//method to get happy
	public boolean getHappy()
	{
		return happy;
	}
	//method to determine if someone is happy
	public boolean isHappy()
	{
		return ( getHappy() ? true : false );
	}
	
	
	//public method to spend
	public void spend( int amtSpent )
	{
		if( amtSpent >= 0 )
		{
			if( amtSpent <= getDollars() )
			{
				setDollars( getDollars() - amtSpent );
			}//end if
			else
			{
				setDollars( getDollars() - getDollars() );
			}//end else
		}//end if
	}//end method spend
	
	//method to determine if person can afford to spend	
	public boolean canAfford( int amount )
	{
		return ( ( getDollars() >= amount ) ? true : false );
	}//end method canAfford
	
	//method to earn money
	public void earned( int amtEarned )
	{
		if( amtEarned >= 0 )
		{
			setDollars( getDollars() + amtEarned );
		}
	}
	
	
	//method to say something
	public void says()
	{
		if( getHappy() == true )
		{
			System.out.println( "My name is " + getPersonName() + " and I am doing great!" );
		}
		else
		{
			System.out.println( "My name is " + getPersonName() + " and I could be better." );
		}
	}
	
	
	
	//method toString overrides superclass toString method.
	@Override
	public String toString()
	{
		return String.format( "%s %s %s %s", getPersonName(), "has",
									getDollars(), "dollars");
	}
		
}// end class Person

Java Code:

// Student.java
// The Student class is just made up of a constructor and a toString method.

class Student extends Person
{
	private String studentMajor;  // keep track of the student's major
	
	// constructor
	public	Student( String name, int money, boolean bool, String major )
	{
		super( name, money, bool );
		
		studentMajor = major;
	}// end Student constructor
	
	// toString method
	@Override	// indicates this method overrides a superclass method
	public	String	toString()
	{
		return String.format( "%s %s, %s %s", "student", super.toString(),
									"major is",	studentMajor );
	}
}

Java Code:

public class Tutor extends Person
{
	private double hourlyRate;  // the hourly rate that the tutor charges	
	
	// constructor
	public	Tutor( String name, int money, boolean bool, double rate )
	{
		super( name, money, bool );
		
		if( rate >= 0.0 )
		{
			hourlyRate = rate;
		}
		else
		{
			hourlyRate = 10.0;
		}
	}// end Tutor constructor
	
	// toString method
	@Override	// indicates this method overrides a superclass method
	public	String	toString()
	{
		return String.format( "%s %s %s %s %s $%.2f %s", "tutor", super.getPersonName(),
									"has", super.getDollars(), "dollars and charges",
									hourlyRate, "per hour" );
	}
}

Java Code:

// SmallRoom.java
// In addition to constructor(s) and toString methods, the SmallRoom class
// will have the following methods:  isEmpty,  isOccupied,  enter,  leave 


public class SmallRoom
{	
	private Person occupant;  // make up a name for the person who is occupying the room
	private String room;  // this field represents the name of the room
	
	// constructor
	public SmallRoom( String roomNumber )
	{
		room = roomNumber;
	}
	
	// toString method
	@Override	// indicates this method overrides a superclass method
	public	String	toString()
	{
		if( room.isEmpty() )
		{
			return String.format( "%s %s %s", "room", room.toString(), "is empty" );
		}
		else
		{
			return String.format( "%s %s %s %s", "room", room.toString(), "is occupied by",
										occupant.toString() );
		}
	}
	
	// isEmpty method
	public	boolean	isEmpty()
	{
		if( occupant == null )
			return true;
		else
			return false;
	}
	
	// isOccupied method
	public	boolean	isOccupied()
	{
		if( occupant != null )
			return true;
		else
			return false;
	}
	
	// enter method
	public	void	enter( Person person )
	{
		occupant = person;
	}
	
	// leave method
		
}

Java Code:

public class TestSmallRoom
{
	public static void main(String [] args)
	{	
		Tutor alan = new Tutor("Jackson", 100, true, 26.00);
		Tutor carol = new Tutor("Bennett", 100, true, -32.00);  // bad rates default to $10.00
		
		Student happy = new Student("Smiles", 500, true, "Software Engineering" );
		Student ming = new Student("Li", 75, false, "Accounting");
		Student prakash = new Student("Ramanan", 110, false, "Nursing" 		);
		Student bill = new Student("Clinton", 5000, true, "Poli Sci");
		
		alan.earned(52);
		bill.spend(3300);
		
		System.out.println(alan);
		System.out.println(carol);
		System.out.println(happy);
		System.out.println(ming);
		
		SmallRoom room1 = new SmallRoom("F105");
		SmallRoom room2 = new SmallRoom("G110");
		SmallRoom room3 = new SmallRoom("H300");
		
		if (room1.isEmpty())
			room1.enter(alan);

		if (room2.isEmpty())
			room2.enter(prakash);

		System.out.println(room1);     
		System.out.println(room2);
		System.out.println(room3);     // this line produces a null pointer exception, BUT
                                                                   // the exception shows the coding for the code
                                                                  // that did print correctly.


// THE FOLLOWING CODE IS COMMENTED OUT BECAUSE THE PROGRAM IS UNFINISHED
		
//		room1.enter(bill);  // rejected since already occupied
		
//		Person p = null;
		
		// The "leave" method set the room to empty and returns a
		// reference to whoever was in the room.  Below, the reference
		// p will point to student prakash and room2 becomes empty.
		// A null reference is returned if the room was already empty
		// when the leave method was called.
		
//		if (room2.isOccupied())
//			p = room2.leave();

			
//		System.out.println("Just left room 2: " + p);
//		System.out.println("Room2 is now: " + room2);
		
//		SmallRoom room4 = new SmallRoom("L002");
//		room3.enter(bill);
		
//		room4.enter(room3.leave());  // Bill moves from room3 to room4
		
//		System.out.println("AT THE END:");
//		System.out.println(room1);
//		System.out.println(room2);
//		System.out.println(room3);
//		System.out.println(room4);
	}
}

**Norm** · 06-26-2011, 06:38 PM

I am not understanding why i am getting a null pointer exception,

Please copy and paste here the full text of the error message.
It has the line number where the error occurred.
If you can find that line in your code, see what variable is null and back track to find out why.
If you can not tell which one is null, Add a println statement before the statement to show all the variables' values used on that line.

**doWhile** · 06-26-2011, 06:39 PM

You've posted tons of code without the full stack trace. Your best bet to help you get advice is to do one or both of the following: 1) post an SSCCE 2) post the full stack trace of the exception whose line numbers line up with the posted code

**Fubarable** · 06-26-2011, 07:29 PM

You're calling isEmpty on the room String which is the wrong entity for that method. You don't care if the String name for the room has text or not, but rather if the room is occupied or not. Perhaps you should call this method on another object (or another redundant method in the class). But I also agree, when you have an error that you need help with, please post the entire exception message and indicate which lines are throwing the exception.

**CGHMN** · 06-27-2011, 08:25 AM

Here is the error he is getting

symbol : method leave()
location: class SmallRoom
p = room2.leave();
^
TestSmallRoom.java:55: cannot find symbol
symbol : method leave()
location: class SmallRoom
room4.enter(room3.leave()); // Bill moves from room3 to room4