nullpointerexception error

[Theresa Marlin]

I had to write a program that created a pantry which was an array of three jams. When I tried to print out my array, I got a nullpointerexception. What is a nullpointer exception? How can they be fixed?
Tester:

   class PantryTester
{
  public static void main ( String[] args )
  {
    Jam goose = new Jam( "Gooseberry", "7/4/86", 12 );
    Jam apple = new Jam( "Crab Apple", "9/30/99", 8 );
    Jam rhub  = new Jam( "Rhubarb", "10/31/99", 3 );

    Pantry hubbard = new Pantry( goose, apple, rhub );
    hubbard.print();

    hubbard.select(1);
    hubbard.spread(2);
    hubbard.print();

    hubbard.select(3);
    hubbard.spread(4);
    hubbard.print();
  }
}
 

Class 1:

  class Pantry
{
  private Jam  jar1 ;
  private Jam  jar2 ;
  private Jam  jar3 ;
  private Jam  selected ;
  Pantry( Jam jar1, Jam jar2, Jam jar3 )
  {
    Jam [] pantry1 = new Jam[3];
    pantry1[0] = jar1;
    pantry1[1] = jar2;
    pantry1[2] = jar3;
  }
  
  public void print()
  {
    Jam [] pantry1 = new Jam[3];
    for(int j = 0; j < pantry1.length; j++)
    {
      
      pantry1[j].print();
    }
  }
  public boolean select( int jarNumber )
  {
    selected = null;
    if ( jarNumber == 1 )
      selected =  jar1 ;

    else if ( jarNumber == 2 )
      selected = jar2 ;

    else 
      selected = jar3 ;
    
    if (selected == null)
      return false;
    else return true;
    
  }

  // spread the selected jam
  public void spread( int oz )
  {
    selected.spread( oz ) ;
  }
  
  public void replace( Jam j, int slot )
  {
    Jam currentSelection;
    if (slot == 1)
      currentSelection = jar1;
    if (slot == 2)
      currentSelection = jar2;
    else 
      currentSelection = jar3;
    currentSelection = j;
    if (currentSelection == jar1)
      jar1 = currentSelection;
    if (currentSelection == jar2)
      jar2 = currentSelection;
    else 
      jar3 = currentSelection;
    }
  
  public void mixedFruit()
  {
    if(jar1.getCapacity() <= 2 && jar2.getCapacity() <= 2 && jar3.getCapacity() <=2)
    {
      Jam mix = new Jam("Mixed Fruit", "7/4/86", (jar1.getCapacity() + 
                                                  jar2.getCapacity() + jar3.getCapacity()));
      jar1 = mix;
      jar2 = null;
      jar3 = null;
    }
  }
}
  

  

Class 2

  import java.io.*;
class Jam
{
  // Instance Variables
  String contents ;   // type of fruit in the jar
  String date ;      // date of canning
  int capacity ;      // amount of jam in the jar


  // Constructors
  Jam( String contents, String date, int size )
  {
    this.contents = contents;
    this.date = date;
    capacity = size;
  }

  // Methods
  public boolean empty ()
  {
    return ( capacity== 0 ) ;
  }

  public void print ()
  {
    System.out.println(contents + "     " + date + "    " + capacity + " fl. oz.");
  }

  public void spread ( int fluidOz)
  {
    if ( !empty() )
    {
      if ( fluidOz <= capacity )
      {
        System.out.println("Spreading " + fluidOz + " fluid ounces of "
            + contents );
        capacity = capacity - fluidOz ;
      }
      else
      {
        System.out.println("Spreading " + capacity + " fluid ounces of "
            + contents );
        capacity =  0 ;
        System.out.println("No jam in the Jar!");
      }
          System.out.println("");
          
     }
    else 
        System.out.println("No jam in the Jar!");
  }
  public int getCapacity()
  {
    return capacity;
  }

}
  

Thank you for your help!!!

[Sebastian Janisch]

hi.

first, please always post the exact exception so it's easier to find the piece of code that causes trouble.

I went through it real quick and think that I found the spot.

  public void print()  
  {  
    Jam [] pantry1 = new Jam[3];  
    for(int j = 0; j < pantry1.length; j++)  
    {  
        
      pantry1[j].print();  
    }  
  }    

What you do is create a Jam array as a local variable but don't put any values in it. Hence the value of each index is null which means it has not gotten a reference yet.

Upon invoking a method on a variable that is null, you'll get a big fat nullpointer exception.

What you want to do is make your Jam array an instance variable and pass the values in the constructor.

[Geoff Prewett]

Sebastian's answer looks like the problem, to me, too. Also, I noticed you're assigning things to the 'patry1' local variable in the Pantry constructor. Since you don't do anything with them, I assume that you probably meant them to persist, in which case you would want to assign them to a member variable. Something like:

class Pantry
{
   private Jam[] items;
   public Pantry(Jam jar1, Jam jar2, Jam jar3)
   {
      items[0] = jar1;
      items[1] = jar2;
      items[2] = jar3;
   }
   public void print()
   {
      for (Jam j : items)
         j.print();
   }
}

(the foreach loop iterates over every item in the array. You probably do not want "for (int i = 0; i < 3; i++)" since if you ever change the length of the array your code will break. "for (int i = 0; i < items.length; ++i)" is better, but the foreach loop is more concise.)

The NullPointerException name is a bit confusing, because what most CS people call "pointers", Java usually calls "references". Basically if you try to do anything will a null reference besides checking to see if it's null, you'll get a NullPointerException. Look at the output window and you'll see a stack trace that points you to the exact line that causes the problem.