NullPointerExcpetion Help

[Jack O'Herrer]

Hey Everyone, I'm hoping that one of you can help me with a problem I'm having. The code I'm trying to run is compiling fine, but I'm getting a NullPointerException error when I'm actually trying to run it. What I think is happening is the stack is for some reason being initialized with a null value at the beginning.

Any help would be greatly appreciated. Heres the code

import java.util.Scanner;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.FileNotFoundException;
import java.io.PrintWriter;
 
 
 
public class P5
{
  public static void main(String[] args)
  {
    String operation = "";
    char operand = ' ';
    Stack stack[] = new Stack[3];
    int linecounter = 0;
    boolean error = false;
    Scanner inFile = null;
    PrintWriter outFile = null;
    
    try
    {
      inFile = new Scanner(new FileInputStream("hanoi.txt"));
      outFile = new PrintWriter(new FileOutputStream("p5.txt"));
    }
    catch(FileNotFoundException e)
    {
      System.out.println("File Hanoi.txt was not found or could not be read");
      System.exit(0);
    }
   
    stack[0] = new Stack(inFile.next());
    stack[1] = new Stack(" ");
    stack[2] = new Stack(" ");
    inFile.nextLine();
    System.out.println(stack[0]);
    
    while(inFile.hasNextLine() == true)
    {
      operation = inFile.next();
      System.out.println(operation + operation.substring(0,3));
      if (operation.substring(0,3).equals("pus"))
      {
        if (operand != ' ')
        {
           int i = operation.charAt(4);
           stack[i].push(operand);
        }
        else
        {
          outFile.println("Explain the illegal error at line " + linecounter);
        }     
      }
      
      else if (operation.substring(0,3).equals("pop"))
      {
        int i = Character.getNumericValue(operation.charAt(3));
        System.out.println(i);
        operand = stack[i].getTop();
        stack[0].pop();
      }
      
      else
      {
        outFile.println("Explain the illegal error at line " + linecounter);
        error = true;
      }
      
      outFile.println("operand = " + operand);
      inFile.nextLine();
      linecounter++;          
    }
    outFile.println("Stack0 = " + stack[0] + "Stack1 = " + stack[1] + "Stack2 = " + stack[2]);
  }
}
 
class Stack
{
  private String stack;
  
  public Stack()
  {
    String stack = " ";
  }
  
  public Stack(String s)
  {
    String stack = s;
  }
  
  public char pop()
  {
    char a = stack.charAt(stack.length() - 1);
    stack = stack.substring(0, stack.length()-1);
    return a;
  }
  
  public void push(char a)
  {
    stack = stack + a;
  }
  
  public char getTop()
  {
    return 'a';//stack.charAt(stack.length() - 1);
  }
  
  public boolean isEmpty()
  {
    if(stack.length() == 0) return true;
    else return false;
  }
  
  public String toString()
  {
    return stack;
  }
  
  public boolean equals(String s)
  {
    if (stack.equals(s) == true) return true;
    else return false;
  }
}
           
           
 
      
       

[ February 21, 2008: Message edited by: Jack O'Herrer ]
[ February 21, 2008: Message edited by: Jack O'Herrer ]

[Garrett Rowe]

class Stack
{
  private String stack;
 
  public Stack()
  {
    String stack = " ";
  }
  

I haven't read through all your code, but you do have a problem here. The String you are assigning in your constructor isn't the same as the String you declare as an instance variable. Can you figure out why?

[Jack O'Herrer]

Aha! Thanks, I didn't realize that I reinitialized the string variable.

Now that thats fixed, I'm having problems with writing to the output file. Even when I try to hardcode the line

outFile.println("This is a test");

I'm getting nothing in the file P5.txt

Thanks again for the help

[Garrett Rowe]

Try closing your PrintWriter when you are done writing, aside from that just being a good habit to get into, it also automatically calls the flush() method to ensure all the data in the buffer gets written to the output file.

[Campbell Ritchie]

Another suggestion: you are not implementing the equals() method correctly; it ought to take Object as a parameter type.
Please do a search for equals(); it is discussed frequently on these boards.