Thread: When I run my program, it says error: java.lang.NullPointerException

Hello,

I need some help, because I just don't see it!

This is my code:

 
class GameOfLife{  // main class
  Field field = new Field();
  int numberOfGenerations;
  Scanner scanner;
  File source;
  String filename = "GameOfLifeInput.txt";
  char[] charArray;
 
  public static void main(String[]args){ // starts up program
    new GameOfLife().play();                                                                   //This is line 18
  } // end of method
 
  void readInitial(String filename){ // reads input from file
    try{
      scanner = new Scanner(source);
      String line = null;
      source = new File(filename);
      int row = 0;
      int col = 0;
      LineNumberReader lnr = new LineNumberReader(new FileReader(new File(filename)));
      lnr.skip(Long.MAX_VALUE);
      lnr.getLineNumber();
      Cell[][] cells = new Cell[lnr.getLineNumber()][];
 
      while (scanner.hasNext()){
        line = scanner.nextLine();
        char[] charArray = line.toCharArray();
        cells[row] = new Cell[charArray.length];
        for (Character c : charArray){
          Cell cell = new Cell();
          if (c == ' '){
            cell.setAlive(false);
          }else{
            cell.setAlive(true);
          }
          cells[row][col] = cell;
          ++col;
        }
      ++row;
      }
      field.setField(cells);
    }catch(FileNotFoundException e){
      System.out.println("Could not find or open file due to \n"+e);
    }catch (IOException e) {}
 
  } // end of method 
 
  void play(){ //displays the generations
    try{ 
      int i = 0;
      numberOfGenerations = 0;
      System.out.println("How many generations do you want to see?");
      numberOfGenerations = scanner.nextInt();                                                                   // this is line 61
      readInitial(filename);
      while(i < numberOfGenerations){
        field.print();
        i++;
      }
 
      Thread.sleep(2000);
    }catch(InterruptedException e){
    }
  }
} // end of class

This is the output:

Welcome to DrJava. Working directory is C:\Users\Esteban\Documents\School\2IS60 - App Programming
> run GameOfLife

How many generations do you want to see?
java.lang.NullPointerException
at GameOfLife.play(GameOfLife.java:61)
at GameOfLife.main(GameOfLife.java:18)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Nativ e Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknow n Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Un known Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at edu.rice.cs.drjava.model.compiler.JavacCompiler.ru nCommand(JavacCompiler.java:272)
>

Null pointer means, that a variable on a corresponding line is set to null. Go to this line, locate all the non-primitive variables that are there and track, whether they are initialized or not.

Originally Posted by angstrem

Null pointer means, that a variable on a corresponding line is set to null. Go to this line, locate all the non-primitive variables that are there and track, whether they are initialized or not.

Thank you for your fast reply!

I tried to locate it, but I just don't see it. Only in this class the variabel numberOfGenerations is used and it is initialized just before the user is asked for the number of generations.

Yes, you have to initialize variables. And, as I can see, you really have code for initializing variables. But the code is executed sequentially, statement after statement. If the statement on 61st line says, that "here, I have variable that is not initialized", then what is it supposed to mean?

Originally Posted by angstrem

Yes, you have to initialize variables. And, as I can see, you really have code for initializing variables. But the code is executed sequentially, statement after statement. If the statement on 61st line says, that "here, I have variable that is not initialized", then what is it supposed to mean?

Stupid of me! scanner = new Scanner (System.in);

Thank you!