Head First Jave - Code Example - NullPointerException

[Michael Francis]

I'm going through the Head First Java book, and I am working on an example code for a mini battleship game.

When I run the program(using eclipse) I get

Exception in thread "main" java.lang.NullPointerException
	at battleship.DotComBust.setUpGame(DotComBust.java:19)
	at battleship.DotComBust.main(DotComBust.java:70)
    

Here are the three class files.

package battleship;

import java.util.*;

public class DotComBust {
	private GameHelper helper = new GameHelper();
	private ArrayList<DotCom> dotComsList;
	private int numOfGuesses = 0;
	
	
	private void setUpGame(){
		//first make some dot coms and give them locations
		DotCom one = new DotCom();
		one.setName("Pets.com");
		DotCom two = new DotCom();
		two.setName("eToys.com");
		DotCom three = new DotCom();
		three.setName("Go2.com");
		dotComsList.add(one);
		dotComsList.add(two);
		dotComsList.add(three);
		
		System.out.println("Your goal is to sink three dot coms.");
		System.out.println("Pets.com, eToys.com, Go2.com");
		System.out.println("Try to sink them all in the fewest number of gueses");
		
		for (DotCom dotComToSet : dotComsList){
			ArrayList<String> newLocation = helper.placeDotCom(3);
			dotComToSet.setLocationCells(newLocation);
		}// close for loop
	}// close setUpGame method
	
	private void startPlaying(){
		while(!dotComsList.isEmpty()){
			String userGuess = helper.getUserInput("Enter a guess");
			checkUserGuess(userGuess);
		}//close while
		finishGame();
	}//close the startPlaying method
	
	private void checkUserGuess(String userGuess){
		numOfGuesses++;
		String result = "miss";
		for (DotCom dotComToTest : dotComsList) {
			result = dotComToTest.checkYourself(userGuess);
			if (result.equals("hit")){
				break;
			}
			if (result.equals("kill")){
				dotComsList.remove(dotComToTest);
				break;
			}
		}//close for
		System.out.println(result);
	}//close method
	
	private void finishGame(){
		System.out.println("All Dot Coms are dead! Your Stock is now worless.");
		if (numOfGuesses <= 18) {
			System.out.println("It only took you " + numOfGuesses + " guesses.");
			System.out.println("You got out before your options sank.");
		} else {
			System.out.println("Took you long enough. "+ numOfGuesses + " guesses.");
			System.out.println("Fish are dancing with your options.");
		}
	}
	
	public static void main (String[] args) {
		DotComBust game = new DotComBust();
		game.setUpGame();
		game.startPlaying();
	}//close method
}
    

package battleship;

import java.util.*;

public class DotCom {
	
	private ArrayList<String> locationCells;
	private String name;

	public void setLocationCells(ArrayList<String> locs){
		locationCells = locs;
	}
	
	public void setName(String n){
		name = n;
	}
	
	public String checkYourself(String userInput){
		
		String result = "miss";
		
		int index = locationCells.indexOf(userInput);
		
		if (index >= 0) {
			locationCells.remove(index);
			
			if (locationCells.isEmpty()) {
				result = "kill";
				System.out.println("Ouch! You sunk " + name + "  :(");
			} else {
				result = "hit";
			}//close if
		
		}// close outer if
		
		
		
		return result;
		
	}// close method
	
	
}// end of class
    

package battleship;

import java.io.*;
import java.util.*;

public class GameHelper {
	
	private static final String alphabet = "abcdefg";
	private int gridLength = 7;
	private int gridSize = 49;
	private int[] grid = new int[gridSize];
	private int comCount=0;
	
	public String getUserInput(String prompt){
		String inputLine = null;
		System.out.print(prompt + " ");
		try {
			BufferedReader is = new BufferedReader(new InputStreamReader(System.in));
			inputLine = is.readLine();
			if (inputLine.length() == 0) return null;
		}
		catch (IOException e) {
			System.out.println("IOException: " + e);
		}
		return inputLine.toLowerCase();
	}
	
public ArrayList<String> placeDotCom(int comSize) {
		ArrayList<String> alphaCells = new ArrayList<String>();
		String[] alphcoords = new String[comSize];
		String temp = null;
		int[] coords = new int [comSize];
		int attempts = 0;
		boolean success = false;
		int location = 0;
		
		comCount++;
		int incr = 1;
		if ((comCount % 2) == 1){
			incr = gridLength;
		}
		
		while (!success & attempts++ < 200){
			location = (int) (Math.random() * gridSize);
			int x = 0;
			success = true;
			while (success && x < comSize) {
				if(grid[location] == 0){
					coords[x++] = location;
					location += incr;
					if (location >= gridSize) {
						success = false;
					}
					if (x>0 && (location % gridLength == 0)){
						success = false;
					}
				
				} else {
					success = false;
				}
			}
		}
		int x = 0;
		int row = 0;
		int column = 0;
		
		while (x< comSize){
			grid[coords[x]] = 1;
			row = (int) (coords[x] / gridLength);
			column = coords[x] % gridLength;
			temp = String.valueOf(alphabet.charAt(column));
			alphaCells.add(temp.concat(Integer.toString(row)));
			x++;
		}
		return alphaCells;
	}
}

	
   

I also notice that in line 30 of GameHelper.java there is this line

		String[] alphcoords = new String[comSize];    

however, alphcoords is never used throughout the program again. Could this be causing the error?

Thanks in advance!

[Rob Spoor]

You never initialize dotComsList. The NullPointerException tells you exactly on which line it occurs (DotComBust.java, line 19). Then you look on that line for any dereferencing. In this case, the line is dotComsList.add(one); and only dotComsList is dereferenced. Therefore it must be null.

[Ireneusz Kordal]

The problem is in line 07:

  private ArrayList<DotCom> dotComsList;     

dotComList is not initialized (is null), and program throws NullPointerException if you try to use it.
You must create an array before you can use it

dotComList = new ArrayList<DotCom>()

[Michael Francis]

Thanks a lot, I should have caught that.

If there are moderators, this thread can be closed.

[Campbell Ritchie]

Welcome to JavaRanch

Aarr, there be moderators . . .

But we don't usually close threads. We simply let them drift off into the archives, in case somebody thinks of something useful to say later on.

[Rob Spoor]

Although of course not too much later