Null Error in Program

[Anai Anderson]

package World;

import java.util.ArrayList;
import java.util.Scanner;

public class Game {
	static ArrayList<Item> inventory = new ArrayList<Item>();

	public static void main(String args[]) {
		Room currentRoom = World.buildWorld();
		Scanner input = new Scanner(System.in);
		System.out.println(currentRoom);
		System.out.println("\nNow we'll move west!");
		currentRoom = currentRoom.getExit('w');
		System.out.println(currentRoom);
		System.out.println("\nNow we'll move east!");
		currentRoom = currentRoom.getExit('e');
		System.out.println(currentRoom);
		System.out.println("\nNow we'll move east!");
		currentRoom = currentRoom.getExit('e');
		System.out.println(currentRoom);
		System.out.println("\nNow we'll move south!");
		currentRoom = currentRoom.getExit('s');
		System.out.println(currentRoom);
		System.out.println("\nNow we'll move north!");
		currentRoom = currentRoom.getExit('n');
		System.out.println(currentRoom);
		System.out.println("\nNow we'll move up!");
		currentRoom = currentRoom.getExit('u');
		System.out.println(currentRoom);
		System.out.println("\nNow we'll move down!");
		currentRoom = currentRoom.getExit('d');
		System.out.println(currentRoom);
		String command;
		do {
			System.out.println(currentRoom);
			System.out.println("What do you want to do?");
			command = input.nextLine();
			String[] words = command.split(" ");
			switch (words[0]) {
			case "e":
			case "n":
			case "s":
			case "w":
			case "u":
			case "d":
				currentRoom = currentRoom.getExit(command.charAt(0));
				break;
			case "x":
				System.out.println("Thanks for walking through my game!");
				break;
			case "take":
				Item i = currentRoom.getItem(words[1]);
				if (i == null)
					System.out.println("There's nothing to take here.");
				else {
					inventory.add(i);
					System.out.println("You picked up " + i.getname());
				}
				break;
			case "look":
				Item i1 = currentRoom.getItem(words[1]);
				if (i1 == null) {
					for (int i2 = 0; i2 < inventory.size(); i2++) {
						Item ite = inventory.get(i2);
						if (words[1].equals(ite.getname())) {
							System.out.println(ite.getdesc());
						}
					}
					System.out.println("There is no such item in room or inventory.");
				} else {
					System.out.println(i1.getdesc());
				}
				break;
			case "i":
				System.out.println("You are carrying: ");
				for (Item it : inventory)
					System.out.println(it);
				break;
			default:
				System.out.println("I don't know what that means.");
			}
		} while (!command.equals("x"));
		input.close();
	}

I'm running into an error on line 26 when I try to run it. It gives this error: Exception in thread "main" java.lang.NullPointerException: Cannot invoke "World.Room.getExit(char)" because "currentRoom" is null at GameRoom/World.Game.main(Game.java:26)

[Carey Brown]

currentRoom = currentRoom.getExit('s');

This call prior to line 26 is the last place 'currrentRoom' has been set so that means the call to getExit('s') is returning null, you'll have to figure out why.

[Mike Simmons]

Well, it's usually a good idea to look at the error message:

because "currentRoom" is null at GameRoom/World.Game.main(Game.java:26)

So, why is currentRoom null?  Is there somewhere in your program where currentRoom is set to something other than null?  Where does that data come from?

Or, you could read what Carey already posted. Oh well, too late...

[Norm Radder]

What is printed by the println statement on line 24?

[Anai Anderson]

Carey Brown wrote:

currentRoom = currentRoom.getExit('s');

This call prior to line 26 is the last place 'currrentRoom' has been set so that means the call to getExit('s') is returning null, you'll have to figure out why.

I tried taking it off or changing it, but I'm still getting the same error...

[Mike Simmons]

Do you have the code to theRoom class?  Is that something you can change?  Or is it something you were given?  Can you at least look at it, and show it to us?

Alternately, looking at your main() method, it looks like you're basically wandering around a maze.  How do you know that it's actually possible to move in that direction?  Is it possible that the method returns null when it's not possible to move in that direction?  If that happens, you should probably not update currentRoom to be what getExit() returns, because if it's null, you no longer have a valid currentRoom reference.  

[Anai Anderson]

Mike Simmons wrote:Do you have the code to theRoom class?  Is that something you can change?  Or is it something you were given?  Can you at least look at it, and show it to us?

Alternately, looking at your main() method, it looks like you're basically wandering around a maze.  How do you know that it's actually possible to move in that direction?  Is it possible that the method returns null when it's not possible to move in that direction?  If that happens, you should probably not update currentRoom to be what getExit() returns, because if it's null, you no longer have a valid currentRoom reference.  

I actually was able to change it to something different now:

package World;
import java.util.Scanner;
import java.util.ArrayList;
import java.util.Scanner;

public class Game {
	static ArrayList<Item> inventory = new ArrayList<Item>();
	public static void main(String args) {
		runGame();
	}
	public static void runGame() {
		Room currentRoom = World.buildWorld();
		Scanner input = new Scanner(System.in);
		
		String command;
		do {
			System.out.println(currentRoom);
			System.out.println("What do you want to do?");
			command = input.nextLine();
			String[] words = command.split(" ");
			switch (words[0]) {
			case "e":
			case "n":
			case "s":
			case "w":
			case "u":
			case "d":
				currentRoom = currentRoom.getExit(command.charAt(0));
				break;
			case "x":
				System.out.println("Thanks for walking through my game!");
				break;
			case "take":
				Item i = currentRoom.getItem(words[1]);
				if (i == null)
					System.out.println("There's nothing to take here.");
				else {
					inventory.add(i);
					System.out.println("You picked up " + i.getname());
				}
				break;
			case "look":
				Item i1 = currentRoom.getItem(words[1]);
				if (i1 == null) {
					for (int i2 = 0; i2 < inventory.size(); i2++) {
						Item ite = inventory.get(i2);
						if (words[1].equals(ite.getname())) {
							System.out.println(ite.getdesc());
						}
					}
					System.out.println("There is no such item in room or inventory.");
				} else {
					System.out.println(i1.getdesc());
				}
				break;
			case "i":
				System.out.println("You are carrying: ");
				for (Item it : inventory)
					System.out.println(it);
				break;
			default:
				System.out.println("I don't know what that means.");
			}
		} while (!command.equals("x"));
		input.close();
	}
}

right now its just saying this error now: Error: Main method not found in class World.Game, please define the main method as: public static void main(String[] args) or a JavaFX application class must extend javafx.application.Application

package World;

public class World {
	public static Room buildWorld() {
		Room bedroom = new Room("You are in your Bedroom.");
		Room bathroom = new Room("You are in the bathroom.");
		Room hallway = new Room("You are in the hallway.");
		Room attic = new Room("You are in the attic.");
		Room livingroom = new Room("You are in the living room.");
		Room kitchen = new Room("You are in the kitchen.");
		bedroom.addExit('e', hallway);
		hallway.addExit('w', bedroom);
		bedroom.addExit('w', bathroom);
		bathroom.addExit('e', bedroom);
		hallway.addExit('u', attic);
		attic.addExit('d', hallway);
		livingroom.addExit('n', hallway);
		livingroom.addExit('w', kitchen);
		Item key = new Item("This is a key.", "key");
		livingroom.setItem(key);
		Item phone = new Item("This is a phone.", "phone");
		bedroom.setItem(phone);
		Item sandwich = new Item("This is a sandwich.", "sandwich");
		kitchen.setItem(sandwich);
		Item mirror = new Item("This is a mirror.", "mirror");
		bedroom.setItem(mirror);
		Item book = new Item("This is a book.", "book");
		bedroom.setItem(book);
		Item flashlight = new Item("This is a flashlight.", "flashlight");
		bedroom.setItem(flashlight);
		
		return bedroom;
	}
}

[Tim Holloway]

Note: I added code tags to the previous post to make it readable.

[Carey Brown]

            case "e":
            case "n":
            case "s":
            case "w":
            case "u":
            case "d":
                currentRoom = currentRoom.getExit(command.charAt(0));
                break;

You still have a problem here that getExit() can return null and you are not handling that.

[Tim Holloway]

Pedantic note here: While it is syntactically valid to start a package name component with an upper-case letter, it is a standard convention that package name components should start with lower-case. Similar conventions apply to class and member/method naming capitalizations.

Also, it's common, though not required to make your package names be world-unique by making the high-level elements of it to reflect the organization you're working for. Now, it's perfectly OK not to make a package name be something like "edu.ucf.cop0999.jsmith.project1", but if you want to share your code with the world, it's helpful.

And of course, while Windows likes to create directories with names that start uppercase, the rest of the world, including Java, generally does not, and Java is case-sensitive, so if you do rename your package from "World" to "world", make sure you package directory name is also changed!

On a less pedantic level, it's worth noting that in practical application setting up a dungeon like this is often easier to do declaratively. If, rather than coding an explicit statement for each room and exit, you had a more general algorithm, you could even load up the architecture from a config file such as a Java properties file or a structured data file such as YAML or XML and be able to completely change the world without altering the program code.