What is causing my NullPointerException?

Question

My program compiles perfectly, but whenever I try to run it throws a NullPointerException. I tried searching this up, and found that the error relates to some value being null while the program is trying to use it, but I re-checked everything and came up blank.

Here's the error:

java.lang.NullPointerException at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source) at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source) at java.lang.reflect.Method.invoke(Unknown Source) at edu.rice.cs.drjava.model.compiler.JavacCompiler.runCommand(JavacCompiler.java:27‌2)

The code:

import javax.swing.*; import java.util.Scanner; public class WorldsMostBoringGame { public void main (String [] args) { System.out.println("You are in a room with a locked door and a key."); Scanner keyboard = new Scanner(System.in); boolean hasKey = false, doorOpen = false, amIDoneYet = false, monsterAlive = true; while (!amIDoneYet) { String userInput = keyboard.nextLine(); if (userInput == "look around") LookAround(hasKey); else if (userInput == "get key") GetKey(hasKey, monsterAlive); else if (userInput == "open door") OpenDoor(doorOpen, hasKey, amIDoneYet); else if (userInput == "kill monster") KillMonster(monsterAlive); else System.out.println(userInput+ " is not a recognized command."); } } public boolean GetKey(boolean hasKey, boolean monsterAlive) { if (hasKey == false && monsterAlive == false) System.out.println("You pick up the key."); else if (hasKey == true && monsterAlive == false) System.out.println("You already picked up the key."); else if (monsterAlive == true) { System.out.println("You must kill the monster first."); return hasKey = false; } return hasKey = true; } public void LookAround(boolean hasKey) { if (!hasKey) System.out.println("You are in a room with a locked door and a key."); else System.out.println("You are in a room with a locked door. You have a key."); } public boolean OpenDoor(boolean doorOpen, boolean hasKey, boolean amIDoneYet) { if (hasKey) { System.out.println("You unlock the door. Game over. You win."); amIDoneYet = true; return doorOpen; } else { System.out.println("The door is locked. Find a key."); return doorOpen = false; } } public boolean KillMonster(boolean monsterAlive) { System.out.println("You kill the monster."); return monsterAlive = false; } }

Could we see the error, including the line at which it occures — Richard Tingle
– Richard Tingle, Commented Oct 28, 2013 at 15:44
Additionally, don't try to compare strings with "==". See tinyurl.com/so-java-string-equality — Jon Skeet
– Jon Skeet, Commented Oct 28, 2013 at 15:45
I thought this is the first day since creation there is no == on Strings :( — Maroun
– Maroun, Commented Oct 28, 2013 at 15:45
@user2928683: Well that's not in your code at all, and it looks like you're using a custom Java compiler. What happens if you try to compile that code with a normal Java compiler and run it? — Jon Skeet
– Jon Skeet, Commented Oct 28, 2013 at 15:45
public void main (String [] args) should be public static void main (String [] args) — ccheneson
– ccheneson, Commented Oct 28, 2013 at 15:47

Reimeus · Accepted Answer · 2013-10-28 16:34:26Z

15

Add a static keyword to the main method so the application has a valid entry point

public static void main (String [] args) {

Edit:

Once this change is made create an instance of WorldsMostBoringGame such that the instance methods can be invoked:

WorldsMostBoringGame game = new WorldsMostBoringGame(); // create this instance while (!amIDoneYet) { String userInput = keyboard.nextLine(); if ("look around".equals(userInput)) { // use String.equals game.lookAround(hasKey); } ...

edited Oct 28, 2013 at 16:34

answered Oct 28, 2013 at 15:47

Reimeus

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5 Comments

Maroun Over a year ago

How did you see that :|

Rahul Over a year ago

Do add the info about using equals() method instead of == also. The user definitely needs to know that as well. :) And now that main is static, as a side effect, all the other methods need to be static as well.

Richard Tingle Over a year ago

Although this breaks everything else because main is written as if its not static. I renamed the existing main and recreated a new real main

user2928683 Over a year ago

Thanks, this fixed it! Made all the methods static, and runs like a champ.

Reimeus Over a year ago

@user2928683 I recommend creating an instance of WorldsMostBoringGame to avoid making those methods static

Junjie · Accepted Answer · 2013-10-29 15:07:42Z

import javax.swing.*; import java.util.Scanner; public class WorldsMostBoringGame { public static void main(String [] args) { System.out.println("You are in a room with a locked door and a key."); Scanner keyboard = new Scanner(System.in); boolean hasKey = false, doorOpen = false, amIDoneYet = false, monsterAlive = true; while (!amIDoneYet) { String userInput = keyboard.nextLine(); if (userInput.equals("look around")) LookAround(hasKey); else if (userInput.equals("get key")) GetKey(hasKey, monsterAlive); else if (userInput.equals("open door")) OpenDoor(doorOpen, hasKey, amIDoneYet); else if (userInput.equals("kill monster")) KillMonster(monsterAlive); else System.out.println(userInput+ " is not a recognized command."); } } public static boolean GetKey(boolean hasKey, boolean monsterAlive) { if (hasKey == false && monsterAlive == false) System.out.println("You pick up the key."); else if (hasKey == true && monsterAlive == false) System.out.println("You already picked up the key."); else if (monsterAlive == true) { System.out.println("You must kill the monster first."); return hasKey = false; } return hasKey = true; } public static void LookAround(boolean hasKey) { if (!hasKey) System.out.println("You are in a room with a locked door and a key."); else System.out.println("You are in a room with a locked door. You have a key."); } public static boolean OpenDoor(boolean doorOpen, boolean hasKey, boolean amIDoneYet) { if (hasKey) { System.out.println("You unlock the door. Game over. You win."); amIDoneYet = true; return doorOpen; } else { System.out.println("The door is locked. Find a key."); return doorOpen = false; } } public static boolean KillMonster(boolean monsterAlive) { System.out.println("You kill the monster."); return monsterAlive = false; } }

change all the method to static, and "==" should n't used to compare composed type

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