Exception in thread "main" java.lang.NullPointerException

[Garrett Nickel]

The code compiles just fine, but when I try to run the Launcher, it gives me the message:

Exception in thread "main" java.lang.NullPointerException
at HundredGame.startGame(HundredGame.java:19)
at GameLaunch.main(HundredGame.java:79)

my code is:

public class HundredGame {
	Player p1;
	Player p2;
	Player p3;
	Player p4;
	public void startGame() {
		int p1guess=0;
		int p2guess=0;
		int p3guess=0;
		int p4guess=0;
		boolean p1isCorrect=false;
		boolean p2isCorrect=false;
		boolean p3isCorrect=false;
		boolean p4isCorrect=false;
		int CorrectNumber=(int) (Math.random() * 101);
		System.out.println("I am thinking of a number from 0 to 100. Whichever player guesses the correct number first wins!");
		while(true) {
			System.out.println("The number I am thinking of is " + CorrectNumber);
			p1.guess();
			p2.guess();
			p3.guess();
			p4.guess();
			p1guess=p1.number;
			p2guess=p2.number;
			p3guess=p3.number;
			p4guess=p4.number;
			System.out.println("Player 1 has guessed the number " + p1guess);
			System.out.println("Player 2 has guessed the number " + p2guess);
			System.out.println("Player 3 has guessed the number " + p3guess);
			System.out.println("Player 4 has guessed the number " + p4guess);
			if (p1guess==CorrectNumber) {
				p1isCorrect=true;
			}
			if (p2guess==CorrectNumber) {
				p2isCorrect=true;
			}
			if (p3guess==CorrectNumber) {
				p3isCorrect=true;
			}
			if (p4guess==CorrectNumber) {
				p4isCorrect=true;
			}
			if (p1isCorrect||p2isCorrect||p3isCorrect||p4isCorrect) {
				System.out.println("Looks like we have a winner!!");
				System.out.println("Player one guessed correctly--" + p1isCorrect);
				System.out.println("Player two guessed correctly--" + p2isCorrect);
				System.out.println("Player three guessed correctly--" + p3isCorrect);
				System.out.println("Player four guessed correctly--" + p4isCorrect);
				System.out.println("Game Over!");
				if (p1isCorrect) {
					System.out.print("Player 1 wins!");
				}
				if (p2isCorrect) {
					System.out.print("Player 2 winS!");
				}
				if (p3isCorrect) {
					System.out.print("Player 3 wins!");
				}
				if (p4isCorrect) {
					System.out.print("Player 4 wins!");
				}
				break;
			} else {
				System.out.println("Nobody got it right! Try Again!");
			}
		}
	}
}
class Player {
	int number=0;
	public void guess() {
		number=(int) (Math.random() * 101);
		System.out.print("I am guessing the number " + number);
	}
}
class GameLaunch {
	public static void main(String[] args) {
		HundredGame game=new HundredGame();
		game.startGame();
	}
}

Please help, I'm very new to java

[Garrett Rowe]

p1 - p4 are declared, but never initialized. Therefore they are null, and the code fails with a NullPointerException. You must initialize all variables before you use them, i.e.:

p1 = new Player();

[Henry Wong]

Exception in thread "main" java.lang.NullPointerException
at HundredGame.startGame(HundredGame.java:19)
at GameLaunch.main(HundredGame.java:79)

The stacktrace gives the full details. On line 19 of the HundredGame.java file, in the startGame() method is code, which when called gives a null pointer exception. A null pointer exception is generally thrown when you are trying to dereference (ie. use) an object which hasn't been instantiated.

Henry

[Garrett Nickel]

Thanks, it works now.