Question

Here’s the code I have for the core program itself:

import java.util.Scanner;
import static java.lang.System.*;

public class RockPaperScissors
{
private String playChoice;
private String compChoice;

public RockPaperScissors()
{



}

public RockPaperScissors(String player)
{
    playChoice = player;
}

public void setPlayers(String player)
{
    playChoice = player;
    compChoice = "";
    int num;
    num = (int) (Math.random()*3);
    switch(num)
    {
    case 0 : compChoice = "R";break;
    case 1 : compChoice = "P";break;
    case 2 : compChoice = "S";break;
    }
    
    System.out.println(num + " " + compChoice);
    out.print(compChoice);
}

public String determineWinner()
{
    String winner="";
    
    if(playChoice == "R")
    {
        switch(compChoice)
        {
        case "R" : winner = "!Draw Game!";break;
        case "P" : winner = "!Computer wins <<Paper Covers Rock>>!";break;
        case "S" : winner = "!Player wins <<Rock Breaks Scissors>>!";break;
        }
    }
    else if(playChoice == "P")
    {
        switch(compChoice)
        {
        case "R" : winner = "!Player wins <<Paper Covers Rock>>!";break;
        case "P" : winner = "!Draw Game!";break;
        case "S" : winner = "!Computer wins <<Scissors Cuts Paper>>!";break;
        }
    }
    else if(playChoice == "S")
    {
        switch(compChoice)
        {
        case "R" : winner = "!Computer wins <<Rock Breaks Scissors>>!";break;
        case "P" : winner = "!Player wins <<Scissors Cuts Paper>>!";break;
        case "S" : winner = "!Draw Game!";break;
        }
    }
    return winner;
}

public String toString()
{
    String output="";
    
    output = "player had " + playChoice + "\n computer had " + compChoice + "\n " + determineWinner();
    
    return output;
}
}

and here’s my runner class:

import java.util.Scanner;
import static java.lang.System.*;

public class Lab10d
{
public static void main(String args[])
{
    Scanner keyboard = new Scanner(System.in);
    char response  ;
    
    //add in a do while loop after you get the basics up and running
    
        String player = "";
    
        out.print("Rock-Paper-Scissors - pick your weapon [R,P,S] :: ");
        
        //read in the player value
        player = keyboard.next();
    
        RockPaperScissors game = new RockPaperScissors(player);
        game.determineWinner();
        out.println(game);
    while(response.equals('y'))
    {
        out.print("Rock-Paper-Scissors - pick your weapon [R,P,S] :: ");
        player = keyboard.next();
        game.setPlayers(player);
        game.determineWinner();
        out.println(game + "\n");
        out.println("would you like to play again? (y/n):: ");
        response=  keyboard.next();
        
        
        
        
    }
    
}
}

here’s an example of what should be resulting:

Rock-Paper-Scissors – pick your weapon[R,P,S]:: R

player had R

computer had P

!Computer wins <>!

Do you want to play again? y

and if you put n for playing again, it stops.

main problem is getting compChoice to not say null

Answer 1

Your code in main method:

RockPaperScissors game = new RockPaperScissors(player);
game.determineWinner();

You initialize the compChoice String in the setPlayers() method, but that method is never called. Then you switch on compChoice, which is null, thus you get the NullPointerException.

This should fix your problem:

RockPaperScissors game = new RockPaperScissors(player);
game.setPlayers(player); // <-- insert this line.
game.determineWinner();

But be aware that you are not saving the String returned by determineWinner(), and then call out.println(game); which will call the determineWinner() method again…

Also this piece of your code

while(response == response)

will always be an endless loop, and i can’t find a break statement in your loop.

Also NEVER compare Strings with ==, use equals, google the explanation, there even are hundreds of SO explanations for that. So this

if (playChoice == "R") // <-- WRONG

should become this:

if (playChoice.equals("R")) // <-- right

or

if ("R".equals(playChoice)) // <-- right