无法使compChoice不返回null

Question

下面是我为核心程序本身编写的代码：

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;
}
}

这是我的跑步课：

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();
        
        
        
        
    }
    
}
}

下面是一个应该产生什么结果的例子：

石头纸-剪刀-挑选你的weaponR，P，S:：r 玩家拥有R 计算机有P 电脑赢了<>！ 你想再玩一次吗？是

如果你再投入比赛，它就停止了。

主要问题是如何使compChoice不表示null

Answer 1

main方法中的代码：

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

您可以在compChoice方法中初始化setPlayers()字符串，但从未调用该方法。然后打开compChoice (为null )，从而得到NullPointerException。

这应该可以解决您的问题：

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

但是请注意，您没有保存determineWinner()返回的字符串，然后调用out.println(game);，它将再次调用determineWinner()方法.

还有这段代码

while(response == response)

将永远是一个没完没了的循环，我在您的循环中找不到一个break语句。

此外，从不将字符串与==进行比较，使用等量，谷歌的解释，甚至有数以百计这样的解释。所以这就是

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

应该变成这样：

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

或

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

Answer 2

我只是快速地浏览了您的代码，实际上并没有测试/运行您的代码。但是我发现:使用strings方法来比较字符串，不要使用==来比较字符串。