我已发布我的程序以供审查代码审查（stackexchange）。 一切都很好，我回家后被告知使用IDE。
我使用Eclipse IDE打开了我的源代码，然后我开始在运行时获取（无论是在IDE上还是没有）这个错误：
Exception in thread "main" java.lang.NullPointerException
at games.Spin.rand(Spin.java:68)
at games.Spin.<init>(Spin.java:10)
at games.GameHandler.<init>(GameHandler.java:8)
at Mains.startGame(Mains.java:16)
at Mains.main(Mains.java:9)
为什么这样做？我的伙伴已经查看了我的代码，但发现它没有任何问题？。
我是java的新手，尝试过深入OO。
我的代码位于代码审查线程（3个类）：
https://codereview.stackexchange.com/questions/28197/improving-my-java-object-oriented-review
它出了什么问题？为什么它给我这个例外？
第68行：return r.nextInt(x);
public int rand(int x) {
return r.nextInt(x);
}
这就是我创建r对象的方式：
/**
* Creating new Random object.
**/
private Random r = new Random();
Mains.java：
导入games.GameHandler; import java.util.Scanner; import java.io。*;
public class Mains {
public static void main (String[] args) {
//Start the game
startGame();
}
private static void startGame() {
//Declares
GameHandler handler = new GameHandler();
Scanner console = new Scanner(System.in);
boolean game = true;
String input = "";
//Print program welcome text
handler.printStart();
//While in game...
while (game) {
//Getting input ready for new commands from the player
input = console.nextLine();
//Checking if input was set.
if (input != null) {
//Selecting the game you want to play.
handler.selectGame(input);
//If game was selected.. then.. let's start playing.
while (handler.inGame) {
//Use will say something.
input = console.nextLine();
//If it was "exit", it will go back and select another game.
if (input.equals("exit")) {
handler.exitGame();
} else {
//Play again.
handler.continueGame(input);
}
}
}
}
}
}
GameHandler.java：
打包游戏; import java.io。*;public class GameHandler {
private String[] games = {"Spin", "Tof"};
private String[] navigation = {"Back", "Start"};
private Spin spin = new Spin();
private boolean spinGame = false;
private boolean tofGame = false;
public boolean inGame = false;
/**
* Method printStart
*
* Will welcome the player to the program.
*/
public void printStart() {
this.print(0, "Welcome to the program!");
this.print(0, "Please select a game: " + this.availableGames());
}
/**
* Method available games
*
* This will print all the games that are located in the games array in one row.
**/
private String availableGames() {
String names = "";
for (int i = 0; i < games.length; i++) {
names = (names + games[i]);
if (i < games.length -1) {
names = (names + ", ");
}
}
return names;
}
/**
* Method selectGame
*
* This will select the given game.
* @param command The entered command.
**/
public void selectGame(String command) {
if (this.inArray(command))
{
if (command.equalsIgnoreCase("spin")) {
this.startGame("spin");
} else if (command.equalsIgnoreCase("tof")) {
this.startGame("tof");
}
} else {
this.print(0, "Could not find game!");
}
}
/**
* Method inArray
*
* This will check if the entered game name is exisiting in the games array.
* If yes, will return a boolean true, else false.
*
* @param value The entered game name.
* @return boolean true/false.
**/
private boolean inArray(String value) {
int returning = 0;
for (String s : games) {
if (value.equalsIgnoreCase(s)) {
returning = 1;
}
}
if (returning == 1) {
return true;
} else {
return false;
}
}
/**
* Method startGame
*
* Will start the game, and print instructions.
* will set the game boolean to true.
**/
private void startGame(String game) {
switch (game) {
case "spin":
this.print(0, "Welcome to spin game!");
this.print(0, "Please click on any key to spin!");
spinGame = true;
break;
case "tof":
break;
}
inGame = true;
}
/**
* Method continueGame
*
* Will continue the game, either spin again, or print new question or even answer.
* @param command The entered command.
**/
public void continueGame(String command) {
while (inGame) {
if (spinGame) {
this.spinWheel();
// Break out of the loop.
break;
}
}
}
/**
* Method exitGame
*
* Exit the game..
**/
public void exitGame() {
spinGame = false;
tofGame = false;
this.printStart();
}
/**
* Method spinWheel
*
* This will spin the wheel.
**/
private void spinWheel() {
this.print(0, spin.spinWheel());
}
/**
* Method print
*
* Prints text using System.out
* @param type printing type (Println/print).
* @param message The message
**/
private void print(int type, String message) {
switch (type) {
case 0:
System.out.println(message);
break;
case 1:
System.out.print(message);
break;
}
}
}
Spin.java：
打包游戏; import java.util.Random;public class Spin {
/**
* The base auth we are going to work with..
**/
private int auth = this.rand(1000) / 5;
/**
* Creating new Random object.
**/
private Random r = new Random();
/**
* Method spinWheel
*
* Spins the damn wheel..
* @return spinned value + if you won or not.
**/
public String spinWheel() {
return this.spinWheel(this.rand(100));
}
/**
* spinWheel
*
* Returning results.
**/
private String spinWheel(int number) {
int result = this.Calculate(this.rand(number));
if (result < 101) {
return "You have won the game!" + result;
} else {
return "You've lost the game!" + result;
}
}
/**
* Method calculate
*
* Calculates the spin.
* @return the spinned number.
**/
private int Calculate(int Number) {
int var = this.rand(101);
int holder = (var * Number) / 2;
return holder + this.auth;
}
/**
* Shortcut for nextInt of Random
**/
public int rand(int x) {
return r.nextInt(x);
}
}
答案 0 :(得分：4)
rand实例Random之前调用 r。切换订单或这两个陈述
private int auth = this.rand(1000) / 5;
private Random r = new Random();
应该是
private Random r = new Random();
private int auth = this.rand(1000) / 5;
答案 1 :(得分：2)
在你的Spinwheel类定义中首先分配r，即在它之前将它放在this.rand（1000）中：
public class Spin {
/**
* Creating new Random object.
**/
private Random r = new Random();
/**
* The base auth we are going to work with..
**/
private int auth = this.rand(1000) / 5;
答案 2 :(得分：1)
r为空，因此您无法在r上调用任何实例方法。确保在使用之前初始化r。
更具体地说，在这一行：
private int auth = this.rand(1000) / 5;
您在初始化r之前调用rand()方法（之后立即初始化）。
答案 3 :(得分：1)
这是导致NullPointerException：
private int auth = this.rand(1000) / 5;
由于在<{1}}的初始化之前，所以在 r初始化之前，您正在调用rand 。在这种情况下r r位于null，这是您的例外。
从堆栈跟踪中可以看出这一点：
rand请注意，初始化程序中发生了异常。从那里，很容易退出正在发生的事情。
首先需要初始化at games.Spin.rand(Spin.java:68)
at games.Spin.<init>(Spin.java:10)
，即在r的初始化行之前移动r的初始化行。因此：
auth答案 4 :(得分：1)
这是因为r在语句private int auth = this.rand(1000) / 5;中实例化之前正在使用。因此，JVM将r视为null，这导致NPE。要在Spin类中解决此问题，请按以下方式声明字段：
private Random r = new Random();
private int auth = this.rand(1000) / 5;