我正在创建一个数学程序，询问用户他们想使用多少位数，然后询问他们想要做什么样的数学。然后它会根据用户的答案向用户提出 10 个数学问题。然后它决定它们是对还是错并显示适当的消息。然后它会计算他们正确答案的百分比，并根据该百分比显示一条消息。
我似乎有数学逻辑工作，但一旦它循环到第二个问题，我得到一个 NullpointerException。
我以前从未遇到过这个问题，但我从四处搜索中收集到的是，这意味着我正在尝试使用一个具有空值的对象（不是 100%？）。谁能告诉我为什么这段代码第一次可以正常工作，但第二次抛出这个异常？
谢谢你的帮助。
在 assignment2.Assignment2.askQuestion(Assignment2.java:104) 在 assignment2.Assignment2.main(Assignment2.java:43) 的线程“主”java.lang.NullPointerException 中的异常
第 104 行读取
operatorText = (selectedOperator == 5) ? operators[(int) Math.floor(Math.random() * operators.length)] : operators[selectedOperator - 1];
第 43 行读取
askQuestion(0, null, 0, 0, null, 0);
完整代码
import java.util.*;
import javax.swing.JOptionPane;
public class Problem2 {
public static void main(String[] args) {
// TODO Auto-generated method stub
Scanner input = new Scanner(System.in);
int difficulty = 1;
String[] operators = {"plus", "minus", "times", "divided by"};
int selectedOperator = 1;
int correctAnswers = 0;
int answeredTyped = 0;
int difficultyInput = Integer.parseInt(
JOptionPane.showInputDialog("Please choose the difficulty. " +
"Enter the number of digits to use in each problem."));
if (difficultyInput > 0) {
difficulty = difficultyInput;
}
int arithmeticMethod = Integer.parseInt(
JOptionPane.showInputDialog(
"Choose an arithmetic problem to study: 1 " +
"= Addition Only, 2 = Subtraction Only, 3 = " +
"Multiplication Only, 4 = Division Only, 5 = Random Problems" ));
selectedOperator = arithmeticMethod;
new Problem2().askQuestion(difficulty, null,
arithmeticMethod, arithmeticMethod,
operators, arithmeticMethod);
while(answeredTyped < 10) {
askQuestion(0, null, 0, 0, null, 0);
if(((float)correctAnswers / answeredTyped) >= 0.75) {
JOptionPane.showMessageDialog(null,
"Congratulations, you are ready to go on to the next level!");
} else {
JOptionPane.showMessageDialog(
null, "Please ask your teacher for extra help.");
}
}
}
public static boolean checkResponse (double primaryInt,
double secondaryInt, String operatorText, double response){
if (operatorText.equals("plus")) {
return (primaryInt + secondaryInt) == response;
} else if (operatorText.equals("minus")) {
return (primaryInt - secondaryInt) == response;
} else if (operatorText.equals("times")) {
return (primaryInt * secondaryInt) == response;
} else if (operatorText.equals("divided by")) {
return (primaryInt / secondaryInt) == response;
}
return false;
}
public static String displayResponse (boolean isCorrect) {
int randomIndex = (int) (Math.floor(Math.random() * (4 - 1 + 1)) + 1);
switch (randomIndex) {
case 1:
return isCorrect ? "Very Good!" : "No. Please try again.";
case 2:
return isCorrect ? "Excellent!" : "Wrong. Try once more.";
case 3:
return isCorrect ? "Nice Work!" : "Don\'t give up!";
case 4:
return isCorrect ? "Keep up the good work!" : "No. Keep trying.";
}
return "Oops...";
}
public static void askQuestion(int difficulty,
String operatorText, int selectedOperator,
int answeredTyped, String[] operators, int correctAnswers){
boolean correctAnswer = false;
double primaryInt = Math.floor(Math.pow(10,
difficulty-1) + Math.random() * 9 * Math.pow(10, difficulty-1));
double secondaryInt = Math.floor(Math.pow(10,
difficulty-1) + Math.random() * 9 * Math.pow(10, difficulty-1));
operatorText = (selectedOperator == 5) ? operators[(int) Math.floor(Math.random() * operators.length)] : operators[selectedOperator - 1];
while(!correctAnswer && answeredTyped < 10) {
double response = Double.parseDouble (
JOptionPane.showInputDialog("How much is " +
primaryInt + " " + operatorText + " " + secondaryInt + "?"));
correctAnswer = checkResponse (primaryInt,
secondaryInt, operatorText, response);
JOptionPane.showMessageDialog(null, displayResponse(correctAnswer));
answeredTyped++;
if(correctAnswer)
correctAnswers++;
}
}
}