来源不明的 java.util.NoSuchElementException

Question

我不确定为什么会收到此错误。当我运行我的程序时，我得到这个

java.util.NoSuchElementException
at java.util.Scanner.throwFor(Unknown Source)
at java.util.Scanner.next(Unknown Source)
at java.util.Scanner.nextDouble(Unknown Source)

我只运行一台扫描仪，它在此 while 循环后关闭。我很好奇是否有人能给我一个关于我应该如何五它的提示。这是一些代码。我还在第 27 行发表了评论，因为我认为这是发生错误的地方。

        try {
        File file = new File("src/text.txt");
        File finalGrades = new File("src/nextText.txt");
        PrintWriter output = new PrintWriter(finalGrades);
        File finalGradesReport = new File("src/nextTextReport.txt");
        PrintWriter output2 = new PrintWriter(finalGradesReport);
        Scanner input = new Scanner(file);

        double totalPointsAvailable = input.nextDouble();
        double homeworkWeight = input.nextDouble() / totalPointsAvailable;
        double projectsWeight = input.nextDouble() / totalPointsAvailable;
        double firstExamWeight = input.nextDouble() / totalPointsAvailable;
        double secondExamWeight = input.nextDouble() / totalPointsAvailable;
        double finalExamWeight = input.nextDouble() / totalPointsAvailable;

        int numA = 0, numB = 0, numC = 0, numD = 0, numF = 0, numStudents = 0;
        double totalPercGrades = 0, averageGrades = 0;

        while (input.hasNext()) {
            double hw = input.nextDouble() * homeworkWeight;
            double pw = input.nextDouble() * projectsWeight; //line 27
            double firstEx = input.nextDouble() * firstExamWeight;
            double secondEx = input.nextDouble() * secondExamWeight;
            double finalEx = input.nextDouble() * finalExamWeight;

Answer 1

发生该错误的原因有两个。

1. 在调用 nextDouble() 之前，您没有检查是否有足够的输入。

2. hasNext() 检查下一个标记，但不会通过调用 nextDouble() 来递增。您想使用 hasNextDouble()

您应该将 nextDouble() 包装在 while 循环中，并在每个 nextDouble() 之前使用计数器或仅使用条件，以确保其中存在双标记您的文件并跟踪您在文件中的位置

应该使用这样的东西:

//...below Scanner input = new Scanner(file); 
int tokenCounter = 0;

//This will set all of your variables on static fields.
while (input.hasNextDouble()) { /* <-- this wraps your nextDouble() call */
  setVariables(input.nextDouble(), tokenCounter++);
}

//several lines of code later 

public static void setVariables(double inputValue, tokenCounter){

  switch(tokenCounter){
   case 0: totalPointsAvailable = inputValue; break;
   case 1: homeworkWeight = inputValue; break;
   case 2: projectsWeight = inputValue; break;
   //Continue the switch/case based on the expected order of doubles
   //and make your variables static sense they seem changes to your code might
   //cause you to break up your main method and static variables will be easier 
   //to access across different methods.
  } 
}