Question

我不断收到错误消息Error: java.lang.NullPointerException at runtime。显然我明白这表明当我引用一些具有空值的变量时它应该有一些值。最好向您展示代码，以便将其置于上下文中。

public class MarathonAdmin
{
   private List<Runner> runners;


   /**
    * Constructor for objects of class MarathonAdmin
    */
   public MarathonAdmin()
   {
      // initialise instance variables
      List<Runner> runners = new ArrayList<>();
   }

    public void readInRunners()
   {
      String pathName = OUFileChooser.getFilename();
      File aFile = new File(pathName);
      Scanner bufferedScanner = null;

      try 
      {
         String runnerName;
         int runnerAge;
         String ageGroup;
         Scanner lineScanner;
         String currentLine;
         bufferedScanner = new Scanner(new BufferedReader(new FileReader(aFile)));
         while (bufferedScanner.hasNextLine())
         {
            currentLine = bufferedScanner.nextLine();
            lineScanner = new Scanner(currentLine);
            lineScanner.useDelimiter(",");
            runnerName = lineScanner.next();
            runnerAge = lineScanner.nextInt();

            Runner runnerObject = new Runner();

            if (runnerAge < 18)
            {
               ageGroup = "junior";
               runnerObject.setAgeGroup(ageGroup);
            }
            else
            if (runnerAge > 54)
            {
               ageGroup = "senior";
               runnerObject.setAgeGroup(ageGroup);
            }
            else
            {
               ageGroup = "standard";
               runnerObject.setAgeGroup(ageGroup);
            }

            runnerObject.setName(runnerName);

            runners.add(runnerObject);


         }
      }

      catch (Exception anException)
      {
         System.out.println("Error: " + anException);
      }

      finally
      {
         try
         {
            bufferedScanner.close();
         }

         catch (Exception anException)
         {
            System.out.println("Error: " + anException);
         }
      }
   }

创建类实例的测试代码是：

MarathonAdmin ma = new MarathonAdmin();
ma.readInRunners();

有一个名为Runners的类正在使用其协议进行设置。该类编译但ma.readInRunner();消息以错误结束。程序要运行的文本文件没有错误。

我对编程有点新鲜，因此很难解决问题。希望有人可以帮助我。

Answer 1

在构造函数中它应该是

public MarathonAdmin()
{
  // initialise instance variables
   this.runners = new ArrayList<>();
}

而不是

public MarathonAdmin()
{
   // initialise instance variables
   List<Runner> runners = new ArrayList<>();
}

Answer 2

问题是您已经创建了一个Try-catch块，但是在catch部分中你没有打印stacktrace ，请参阅：

  catch (Exception anException)
  {
     System.out.println("Error: " + anException); // just prints: Error: java.lang.NullPointerException
  }

改进此catch块以进行调试，例如：

  catch (Exception e) {
      e.printStackTrace(); //prints the full stacktrace
  }

现在你应该得到一个完整的堆栈跟踪，它也会显示行号，你可以自己调试它; - ）

实际上你可能应该删除那些try-catch块。

Java：lists，bufferedScanner和Error：java.lang.NullPointerException

2 个答案: