如何在 Java 中正确使用 try-catch-finally 块？

Question

使用 finally 块关闭我所在的文件的正确方法是什么：event.dat。如果您能帮助我理解这一点，我将不胜感激。我确实花了 4 个小时来移动东西，玩代码并在线搜索答案，但无济于事。没有那部分代码可以很好地工作，但我需要知道它在未来的实例中是如何工作的。谢谢你，有一个美好的一天！

我遇到了这个块的问题：

          finally{
              fstream.close();        
          } 

位于以下代码中：

    String answer;
    String letter;
    String inType = "";
    double inAmount = 0;
    double amount;

    description();
    GironEventClass newInput = new GironEventClass();
    try{
        File infile = new File("event.dat");
        Scanner fstream = new Scanner(infile);

        System.out.println("File Contents ");

        while(fstream.hasNext())
        {
            inAmount = fstream.nextInt();
            inType = fstream.next();

            try{
                newInput.donations(inType, inAmount);

            }
            catch(IllegalArgumentException a){
                System.out.println("Just caught an illegal argument exception. ");
            }  
           finally{
                   fstream.close();        
          }  
        }

        System.out.println("Total Sales: " + newInput.getSale());
        System.out.println("Donations: " + newInput.getDonated());
        System.out.println("Expenses: " + newInput.getExpenses());



    }

    catch (FileNotFoundException e){
        System.out.println("\nEvent.dat could not be opened. ");
    }

    do{
        System.out.print("Are there any more items to add that were not in the text file? (Type 'Y' or 'N')");
        answer = keyboard.next();
        if (("Y".equals(answer)) || ("y".equals(answer)))
        {
            letter = inLetter();
            amount = inAmount();

            newInput.donations(letter, amount);
        }

    }while (("Y".equals(answer)) || ("y".equals(answer)));

    newInput.display();
}

public static String inLetter(){
    Scanner keyboard = new Scanner(System.in);
    String result;
    String resultTwo;

    System.out.println("T = Tiket Sales");
    System.out.println("D = Donations");
    System.out.println("E = Expenses");
    System.out.print("Please input an identifier ");
    result = keyboard.nextLine();
    resultTwo = result.toUpperCase();

    return resultTwo;    
}

public static double inAmount(){
    Scanner keyboard = new Scanner(System.in);
    double result;

    System.out.println("Please input an amount ");
    result = keyboard.nextInt();

    if(result <= 0.0){
        System.out.print("Please input a positive and non-zero amount ");
        result = keyboard.nextInt();
    }

    return result;
}

public static void description(){
    System.out.println("The program will ask you what amount is being spent on what.");
    System.out.println("    ex: expenses, ticket sales, and profts.");
    System.out.println("This program will help determine whether the event generated or lost money.");      
}

Answer 1

扫描仪应该是这样工作的：

while scanner has object
   read them ( one object per method's call) 
when objects are done
   close the reader.

您的问题是当 while 结论为真时使用 close 函数。所以你应该把它放在while循环之外

Answer 2

The Try block 尝试做某事。

The Catch block 仅在 Try 块期间出现问题时执行。

The Finally block 每次都在 Try 块（和 Catch 块，如果执行）之后执行。

您的问题是您尝试在 while 循环内关闭 fstream。

while(fstream.hasNext())  <----- referencing fstream
        {
            inAmount = fstream.nextInt();
            inType = fstream.next();

            try{
                newInput.donations(inType, inAmount);

            }
            catch(IllegalArgumentException a){
                System.out.println("Just caught an illegal argument exception. ");
            }  
           finally{
                   fstream.close();        <---- closing said stream
          }  
        }

由于您已经关闭了流，因此这个 while 循环应该只执行一次。话虽如此，关闭while循环外的fstream，你的程序应该恢复正常。

您还可以将 while 循环移到 try 块内，这也可以。

Answer 3

你需要在try块之外声明fstream，否则它在finally块中是不可见的。

File infile = null;
File infile = null;
try {
    infile = new File("event.dat");
    fstream = new Scanner(infile);

    ...
} catch ( ) {
    ...
} finally {
   // You need to handle exceptions also here
   infile.close();
   fstream.close();
}

您还可以使用新的 try with resources 语法并将流的关闭留给 jvm。