java - Is this file object left open?

Question

As an exercise for school I wrote a method in Java that searches for a character in a file. Here is the code:

 public static void countLetter(char needle, String hayStack) throws IOException {

        File f = new File(hayStack);                                           
        try (Scanner in = new Scanner(f)) {                                    
            String str = null;                                                 
            while (in.hasNext()){                                              
                str += in.next();                                              
            }                                                                  
            char[] charArr = str.toCharArray();                                
            int counter = 0;                                                   
            for (char c: charArr) {                                            
                if (c == needle){                                              
                    counter++;                                                 
                }                                                              
            }                                                                  
            System.out.println(counter);                                       
        }                                                                      
    }

This does what I need it to but I have a question. Is the file object ever opened? And if it is, does it ever closed? I used try-with-resources on the Scanner object so I'm pretty sure I don't have to explicitly close that, but what about the file object?

Answer 1

文件对象只是路径名的抽象表示，它与打开文件无关。所以不能关闭。

Answer 2

该File对象无法打开（因此无法关闭），因为它是文件路径的表示，而不是文件的表示。

该Scanner课程打开您的文件以阅读它。您的程序不会调用in.close()，因此当您的方法返回Scanner时不会被垃圾收集，因为它仍然具有对打开文件的引用。您也可能会锁定文件，具体取决于Scanner.

为了使类有资格自动关闭 try-resource 块中的资源，该类必须实现java.lang.AutoCloseable（或java.io.Closeable）。Scanner没有实现这些接口，所以当你退出 try-resource 块时它不会close()调用它的方法。

将代码更改为：

public static void countLetter(char needle, String hayStack) throws IOException {

    File f = new File(hayStack);                                           
    try (Scanner in = new Scanner(f)) {                                    
        String str = null;                                                 
        while (in.hasNext()){                                              
            str += in.next();                                              
        }                                                                  
        char[] charArr = str.toCharArray();                                
        int counter = 0;                                                   
        for (char c: charArr) {                                            
            if (c == needle){                                              
                counter++;                                                 
            }                                                              
        }                                                                  
        System.out.println(counter);
        in.close();
    }
}

Answer 3

由于该变量是您的函数的本地变量，因此一旦函数存在，它将超出范围，并最终进行垃圾收集。您的问题的答案是否定的，在您的情况下，文件对象从未打开过。