在 Java 尝试块中找不到符号 [重复]

Question

对 Java 相当陌生，我正在尝试使用 fileChooser 打开文件并读取信息。我正处于尝试创建 fileInputStream 和inputStreamReader 的阶段。创建时，尽管文件存在，但我得到了FileNotFoundException。不太清楚为什么会发生这种情况，但我已将此代码放入 try/catch 块中以解决它。不幸的是，在编译变量“in”期间，我仍然收到cannot find symbol 错误。如果有人能对这些问题做出解释，将不胜感激。

openFileBtn.addActionListener(new ActionListener() {
    public void actionPerformed(ActionEvent e) {    
        JFileChooser fileChooser = new JFileChooser(); 
        fileChooser.setCurrentDirectory(new File("."));
        fileChooser.setDialogTitle("Open your Rain Data.");
        int returnVal = fileChooser.showOpenDialog(null);   
        //Handles when a file is opened by the user. 
        if (returnVal == JFileChooser.APPROVE_OPTION) {     
            String absolutePath = fileChooser.getSelectedFile().getAbsolutePath();
            File file = new File(absolutePath);
            try {
                FileInputStream in = new FileInputStream(file); 
                InputStreamReader reader = new InputStreamReader(in); 
            } catch (FileNotFoundException ex) {
                System.out.println("Error - the file was unable to be read by the rain handler. Check permissions and the file.");
            } finally {
                System.out.println(file.canRead()); 
                System.out.println(file.exists());
                System.out.println(in.available());     
            }   
        } 
    }
});

Answer 1

在特定范围内定义的变量，例如try body，仅在此范围内可见。所以在try 正文之外，该变量是不可访问的。
其实你应该在之前声明它：

File file = new File(absolutePath);
FileInputStream in = null;
try {
    in = new FileInputStream(file); 
    InputStreamReader reader = new InputStreamReader(in); 

} catch (FileNotFoundException ex) {
    System.out.println("Error - the file was unable to be read by the rain handler. Check permissions and the file.");

} finally {
    System.out.println(file.canRead()); 
    System.out.println(file.exists());
    System.out.println(in.available());     
}   

请注意，finally 的用法不好。
finally 子句确保：

finally 块在 try 块和任何 catch 块之后执行 无论控制如何离开 try 块或 捕获块。

通常您使用它来清除在try 正文中打开/使用的资源。
实际上file 或in 可能是null。因此，此代码在运行时可能会因NullPointerException 而失败。
除了您的代码抛出一些 IOException 之外，您还应该在错误处理中捕获它们，而不仅仅是 FileNotFoundException：

这种处理只能在try中执行如：

try {
    in = new FileInputStream(file); 
    InputStreamReader reader = new InputStreamReader(in); 
    System.out.println(file.canRead()); 
    System.out.println(file.exists());
    System.out.println(in.available());     

} 
 catch (FileNotFoundException ex) {
    System.out.println("Error - the file was unable to be read by the rain handler. Check permissions and the file.");
} 
 catch (IOException ex) {
        System.out.println("Error...");
 } 

还有一个更好的方法是依靠资源尝试自动关闭流资源：

try (FileInputStream in = new FileInputStream(file);
      InputStreamReader reader = new InputStreamReader(in)){
     // ...
}

Answer 2

标准方法是：

FileInputStream fileInputStream = null;
try {
    fileInputStream = new FileInputStream(...);
    // do something with the inputstream
} catch (IOException e) {
    // handle an exception
} finally { //  finally blocks are guaranteed to be executed
    // close() can throw an IOException too, so we got to wrap that too
    try {
        if (fileInputStream != null) {
            fileInputStream.close();
        }        
    } catch (IOException e) {
        // handle an exception, or often we just ignore it
    }
}

所以在你的情况下：

public void actionPerformed(ActionEvent e) {
     JFileChooser fileChooser = new JFileChooser();
     fileChooser.setCurrentDirectory(new File("."));
      fileChooser.setDialogTitle("Open your Rain Data.");
      int returnVal = fileChooser.showOpenDialog(null);
      //Handles when a file is opened by the user. 
      if (returnVal == JFileChooser.APPROVE_OPTION) {
          String absolutePath = fileChooser.getSelectedFile().getAbsolutePath();
          File file = new File(absolutePath);
          FileInputStream in = null;
          try {
              in = new FileInputStream(file);
              InputStreamReader reader = new InputStreamReader(in);
          } catch (FileNotFoundException ex) {
              System.out.println("Error - the file was unable to be read by the rain handler. Check permissions and the file.");
          } finally {
              System.out.println(file.canRead());
              System.out.println(file.exists());
              try {
                  System.out.println(in.available());
              } catch (IOException ex) {
                  // handle an exception, or often we just ignore it  
              }
          }
      }
  }

Answer 3

FileInputStream in = new FileInputStream(file); 

在发布的问题中，引用“in”是在 try 块中创建的，因此它在该块内具有本地范围。首选是在 try 块之前声明它，以便有更大的范围。