Passing FileWriter as a parameter to a method

问题:

I'm sure there is a fairly simple answer to this question, so here we go.

I'm trying to use a FileWriter to write text to a file. My program reads text in from an already existing file, specified by the user and then asks whether to print the text to the console or to a new file, also to be named by the user.

I believe my problem is with passing the FileWriter to the "FileOrConsole" method. Am I not passing or declaring the FileWriter in the "FileOrConsole" method correctly? The file is always created but nothing is written to it.

Here is the code:

import java.io.*;
import java.util.*;

public class Reader {

public static void main(String[] args) throws IOException {
    Scanner s = null, input = new Scanner(System.in);
    BufferedWriter out = null;

    try {
        System.out.println("Would you like to read from a file?");
        String answer = input.nextLine();

        while (answer.startsWith("y")) {
            System.out.println("What file would you like to read from?");
            String file = input.nextLine();
            s = new Scanner(new BufferedReader(new FileReader(file)));

            System.out
                    .println("Would you like to print file output to console or file?");
            FileOrConsole(input.nextLine(), s, input, out);
            System.out
                    .println("\nWould you like to read from the file again?");
            answer = input.nextLine();
        }
        if (!answer.equalsIgnoreCase("yes")) {
            System.out.println("Goodbye!");
        }

    } catch (IOException e) {
        System.out.println("ERROR! File not found!");
        // e.printStackTrace();
    } finally {
        if (s != null) {
            s.close();
        }
        if (out != null) {
            out.close();
        }
    }
}

public static void FileOrConsole(String response, Scanner s, Scanner input,
        BufferedWriter out) {
    if (response.equalsIgnoreCase("console")) {
        while (s.hasNext()) {
            System.out.println(s.nextLine());
        }
    } else if (response.equalsIgnoreCase("file")) {
        System.out.println("Name of output file?");
        response = input.nextLine();
        try {
            out = new BufferedWriter(new FileWriter(response));
        } catch (IOException e) {
            e.printStackTrace();
        }
        while (s.hasNext()) {
            try {
                out.write(s.nextLine());
                out.newLine();
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
        try {
            out.close();
        } catch (IOException e) {
            e.printStackTrace();
        }
    } else {
        System.out.println("Sorry, invalid response. File or console?");
        response = input.nextLine();
        FileOrConsole(response, s, input, out);
    }
  }
}

回答1:

you make classic error forgetting that parameters passed by value in case of java it is a value of the reference. The thing is that your assignment

out = new BufferedWriter(new FileWriter(response));

actually does not change the variable declared in main() it stays null

BufferedWriter out = null; and then in finally it skips the close() by the if(out==null) and as it is Buffered and you do no flush nothing is written to file. what you got to do is out.close(); in side the FileOrConsole method call

OR

do the out = new BufferedWriter(new FileWriter(response)); outside of it. You choose :-)

回答2:

Try flushing your stream, but more importantly, remember to close it.

Here's a code example of recommended practice for handling streams. Same approach can be used for input streams too and things like database code where it's important to always clean up after yourself to get the expected results.

BufferedWriter out = null;
try {
    out = // ... create your writer

    // ... use your writer
} catch(IOException ex) {

    // maybe there was a problem creating or using the writer

} finally {
   if (null != out) {
       out.flush();
       out.close();
       out = null;
   }
}