使用try-with-resources读取和写入同一文件

Question

I am trying to make a program that receives a specified String , and removes every occurence of this String in a text document. The text file that is used to read / write is the same. The arguments used are received from cmd, in this order:

inputString filename

The program compiles fine, but after running it leaves the original text file blank. If i make a try-catch block for input handling, and a try-catch block for output handling, I am able to read and write to the same file. If i use a try-with-resources block, I am able to read a file, and save the output to a different file than the original, with all occurences of inputString from cmd removed. But it seems like I can't read and write to the same file using try-with-resources, and also the input.hasNext() statement returns false when I try to do it this way.

Code example below:

package ch12;
import java.io.*;
import java.util.*;

public class Chapter_12_E11_RemoveText {

    public static void main(String[] args) throws Exception {

        if (args.length != 2) {
            System.out.println("Usage java ch12.Chapter_12_E11_RemoveText inputString filename");
            System.exit(1);
        }

        File filename = new File(args[1]);
        if (!filename.exists()) {
            System.out.println("Target file " + args[1] + " does not exist");
            System.exit(2);
        }

        try (
            Scanner input = new Scanner(filename);
            PrintWriter output = new PrintWriter(filename); 
        ) {

            System.out.println("hasNext() is " + input.hasNext());
            System.out.println("hasNextLine() is " + input.hasNextLine());

            while (input.hasNext()) {

                String s1 = input.nextLine();
                System.out.println("String fetched from input.nextLine() " + s1);
                System.out.println("Attemping to replace all words equal to " + args[0] + " with \"\"");
                String s2 = s1.replaceAll(args[0], "");
                output.println(s2);
            }
        }

    }
}  

I am suspecting that when I create a new PrintWriter object with the argument filename , the original file is overwritten by a blank file before the while-loop executes. Am i right here? Is it possible to read and write to the same file using try-with-resources?

Answer 1

From the PrintWriter docs :

If the file exists then it will be truncated to zero size; otherwise, a new file will be created.

So you are correct, by the time you initialize your PrintWriter , your Scanner has nothing to scan.

I would remove the PrintWriter initialization from the resources initialization block, build your file representation in memory, then replace the file contents in another block (or nest it).

That is, if the file has a reasonable size for your memory to handle the replacement.