write reversed lines to a file

[Dave Wilson]

I have a file named apples.txt that has 3 sentences in it. The code below reads the first sentence and reverses it, but does not write the reversed sentence to the new output file. I want it to read and reverse all 3 sentences and then write the reversed sentences to the file revapples.txt. Any suggestions? Thanks.

import java.io.*;
import java.util.*;
 
public class LineReverse2 { 
public static void main(String[] args) throws IOException { 
  File inputFile = new File("apples.txt"); 
  File outputFile = new File("revapples.txt");
  
  FileReader in = new FileReader(inputFile);
  FileWriter out = new FileWriter(outputFile);
 
  BufferedReader br = new BufferedReader(new FileReader("apples.txt"));
  StringBuffer sb = new StringBuffer(br.readLine());
  System.out.println(sb.reverse().toString());
 
          int c;
  while ((c=in.read()) != -1){
  sb.reverse().toString();
out.write(c);
  }
  in.close();
  out.close();
}
}

[Jesper de Jong]

Look carefully at this part of your program:

...
System.out.println(sb.reverse().toString());
 
int c;
while ((c=in.read()) != -1){
sb.reverse().toString();
    out.write(c);
}
...

Think about what the program is doing, statement by statement, and how that corresponds (or not) to what you want your program to do.

For example:
- What does in.read() do?
- What does the line "sb.reverse().toString()" do?
- What are you writing to the output file in the line "out.write(c)"?

[Joe Ess]

Welcome to the Javaranch, Lostin. We are a friendly bunch here, so we ask that you use your real name. As our Naming Policy states, accounts with obviously fictitious names will be closed.
You can change your displayed name here.
Thanks

[Dave Wilson]

Sorry about the user name. I'm so used to using nicknames.

[Dave Wilson]

In answer to your questions:

- What does in.read() do? This opens the apple.txt document and reads the contents.
- What does the line "sb.reverse().toString()" do? This reverses the first line of the document. Not exactly what I want, but if I can reverse the first line, it would be a minor success. I thought if I placed it after the in.read command it would store the reverse in the buffer and then use the next line to write it to the output file.
- What are you writing to the output file in the line "out.write(c)"? The contents of the apple.txt document

[Dave Wilson]

Thanks for the help. I tried an array and bufferedwriter and it works.

import java.io.*;
import java.util.*;
 
public class LineReverse2 { 
public static void main(String[] args) throws IOException { 
  File inputFile = new File("apples.txt"); 
  File outputFile = new File("revapples.txt");
  
  FileReader in = new FileReader(inputFile);
  FileWriter out = new FileWriter(outputFile);
 
  BufferedReader br = new BufferedReader(new InputStreamReader(new FileInputStream(inputFile)));
  BufferedWriter bw = new BufferedWriter(new OutputStreamWriter(new FileOutputStream(outputFile)));
  ArrayList al = new ArrayList(0);
  String s = "";
  while ((s = br.readLine()) !=null) {
al.add(s);
}
  String lines[] = new String[al.size()];
 
  for (int j = (lines.length - 1); j >= 0; j--) {
  lines[j] = (String)al.get(al.size()-(j+1));
System.out.println(lines[j]);
}
      for (int j = 0;j < lines.length;j++) {
System.out.println(lines[j]);
  bw.write(lines[j]);
  bw.write ("\n");
}
bw.flush();
bw.close();
}
}
 

[Jesper de Jong]

- What does in.read() do? This opens the apple.txt document and reads the contents.

No, it reads one character from the input stream.

- What does the line "sb.reverse().toString()" do? This reverses the first line of the document. Not exactly what I want, but if I can reverse the first line, it would be a minor success. I thought if I placed it after the in.read command it would store the reverse in the buffer and then use the next line to write it to the output file.

With the line "StringBuffer sb = new StringBuffer(br.readLine());" you are reading the first line of the file. Calling "sb.reverse()" reverses the characters in the first line.

Inside the while loop, the line "sb.reverse().toString();" reverses the content of the StringBuffer again, converts the content to a String and then throws away the String (because you're not assigning the return value to a variable).

Nowhere in your program you're reading the input file line by line, so you keep reversing the first line, which is still in the StringBuffer, with each iteration of the loop.

- What are you writing to the output file in the line "out.write(c)"? The contents of the apple.txt document

No, you are just writing the character that you read in the line "while ((c=in.read()) != -1){" to the output file. So you're just copying the input file to the output file, character by character.


By the way, what was it that your program should do? Should it reverse each line (e.g., if line 1 of the input file contains "jesper" should it write "repsej" in line 1 of the output file), or should it just reverse the order of the lines? The last piece of code you posted is doing the second.

Why do you have these lines in your program:

FileReader in = new FileReader(inputFile);
FileWriter out = new FileWriter(outputFile);

You're not using in and out anywhere in your program.