Java File: Problem

Problem:

• The input file (/home/tmp/input.txt) contains multiple lines with leading spaces (indentation).

  • Find the number of leading spaces in the first line.

  • Remove that many spaces from the start of all lines, preserving relative indentation.

• The input file (/home/tmp/input.txt):

     first line

        2nd line

        something else goes here

           3rd intent here

     some other line

           another long intent

     finally the last line

• So, the output file (/home/tmp/output.txt) with the modified content like the following:

first line

   2nd line

   something else goes here

      3rd intent here

some other line

      another long intent

finally the last line

Solution:

• Using Java NIO + Streams + Lambda + Regex

import java.io.*;

import java.nio.charset.StandardCharsets;

import java.nio.file.*;

import java.util.concurrent.atomic.AtomicBoolean;

import java.util.concurrent.atomic.AtomicInteger;

import java.util.stream.Stream;

public class MyMain {

    public static void main(String... args) {

        Path pIn = Path.of("/home/temp/input.txt");

        Path pOut = Path.of("/home/temp/output.txt");

        // Using atomic variables because we're inside a stream (lambda)

        AtomicBoolean firstLine = new AtomicBoolean(true);

        AtomicInteger leadingSpaces = new AtomicInteger(0);

        try (Stream<String> lines = Files.lines(pIn, StandardCharsets.UTF_8);

             BufferedWriter writer = Files.newBufferedWriter(pOut, StandardCharsets.UTF_8)) {

            // Streams + Lambda

            lines.map(line -> {

                if (firstLine.get()) {

                    String trimmed = line.replaceFirst("^\\s*", ""); // Regex

                    leadingSpaces.set(line.length() - trimmed.length());

                    firstLine.set(false);

                }

                // Remove up to 'leadingSpaces' from each line; using Regex

                return line.replaceFirst("^\\s{1," + leadingSpaces.get() + "}", "");

            }).forEach(line -> {

                try {

                    writer.write(line);

                    writer.newLine();

                } catch (IOException e) {

                    throw new UncheckedIOException(e);

                }

            });

            System.out.println("File processed successfully: " + pOut);

        } catch (IOException e) {

            e.printStackTrace();

        }

    }

}

• Files.lines() → returns a Stream<String>, reading lazily (line by line).

• AtomicBoolean and AtomicInteger → allow us to mutate values inside a lambda expression.

• The first line determines how many leading spaces to remove (leadingSpaces).

• Each line is transformed with .map(...) and written to output.txt.