Reading and Writing Text Files

Handling text files in Java involves performing operations like reading from, writing to, and appending to files. Here are some common techniques and examples for handling text files in Java:

FileReader and BufferedReader are classes in Java used for reading text from files. Here’s a detailed explanation of each:

FileReader

FileReader is a class in Java that is used to read character files. It is a convenient class for reading text files using the default character encoding of the operating system.

Key Points:
  • Purpose: To read text files.
  • Inheritance: It extends InputStreamReader, which in turn extends Reader.
  • Encoding: Uses the default character encoding of the operating system unless specified otherwise.
  • Basic Usage: Suitable for simple file reading tasks.
Example:
create a text file "example.txt" using any text editor and then run this program.In this example, FileReader reads the file character by character.

import java.io.FileReader;
import java.io.IOException;

public class FileReaderExample {
    public static void main(String[] args) {
        try {
            FileReader fileReader = new FileReader("example.txt");
            int character;
            while ((character = fileReader.read()) != -1) {
                System.out.print((char) character);
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

BufferedReader

BufferedReader is a class that reads text from an input stream, buffering characters to provide efficient reading of characters, arrays, and lines. It can wrap around other Reader classes, such as FileReader, to enhance their performance.

Key Points:
  • Purpose: To read text efficiently by buffering input.
  • Inheritance: It extends Reader.
  • Buffering: Buffers input to provide efficient reading of characters, arrays, and lines.
  • Line Reading: Provides a method readLine() to read a line of text, which is very convenient for reading text files line by line.
import java.io.FileReader;
import java.io.BufferedReader;
import java.io.IOException;

public class FileReadExample {
    public static void main(String[] args) {
        try {
            FileReader fileReader = new FileReader("example.txt");
            BufferedReader bufferedReader = new BufferedReader(fileReader);

            String line;
            while ((line = bufferedReader.readLine()) != null) {
                System.out.println(line);
            }

        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

Writing to a Text File
Using FileWriter character by character
import java.io.FileWriter;
import java.io.IOException;

public class WriteCharacterByCharacterExample {
    public static void main(String[] args) {
        FileWriter fileWriter = null;
         try{ // true to append, false to overwrite
            fileWriter = new FileWriter("example.txt", true);
            fileWriter.write('H');
            fileWriter.write('e');
            fileWriter.write('l');
            fileWriter.write('l');
            fileWriter.write('o');
            fileWriter.write(',');
            fileWriter.write(' ');
            fileWriter.write('W');
            fileWriter.write('o');
            fileWriter.write('r');
            fileWriter.write('l');
            fileWriter.write('d');
            fileWriter.write('!');
            fileWriter.write('\n');
            fileWriter.close();
        } catch (IOException e) {
            e.printStackTrace();
        }
   }
}
Explanation:
  • FileWriter: Used to write characters to a file. The true parameter in the FileWriter constructor indicates that the file should be appended to rather than overwritten.
  • write(char c): Writes a single character to the file.
  • '\n': Writes a newline character to the file.
Using FileWriter and BufferedWriter
import java.io.FileWriter;
import java.io.BufferedWriter;
import java.io.IOException;

public class FileWriteExample {
    public static void main(String[] args) {
        try {
            FileWriter fileWriter = new FileWriter("example.txt", true);
            BufferedWriter bufferedWriter = new BufferedWriter(fileWriter);

            bufferedWriter.write("Hello, World!");
            bufferedWriter.newLine();
            bufferedWriter.write("Welcome to file handling in Java.");
            bufferedWriter.newLine();
            bufferedWriter.close();
            fileWriter.close();

        } catch (IOException e) {
            e.printStackTrace();
        }
   }
}
Explanation:
  • FileWriter writes characters to a file.
  • BufferedWriter wraps FileWriter to provide efficient writing.
  • newLine() method is used to insert a new line.
Programs to try
Reading a File and and write to another (copy file)

import java.io.FileReader;
import java.io.BufferedReader;
import java.io.FileWriter;
import java.io.BufferedWriter;
import java.io.IOException;

public class FileReadWriteExample {
    public static void main(String[] args) {
        String inputFilePath = "input.txt";
        String outputFilePath = "output.txt";

        BufferedReader bufferedReader = null;
        BufferedWriter bufferedWriter = null;

        try {
            // Open the input file for reading
            FileReader fileReader = new FileReader(inputFilePath);
            bufferedReader = new BufferedReader(fileReader);

            // Open the output file for writing
            FileWriter fileWriter = new FileWriter(outputFilePath);
            bufferedWriter = new BufferedWriter(fileWriter);

            // Read from the input file and write to the output file
            String line;
            while ((line = bufferedReader.readLine()) != null) {
                bufferedWriter.write(line);
                bufferedWriter.newLine();
            }
            System.out.println("File read and write operation completed successfully.");
        } catch (IOException e) {
            System.err.println("An IOException occurred: " + e.getMessage());
        } finally {
            // Close the BufferedReader
            if (bufferedReader != null) {
                try {
                    bufferedReader.close();
                } catch (IOException e) {
                    System.err.println("Failed to close BufferedReader: " + e.getMessage());
                }
            }

            // Close the BufferedWriter
            if (bufferedWriter != null) {
                try {
                    bufferedWriter.close();
                } catch (IOException e) {
                    System.err.println("Failed to close BufferedWriter: " + e.getMessage());
                }
            }
        }
    }
}

Write a Java program that reads a file and displays the file on the screen, with a line number before each line.

import java.io.FileReader;
import java.io.BufferedReader;
import java.io.IOException;

public class FileDisplayWithLineNumbers {
    public static void main(String[] args) {
        String inputFilePath = "input.txt"; // Replace with your file path
        BufferedReader bufferedReader = null;

        try {
            // Open the input file for reading
            FileReader fileReader = new FileReader(inputFilePath);
            bufferedReader = new BufferedReader(fileReader);

            // Read from the input file and display with line numbers
            String line;
            int lineNumber = 1;
            while ((line = bufferedReader.readLine()) != null) {
                System.out.println(lineNumber + ": " + line);
                lineNumber++;
            }

        } catch (IOException e) {
            System.err.println("An IOException occurred: " + e.getMessage());
        } finally {
            // Close the BufferedReader
            if (bufferedReader != null) {
                try {
                    bufferedReader.close();
                } catch (IOException e) {
                    System.err.println("Failed to close BufferedReader: " + e.getMessage());
                }
            }
        }
    }
}


Program to Count Characters, Lines, and Words in a Text File
import java.io.BufferedReader;
import java.io.FileReader;
import java.io.IOException;
import java.util.StringTokenizer;

public class FileStatistics {
    public static void main(String[] args) {
        String inputFilePath = "input.txt"; // Replace with your file path
        BufferedReader bufferedReader = null;

        int characterCount = 0;
        int lineCount = 0;
        int wordCount = 0;

        try {
            // Open the input file for reading
            FileReader fileReader = new FileReader(inputFilePath);
            bufferedReader = new BufferedReader(fileReader);

            // Read from the input file
            String line;
            while ((line = bufferedReader.readLine()) != null) {
                lineCount++;
                characterCount += line.length();

                // Use StringTokenizer to count words in the line
                StringTokenizer tokenizer = new StringTokenizer(line);
                while (tokenizer.hasMoreTokens()) {
                    tokenizer.nextToken();
                    wordCount++;
                }
            }

            // Display the results
            System.out.println("Number of characters: " + characterCount);
            System.out.println("Number of lines: " + lineCount);
            System.out.println("Number of words: " + wordCount);

        } catch (IOException e) {
            System.err.println("An IOException occurred: " + e.getMessage());
        } finally {
            // Close the BufferedReader
            if (bufferedReader != null) {
                try {
                    bufferedReader.close();
                } catch (IOException e) {
                    System.err.println("Failed to close BufferedReader: " + e.getMessage());
                }
            }
        }
    }
}

Java program that reads numbers from an input file, separates the odd and even numbers, and writes them into two different files: one for odd numbers and one for even numbers.

import java.io.BufferedReader;
import java.io.BufferedWriter;
import java.io.FileReader;
import java.io.FileWriter;
import java.io.IOException;

public class SeparateOddEven {
    public static void main(String[] args) {
        String inputFilePath = "numbers.txt"; // Replace with your input file path
        String oddFilePath = "odd_numbers.txt"; // File to write odd numbers
        String evenFilePath = "even_numbers.txt"; // File to write even numbers

        BufferedReader bufferedReader = null;
        BufferedWriter oddWriter = null;
        BufferedWriter evenWriter = null;

        try {
            // Open the input file for reading
            FileReader fileReader = new FileReader(inputFilePath);
            bufferedReader = new BufferedReader(fileReader);

            // Open the output files for writing
            FileWriter oddFileWriter = new FileWriter(oddFilePath);
            FileWriter evenFileWriter = new FileWriter(evenFilePath);
            oddWriter = new BufferedWriter(oddFileWriter);
            evenWriter = new BufferedWriter(evenFileWriter);

            // Read numbers from the input file and separate them
            String line;
            while ((line = bufferedReader.readLine()) != null) {
                String[] numbers = line.split("\\s+"); // Split the line into numbers
                for (String numberStr : numbers) {
                    try {
                        int number = Integer.parseInt(numberStr.trim());
                        if (number % 2 == 0) {
                            evenWriter.write(number + " ");
                        } else {
                            oddWriter.write(number + " ");
                        }
                    } catch (NumberFormatException e) {
                        System.err.println("Skipping invalid number: " + numberStr);
                    }
                }
            }

            System.out.println("Odd and even numbers have been separated into different files.");

        } catch (IOException e) {
            System.err.println("An IOException occurred: " + e.getMessage());
        } finally {
            // Close the BufferedReader
            if (bufferedReader != null) {
                try {
                    bufferedReader.close();
                } catch (IOException e) {
                    System.err.println("Failed to close BufferedReader: " + e.getMessage());
                }
            }

            // Close the BufferedWriter for odd numbers
            if (oddWriter != null) {
                try {
                    oddWriter.close();
                } catch (IOException e) {
                    System.err.println("Failed to close oddWriter: " + e.getMessage());
                }
            }

            // Close the BufferedWriter for even numbers
            if (evenWriter != null) {
                try {
                    evenWriter.close();
                } catch (IOException e) {
                    System.err.println("Failed to close evenWriter: " + e.getMessage());
                }
            }
        }
    }
}



Explanation:
  1. File Paths: The input file path (numbers.txt) and the output file paths for odd and even numbers (odd_numbers.txt and even_numbers.txt) are specified.
  2. BufferedReader and BufferedWriter: Declared outside the try block for visibility in the finally block.
  3. Reading and Separating Numbers:
    • The program reads each line from the input file using BufferedReader.
    • Each line is split into numbers using split("\\s+"), which splits the line based on whitespace.
    • Each number is checked whether it's even or odd and written to the corresponding file.
  4. Exception Handling: Catches and prints any IOException that occurs during the file operations.
  5. Finally Block: Ensures that the BufferedReader and BufferedWriter are closed, even if an exception occurs.

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