Java Program to Append a String in an Existing File

The Java IO package offers several methods to append a string to an existing file in Java. Appending a string to a file means adding new data to the end of an existing file without overwriting or deleting the previous data.

Approach: Appending string to a file.

Appending a string to a file is a simple and effective way to modify its contents. It allows for the easy addition of new data to the end of a file without having to rewrite the entire file, which can be time-consuming and inefficient.

Algorithm

Initialize the name of the file to be modified.
Use a BufferedReader to read the existing content of the file and print it to the console.
Use a custom method appendStrToFile to append a new string to the file with a newline character.
Use a BufferedReader again to read the modified content of the file and print it to the console.
Handle any possible IOException that may occur during file reading and writing.

Implementation

Filename: AppendStringToFile.java

import java.io.*;

public class AppendStringToFile {

    // Method to append a string to a file
    public static void appendStrToFile(String fileName, String str) {
        try {
            // Create a BufferedWriter object with FileWriter to append to the file
            BufferedWriter writer = new BufferedWriter(new FileWriter(fileName, true));
            
            // Write the string to the file and add a new line character
            writer.write(str);
            writer.newLine();
            
            // Close the writer
            writer.close();
        }
        catch (IOException e) {
            // Catch any exceptions that occur and print the error message
            System.out.println("Exception occurred: " + e);
        }
    }

    public static void main(String[] args) {
        String fileName = "Watcher.txt";

        try {
            // Create a BufferedReader object with FileReader to read the file
            BufferedReader reader = new BufferedReader(new FileReader(fileName));
            String line;

            // Print the existing content of the file
            System.out.println("Existing content of the file:");
            while ((line = reader.readLine()) != null) {
                System.out.println(line);
            }
            reader.close();
        }
        catch (IOException e) {
            // Catch any exceptions that occur and print the error message
            System.out.println("Exception occurred: " + e);
        }

        // Append the new content to the file
        String str = "WELCOME";
        appendStrToFile(fileName, str);

        // Print the modified content of the file
        try {
            // Create a BufferedReader object with FileReader to read the file again
            BufferedReader reader = new BufferedReader(new FileReader(fileName));
            String line;

            // Print the existing and appended content of the file
            System.out.println("\nModified content of the file:");
            while ((line = reader.readLine()) != null) {
                System.out.println(line);
            }
            reader.close();
        }
        catch (IOException e) {
            // Catch any exceptions that occur and print the error message
            System.out.println("Exception occurred: " + e);
        }
    }
}

Output:

Existing content of the file:
Java is a high-level programming language. Java is portable, robust, and dynamic, with the ability to fit the needs of all types of applications.

Modified content of the file:
Java is a high-level programming language. Java is portable, robust, and dynamic, with the ability to fit the needs of all types of applications.
WELCOME

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