Java Program to Append a String in an Existing File

In Java, we can append a string in an existing file using FileWriter which has an option to open a file in append mode. Java FileWriter class is used to write character-oriented data to a file. It is a character-oriented class that is used for file handling in Java. Unlike FileOutputStream class, we don’t need to convert the string into a byte array because it provides a method to write a string directly. 

Note: The buffer size may be specified, or the default size may be used. A Writer sends its output immediately to the underlying character or byte stream.

Let us see constructors used later on adhering towards as usual methods of this class 

Constructor: FileWriter(File file, boolean append): 

It Constructs a FileWriter object given a File object in append mode. Now let us toggle onto methods of this class which is invoked here and play a crucial role in appending a string in an existing file as follows:

Method 1: write()

This method writes a portion of a String

Syntax:

void write(String s,int off,int len);

Return Type: Void

Parameters: 

• Input string
• int off
• String length

Method 2: close()

This method closes the stream after flushing it.

Return Type: Void 

Example 

Java

// Java Program to Append a String to the
// End of a File

// Importing input output classes
import java.io.*;

// Main class
class GeeksforGeeks {

    // Method 1
    // TO append string into a file
    public static void appendStrToFile(String fileName,
                                       String str)
    {
        // Try block to check for exceptions
        try {

            // Open given file in append mode by creating an
            // object of BufferedWriter class
            BufferedWriter out = new BufferedWriter(
                new FileWriter(fileName, true));

            // Writing on output stream
            out.write(str);
            // Closing the connection
            out.close();
        }

        // Catch block to handle the exceptions
        catch (IOException e) {

            // Display message when exception occurs
            System.out.println("exception occurred" + e);
        }
    }

    // Method 2
    // main driver method
    public static void main(String[] args) throws Exception
    {
        // Creating a sample file with some random text
        String fileName = "Geek.txt";

        // Try block to check for exceptions
        try {

            // Again operating same operations by passing
            // file as
            // parameter to read it
            BufferedWriter out = new BufferedWriter(
                new FileWriter(fileName));

            // Writing on. file
            out.write("Hello World:\n");

            // Closing file connections
            out.close();
        }

        // Catch block to handle exceptions
        catch (IOException e) {

            // Display message when error occurs
            System.out.println("Exception Occurred" + e);
        }

        // Now appendinggiven str to above
        // created file
        String str = "This is GeeksforGeeks";

        // Calling the above method
        appendStrToFile(fileName, str);

        // Let us print modified file
        try {
            BufferedReader in = new BufferedReader(
                new FileReader("Geek.txt"));

            String mystring;

            // TIll there is content in string
            // condition holds true
            while ((mystring = in.readLine()) != null) {
                System.out.println(mystring);
            }
        }

        // Catch block to handle IO exceptions
        catch (IOException e) {
            System.out.println("Exception Occurred" + e);
        }
    }
}

Output: