Let the given two files be file1.txt and file2.txt. Our task is to merge the two files into a third file in java, such as file3.txt. The following are the merge steps.

1. Create PrintWriter object for file3.txt
2. Open BufferedReader for file1.txt
3. Run a loop to repeat each line of file1.txt to file3.txt
4. Open BufferedReader for file2.txt
5. Run a loop to repeat each line of file2.txt to file3.txt
6. Flush PrintWriter stream and shut resources.

In order to run the following java program successfully, file1.txt and file2.txt must go to the same folder OR provide the full path for the program.

// Java program to merge two
// files into third file

import java.io.*;

public class FileMerge
{
	public static void main(String[] args) throws IOException
	{
		// PrintWriter object for file3.txt
		PrintWriter pw = new PrintWriter("file3.txt");
		
		// BufferedReader object for file1.txt
		BufferedReader br = new BufferedReader(new FileReader("file1.txt"));
		
		String line = br.readLine();
		
		// loop to copy each line of
		// file1.txt to file3.txt
		while (line != null)
		{
			pw.println(line);
			line = br.readLine();
		}
		
		br = new BufferedReader(new FileReader("file2.txt"));
		
		line = br.readLine();
		
		// loop to copy each line of
		// file2.txt to file3.txt
		while(line != null)
		{
			pw.println(line);
			line = br.readLine();
		}
		
		pw.flush();
		
		// closing resources
		br.close();
		pw.close();
		
		System.out.println("Merged file1.txt and file2.txt into file3.txt");
	}
}

Output:

Merged file1.txt and file2.txt into file3.txt

Note :