How to read all files in a folder to a single file using Java?

The listFiles() method of the File class returns an array holding the objects (abstract paths) of all the files (and directories) in the path represented by the current (File) object.

To read the contents of all the files in a folder into a single file −

• Create a file object by passing the required file path as a parameter.
• Read the contents of each file using Scanner or any other reader.
• Append the read contents into a StringBuffer.
• Write the StringBuffer contents into the required output file.

Example

import java.io.DataOutputStream;
import java.io.File;
import java.io.FileOutputStream;
import java.io.IOException;
import java.util.Scanner;
public class Test {
   public static void main(String args[]) throws IOException {
      //Creating a File object for directory
      File directoryPath = new File("D:\\SampleDirectory");
      //List of all files and directories
      File filesList[] = directoryPath.listFiles();
       System.out.println("List of files and directories in the specified directory:");
      Scanner sc = null;
      StringBuffer sb = new StringBuffer();
      for(File file : filesList) {
         System.out.println("File name: "+file.getName());
         System.out.println("File path: "+file.getAbsolutePath());
         System.out.println("Size :"+file.getTotalSpace());
         //Instantiating the Scanner class
         sc= new Scanner(file);
         String input;
         while (sc.hasNextLine()) {
            input = sc.nextLine();
            sb.append(input+" ");
         }
         System.out.println("Contents of the file: "+sb.toString());
         System.out.println(" ");        
           //Instantiating the FileOutputStream class
         FileOutputStream fileOut = new FileOutputStream("D:\\output.txt");
         //Instantiating the DataOutputStream class
         DataOutputStream outputStream = new DataOutputStream(fileOut);
         //Writing UTF data to the output stream
         outputStream.write(sb.toString().getBytes());
         outputStream.flush();
         System.out.println("Data entered into the file");
      }
   }
}

Output

List of files and directories in the specified directory:
File name: sample1.txt
File path: D:\SampleDirectory\sample1.txt
Contents of the file: sample text file1

Data entered into the file
File name: sample2.txt
File path: D:\SampleDirectory\sample2.txt
Contents of the file: sample text file2

Data entered into the file
File name: sample3.txt
File path: D:\SampleDirectory\sample3.txt
Contents of the file: sample text file3

Data entered into the file