==================================Reading a text file line by line:

import java.io.BufferedReader;
import java.io.FileReader;
import java.io.IOException;
 
/**
 * This program demonstrates how to read characters from a text file
 * using a BufferedReader for efficiency.
 * @author www.codejava.net
 *
 */
public class TextFileReadingExample3 {
 
    public static void main(String[] args) {
        try {
            FileReader reader = new FileReader("MyFile.txt");
            BufferedReader bufferedReader = new BufferedReader(reader);
 
            String line;
 
            while ((line = bufferedReader.readLine()) != null) {
                System.out.println(line);
            }
            reader.close();
 
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
 
}
 ======================================Text File Writer:
Text File Writer: - appends if file exists:
import java.io.FileWriter;
import java.io.IOException;
 
/**
 * This program demonstrates how to write characters to a text file.
 * @author www.codejava.net
 *
 */
public class TextFileWritingExample1 {
 
    public static void main(String[] args) {
        try {
            FileWriter writer = new FileWriter("MyFile.txt", true);
            writer.write("Hello World");
            writer.write("\r\n");   // write new line
            writer.write("Good Bye!");
            writer.close();
        } catch (IOException e) {
            e.printStackTrace();
        }
 
    }
 
}
Note that, a writer uses default character encoding of the operating system by default. 
It also creates a new file if not exits, or overwrites the existing one. If you want to 
append text to an existing file, pass a boolean flag of true to constructor of the writer class:

FileWriter writer = new FileWriter("MyFile.txt", true);

======================================================Buffered File Writer:
Buffered file writer (preferred )
import java.io.BufferedWriter;
import java.io.FileWriter;
import java.io.IOException;
 
/**
 * This program demonstrates how to write characters to a text file
 * using a BufferedReader for efficiency.
 * @author www.codejava.net
 *
 */
public class TextFileWritingExample2 {
 
    public static void main(String[] args) {
        try {
            FileWriter writer = new FileWriter("MyFile.txt", true);
            BufferedWriter bufferedWriter = new BufferedWriter(writer);
 
            bufferedWriter.write("Hello World");
            bufferedWriter.newLine();
            bufferedWriter.write("See You Again!");
 
            bufferedWriter.close();
        } catch (IOException e) {
            e.printStackTrace();
        }
 
    }
 
}

==============================================================Buffered File Writer with UTF encoding
And the following example specifies specific character encoding (UTF-16) when writing to the file:

import java.io.BufferedWriter;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.OutputStreamWriter;
 
/**
 * This program demonstrates how to write characters to a text file using
 * a specified charset.
 * @author www.codejava.net
 *
 */
public class TextFileWritingExample3 {
 
    public static void main(String[] args) {
        try {
            FileOutputStream outputStream = new FileOutputStream("MyFile.txt");
            OutputStreamWriter outputStreamWriter = new OutputStreamWriter(outputStream, "UTF-16");
            BufferedWriter bufferedWriter = new BufferedWriter(outputStreamWriter);
             
            bufferedWriter.write("Xin chào");
            bufferedWriter.newLine();
            bufferedWriter.write("Hẹn gặp lại!");
             
            bufferedWriter.close();
        } catch (IOException e) {
            e.printStackTrace();
        }
         
    }
}
===================================================================Listing recursive directory objects
Listing files in a directory recursively
To list all sub files and sub directories nested in all levels, one solution is to use recursion algorithm:

List files in the first level.
For each file in the first level list files:
If the file is a directory:
Print out directory name.
Repeat the step 1 and 2 with the current directory.
If the file is a file:
Print out file name.
Continue with next file.
The following method implements the above algorithm:
import java.io.File;
/**
 *
 * @author www.codejava.net
 *
 */
public class ListDirectoryRecurisve {
    public void listDirectory(String dirPath, int level) {
        File dir = new File(dirPath);
        File[] firstLevelFiles = dir.listFiles();
        if (firstLevelFiles != null && firstLevelFiles.length > 0) {
            for (File aFile : firstLevelFiles) {
                for (int i = 0; i < level; i++) {
                    System.out.print("\t");
                }
                if (aFile.isDirectory()) {
                    System.out.println("[" + aFile.getName() + "]");
                    listDirectory(aFile.getAbsolutePath(), level + 1);
                } else {
                    System.out.println(aFile.getName());
                }
            }
        }
    }
    public static void main(String[] args) {
        ListDirectoryRecurisve test = new ListDirectoryRecurisve();
        String dirToList = System.getProperty("user.home") + File.separator + "Documents";
        test.listDirectory(dirToList, 0);
    }
}