Java Files

FileReader and FileWriter are classes in the Java IO API (Application Programming Interface) that allow you to read and write files, respectively. They provide a convenient way to work with files as character streams.

FileReader is used to read the contents of a file as a stream of characters. It is a subclass of the InputStreamReader class, which is itself a subclass of Reader. The FileReader class takes a String or a File object as an argument to its constructor, specifying the name of the file to be read.

FileWriter is used to write the contents of a file as a stream of characters. It is a subclass of OutputStreamWriter class, which is itself a subclass of Writer. The FileWriter class takes a String or a File object as an argument to its constructor, specifying the name of the file to be written.

BufferedReader and BufferedWriter are classes that provide buffering for input and output streams, respectively. They can be used to improve the performance of reading and writing to a file.

BufferedReader wraps an existing Reader and buffers the input. It reads a large block of characters from the underlying stream and then allows the program to read them one at a time, which is much more efficient than reading one character at a time from the underlying stream. BufferedReader has a readLine() method that makes it easy to read the contents of a file one line at a time.

BufferedWriter wraps an existing Writer and buffers the output. It writes a large block of characters to the underlying stream, rather than writing one character at a time. This improves performance by reducing the number of calls to the underlying stream. BufferedWriter has a newLine() method that can be used to add a new line after each line of text that is written to a file.

Example program for reading and writing a file

import java.io.*;

public class FileReadWrite {
    public static void main(String[] args) {
        // The name of the file to open
        String fileName = "examplefile.txt";

        // This will reference one line at a time
        String line = null;

        try {
            // FileReader reads text files in the default encoding
            FileReader fileReader = new FileReader(fileName);

            // Always wrap FileReader in BufferedReader
            BufferedReader bufferedReader = new BufferedReader(fileReader);

            // The name of the new file to create
            String newFileName = "duplicatefile.txt";

            // This will output the contents of the file to the new file
            FileWriter fileWriter = new FileWriter(newFileName);
            BufferedWriter bufferedWriter = new BufferedWriter(fileWriter);

            while((line = bufferedReader.readLine()) != null) {
                bufferedWriter.write(line);
                bufferedWriter.newLine();
            }

            // Always close files
            bufferedReader.close();
            bufferedWriter.close();
        }
        catch(FileNotFoundException ex) {
            System.out.println("Unable to open file '" + fileName + "'");
        }
        catch(IOException ex) {
            System.out.println("Error reading file '" + fileName + "'");
            // Or we could just do this:
            // ex.printStackTrace();
        }
    }
}

This program first defines the name of the file that it will read and the name of the new file that it will write. It then creates a FileReader object to read the contents of the original file, and wraps it in a BufferedReader object. This makes it easy to read the contents of the file one line at a time, using the readLine() method of the BufferedReader object.

The program then creates a FileWriter object to write the contents of the file to the new file, and wraps it in a BufferedWriter object. The write() method is used to write the contents of the original file to the new file. The newLine() method is used to add a new line after each line of text that is written to the new file.

The program uses a try-catch block to handle any exceptions that may be thrown. If the file is not found, the program will print an error message to the console. If there is an error reading the file, the program will print another error message.

You can change the input file name and the outfile name, this is just a basic example to understand the flow, you can use different way to read and write a file as well, like FileInputStream and FileOutputStream, Path, etc.