Java Programming File Handling Tutorial Study Guide

Java File Handling - Complete Tutorial

Master Java File I/O: Learn to read, write, create, delete files and directories using File, FileReader, FileWriter, BufferedReader, BufferedWriter, FileInputStream, FileOutputStream classes with practical examples.

1. Introduction to File Handling

File handling lets programs read and write persistent data. Java provides java.io for streams and java.nio.file for modern path-based operations.

Files survive program termination
Character streams for text
Byte streams for binary data
Always handle IOException

File intro

import java.io.File;

public class FileIntro {
    public static void main(String[] args) {
        File f = new File("data.txt");
        System.out.println("Exists: " + f.exists());
    }
}

2. The File Class

The File class represents file and directory pathnames. It can check existence, create directories, list contents, and delete files.

exists(), isFile(), isDirectory()
getName(), getPath(), length()
mkdir() and mkdirs() for folders
delete() removes file or empty dir

File operations

import java.io.File;

public class FileOps {
    public static void main(String[] args) {
        File dir = new File("logs");
        if (!dir.exists()) dir.mkdir();
        System.out.println(dir.isDirectory());
    }
}

3. FileReader and FileWriter

FileReader and FileWriter handle character-based text file I/O. They are convenient for small text files but less efficient than buffered variants.

Character streams for text
Extend InputStreamReader/OutputStreamWriter
Specify encoding carefully in production
Use try-with-resources

FileWriter example

import java.io.*;

public class FileWriteText {
    public static void main(String[] args) throws IOException {
        try (FileWriter fw = new FileWriter("note.txt")) {
            fw.write("Hello File");
        }
    }
}

4. BufferedReader and BufferedWriter

Buffered streams read and write in chunks, reducing system calls. BufferedReader.readLine() is ideal for line-oriented text files.

Wrap FileReader/FileWriter
readLine() returns null at EOF
newLine() writes platform line separator
Much faster for large files

BufferedReader

import java.io.*;

public class ReadLines {
    public static void main(String[] args) throws IOException {
        try (BufferedReader br = new BufferedReader(
                new FileReader("note.txt"))) {
            String line;
            while ((line = br.readLine()) != null) {
                System.out.println(line);
            }
        }
    }
}

5. FileInputStream and FileOutputStream

Byte streams read and write raw bytes for images, audio, and serialized data. They work with binary files where character encoding does not apply.

Binary file copy
read() returns -1 at end
Use byte array buffer in loops
Pair with BufferedInputStream for speed

Copy bytes

import java.io.*;

public class BinaryCopy {
    public static void main(String[] args) throws IOException {
        try (FileInputStream in = new FileInputStream("a.dat");
             FileOutputStream out = new FileOutputStream("b.dat")) {
            int b;
            while ((b = in.read()) != -1) out.write(b);
        }
    }
}

6. Built-in File Utilities

Java NIO Files class provides static helpers: copy, move, delete, readAllBytes, and walk for directory trees with less boilerplate.

Files.readString(Path) — Java 11+
Files.writeString for quick writes
Paths.get() builds path objects
StandardOpenOption for append/create

NIO Files API

import java.nio.file.*;

public class NioFiles {
    public static void main(String[] args) throws Exception {
        Path path = Paths.get("hello.txt");
        Files.writeString(path, "NIO write");
        String text = Files.readString(path);
        System.out.println(text);
    }
}

7. Practical File Examples

Common tasks include logging to files, reading configuration, and copying backups. Combine streams with exception handling for robust code.

Append log entries with BufferedWriter
Count lines in a text file
Copy file with Files.copy
Check file size before processing

Append log

import java.io.*;

public class LogAppend {
    public static void main(String[] args) throws IOException {
        try (BufferedWriter bw = new BufferedWriter(
                new FileWriter("app.log", true))) {
            bw.write("Server started");
            bw.newLine();
        }
    }
}

8. File Handling Best Practices

Reliable file code closes resources, handles missing files gracefully, and uses appropriate APIs for text vs binary data.

Always use try-with-resources
Check exists() before read if needed
Prefer NIO Files for simple operations
Specify Charset for text encoding
Validate paths to prevent directory traversal