I/O Streams

Streams Overview

Java I/O is based on streams — a sequence of data flowing from a source to a destination. All I/O classes are in the java.io package.

Byte Streams

import java.io.*;

public class ByteStreamDemo {
    public static void main(String[] args) throws IOException {
        // Writing bytes
        FileOutputStream fos = new FileOutputStream("data.bin");
        fos.write(65);  // writes byte value 65 ('A')
        fos.write(new byte[]{72,101,108,108,111}); // "Hello"
        fos.close();

        // Reading bytes
        FileInputStream fis = new FileInputStream("data.bin");
        int b;
        while ((b = fis.read()) != -1) {
            System.out.print((char) b);
        }
        fis.close();
    }
}

Character Streams

import java.io.*;

public class CharStreamDemo {
    public static void main(String[] args) throws IOException {
        // Writing characters
        FileWriter fw = new FileWriter("notes.txt");
        fw.write("Java I/O is simple!
");
        fw.write("Character streams handle Unicode.");
        fw.close();

        // Reading characters
        FileReader fr = new FileReader("notes.txt");
        int ch;
        while ((ch = fr.read()) != -1) {
            System.out.print((char) ch);
        }
        fr.close();
    }
}

BufferedReader & BufferedWriter

Buffered streams wrap other streams and add an internal buffer, drastically improving I/O performance by reducing the number of disk operations.

import java.io.*;

public class BufferedDemo {
    public static void main(String[] args) throws IOException {
        // Buffered writing
        BufferedWriter bw = new BufferedWriter(new FileWriter("output.txt"));
        bw.write("Line 1: Java is powerful");
        bw.newLine();
        bw.write("Line 2: I/O is efficient");
        bw.flush();   // ensure buffer is written to disk
        bw.close();

        // Buffered reading — line by line
        BufferedReader br = new BufferedReader(new FileReader("output.txt"));
        String line;
        while ((line = br.readLine()) != null) {
            System.out.println(line);
        }
        br.close();
    }
}

File Class

The File class represents a file or directory path. It does NOT read or write content — it works with file metadata.

import java.io.File;

File f = new File("output.txt");

System.out.println(f.exists());       // true/false
System.out.println(f.getName());      // output.txt
System.out.println(f.length());      // size in bytes
System.out.println(f.isFile());       // true
System.out.println(f.isDirectory()); // false

// Create directory
new File("myFolder").mkdir();

// List files in a directory
File dir = new File(".");
for (String name : dir.list()) System.out.println(name);

Append Mode & try-with-resources

// Append to existing file (second argument = true)
FileWriter fw = new FileWriter("log.txt", true);
fw.write("New entry appended\n");
fw.close();

// try-with-resources — auto closes stream (Java 7+)
try (BufferedReader br = new BufferedReader(new FileReader("log.txt"))) {
    String line;
    while ((line = br.readLine()) != null)
        System.out.println(line);
} // br.close() called automatically