☕ JavaFile I/O

File Handling in Java

Most real applications read from and write to files — config files, logs, exports, uploads. Java gives you classic I/O streams for fine-grained control and the modern java.nio.file API for clean, readable code. Know both.

Writing to a File

The try-with-resources pattern (try with the resource declared in parentheses) automatically closes the file when you're done — even if an exception occurs. Always use this over manually calling .close().

Java

import java.io.*;

// try-with-resources — file is automatically closed after the block
try (FileWriter fw = new FileWriter("log.txt", true);  // 'true' = append mode
     BufferedWriter bw = new BufferedWriter(fw)) {

    bw.write("Application started at: " + new java.util.Date());
    bw.newLine();
    bw.write("User logged in: alice@example.com");
    bw.newLine();

} catch (IOException e) {
    System.err.println("Failed to write log: " + e.getMessage());
}
// BufferedWriter is closed automatically — no need for finally block

Reading from a File

Java

import java.io.*;

// BufferedReader wraps FileReader for line-by-line reading (much more efficient)
try (FileReader fr = new FileReader("config.txt");
     BufferedReader br = new BufferedReader(fr)) {

    String line;
    while ((line = br.readLine()) != null) {  // readLine returns null at end of file
        System.out.println(line);
        // Parse config: String[] parts = line.split("=");
    }

} catch (FileNotFoundException e) {
    System.err.println("File not found: " + e.getMessage());
} catch (IOException e) {
    System.err.println("Error reading file: " + e.getMessage());
}

java.nio.file — The Modern Approach (Recommended)

The java.nio.file package (introduced in Java 7, enhanced in Java 8+) is cleaner, more powerful, and handles large files better. For new code, prefer this over the classic java.io approach.

Java

import java.nio.file.*;
import java.util.List;

// Write a file (overwrites by default)
Files.writeString(Path.of("output.txt"), "Hello from NIO!
Second line");

// Append to a file
Files.writeString(Path.of("output.txt"), "
Appended line",
    StandardOpenOption.APPEND);

// Read all lines into a List
List<String> lines = Files.readAllLines(Path.of("output.txt"));
lines.forEach(System.out::println);

// Read entire file as one String
String content = Files.readString(Path.of("output.txt"));

// File existence and metadata
Path p = Path.of("output.txt");
System.out.println(Files.exists(p));       // true
System.out.println(Files.size(p));         // file size in bytes
System.out.println(Files.isReadable(p));   // true

// Delete
Files.deleteIfExists(p);  // safer than delete() — won't throw if file doesn't exist

Synchronization