Lesson 19 of 20

File Handling

Read, write, append, and organise text data using Java's modern file APIs.

Learning objectives

• Represent file locations with Path.
• Write and read text files with Files.
• Append text without overwriting existing content.
• Handle IOException.
• Build a small persistent notes application.

1. Path and Files

The java.nio.file package provides modern file APIs. Path represents a location; Files provides operations such as read, write, copy, and create-directory.

Example: write a text file

import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Path;

public class WriteFile {
    public static void main(String[] args) {
        Path file = Path.of("message.txt");

        try {
            Files.writeString(file, "Hello from Java!
");
            System.out.println("File written.");
        } catch (IOException e) {
            System.out.println("Write failed: " + e.getMessage());
        }
    }
}

2. Read a text file

Path file = Path.of("message.txt");

try {
    String content = Files.readString(file);
    System.out.println(content);
} catch (IOException e) {
    System.out.println("Read failed: " + e.getMessage());
}

3. Append instead of overwrite

import java.nio.file.StandardOpenOption;

Files.writeString(
    Path.of("log.txt"),
    "Application started
",
    StandardOpenOption.CREATE,
    StandardOpenOption.APPEND
);

CREATE creates the file if necessary. APPEND adds text at the end instead of replacing the existing file.

4. Working with multiple lines

import java.util.List;

List<String> students = List.of(
    "S001,Amina,82",
    "S002,Daniel,74",
    "S003,Mei,91"
);

Files.write(Path.of("students.csv"), students);

Mini project: persistent notes

import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.StandardOpenOption;
import java.util.Scanner;

public class NotesApp {
    public static void main(String[] args) {
        Path notesFile = Path.of("notes.txt");

        try (Scanner input = new Scanner(System.in)) {
            System.out.print("Enter a note: ");
            String note = input.nextLine();

            Files.writeString(
                notesFile,
                note + System.lineSeparator(),
                StandardOpenOption.CREATE,
                StandardOpenOption.APPEND
            );

            System.out.println("
All notes:");
            System.out.println(Files.readString(notesFile));

        } catch (IOException e) {
            System.out.println("File error: " + e.getMessage());
        }
    }
}

Common mistakes

Summary

You can now persist information beyond one program run. Lesson 20 introduces collections and generics, which are more flexible than fixed-size arrays for many real applications.