Files with java.nio.file

Everything you've written so far dies when the program ends. Files make data outlive the process. Modern Java does file I/O with the java.nio.file API - two classes carry almost everything: Path (a reference to a file) and Files (the operations on it).

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

Path notes = Path.of("notes.txt");

// write (replaces the file)
Files.writeString(notes, "first line\nsecond line\n");

// read whole file as one String
String content = Files.readString(notes);

// read as a list of lines
java.util.List<String> lines = Files.readAllLines(notes);

// check before you act
if (Files.exists(notes)) { ... }

Every read/write method declares throws IOException - the checked exception from Module 11 finally earning its keep: the compiler forces you to decide what happens when the disk disagrees with you.

try {
    String config = Files.readString(Path.of("config.txt"));
} catch (java.io.IOException e) {
    System.out.println("no config file, using defaults");
}

Appending, walking, and the small print

// append: CREATE first (if missing), then APPEND
Files.writeString(notes, "another line\n",
    java.nio.file.StandardOpenOption.CREATE,
    java.nio.file.StandardOpenOption.APPEND);

// visit every file under a directory
try (var walk = Files.walk(Path.of("logs"))) {
    walk.filter(p -> p.toString().endsWith(".txt"))
        .forEach(System.out::println);
}

Files.walk returns a stream - the try-with-resources block closes the directory handle when done. And remember the defensive habits from the parsing lesson: file content is dirty data. Every line you read gets the same null-guard, trim, and parse-check treatment as a CSV import.