Reading and writing files

import java.nio.file.*;

Path data = Path.of("data", "orders.csv");          // data/orders.csv (or data\orders.csv on Windows)
System.out.println(data.getFileName());              // orders.csv
System.out.println(data.toAbsolutePath());          // full path
System.out.println(Files.exists(data));             // true or false
Path backup = data.resolveSibling("orders-backup.csv");

import java.io.IOException;
import java.nio.charset.StandardCharsets;
import java.util.List;

try {
    String all = Files.readString(data);                         // whole file as one String (UTF-8)
    List<String> lines = Files.readAllLines(data, StandardCharsets.UTF_8);
    System.out.println(lines.size() + " lines");
} catch (NoSuchFileException e) {
    System.out.println("File not found: " + e.getFile());
} catch (IOException e) {
    System.out.println("Could not read: " + e.getMessage());
}

Files.createDirectories(Path.of("reports"));
Files.writeString(Path.of("reports", "summary.txt"), "Total: Rs 1350\n");          // creates or replaces
Files.writeString(Path.of("reports", "log.txt"), "run at 10:00\n",
        StandardOpenOption.CREATE, StandardOpenOption.APPEND);                       // append
Files.write(Path.of("reports", "cities.txt"), List.of("Pune", "Delhi", "Goa"));     // one line each

try (Stream<String> lines = Files.lines(Path.of("access.log"))) {
    long serverErrors = lines.filter(l -> l.contains(" 500 ")).count();
    System.out.println("500 errors: " + serverErrors);
}

try (BufferedWriter w = Files.newBufferedWriter(Path.of("out.csv"))) {
    w.write("id,amount");
    w.newLine();
}

record Student(String name, String city, int marks) {}

List<Student> students;
try (Stream<String> lines = Files.lines(Path.of("students.csv"))) {
    students = lines.skip(1)                                  // skip the header row
            .filter(l -> !l.isBlank())
            .map(l -> l.split(","))
            .map(p -> new Student(p[0].strip(), p[1].strip(), Integer.parseInt(p[2].strip())))
            .toList();
}