Dateien und Ein-/Ausgabe

import java.io.IOException;
import java.nio.file.*;
import java.util.*;

public class Dateien {
    public static void main(String[] args) throws IOException {
        Path ordner = Files.createTempDirectory("kurs");
        Path datei = ordner.resolve("notiz.txt");               // Pfad zusammensetzen

        Files.writeString(datei, "Zeile 1\nZeile 2\n");
        Files.writeString(datei, "Zeile 3\n", StandardOpenOption.APPEND);

        String inhalt = Files.readString(datei);
        List<String> zeilen = Files.readAllLines(datei);
        System.out.println(zeilen + " " + inhalt.length());
        System.out.println(Files.exists(datei) + " " + Files.size(datei) + " " + datei.getFileName());

        Path kopie = ordner.resolve("kopie.txt");
        Files.copy(datei, kopie);
        try (var strom = Files.list(ordner)) {
            strom.map(p -> p.getFileName().toString()).sorted().forEach(System.out::println);
        }
        Files.delete(kopie);
        Files.delete(datei);
        Files.delete(ordner);
        System.out.println(Files.exists(ordner));
    }
}

import java.io.*;
import java.nio.charset.StandardCharsets;
import java.nio.file.*;

public class Zeilen {
    public static void main(String[] args) throws IOException {
        Path p = Files.createTempFile("daten", ".txt");
        Files.write(p, java.util.List.of("Mia;17", "Tom;19", "Zoe;21"), StandardCharsets.UTF_8);

        try (BufferedReader r = Files.newBufferedReader(p)) {
            String zeile;
            int summe = 0;
            while ((zeile = r.readLine()) != null) {
                String[] teile = zeile.split(";");
                summe += Integer.parseInt(teile[1]);
            }
            System.out.println("Summe der Alter: " + summe);
        }
        try (var stream = Files.lines(p)) {                      // als Stream
            System.out.println(stream.filter(z -> z.contains("o")).count());
        }
        try (BufferedWriter w = Files.newBufferedWriter(p)) {
            w.write("neu");
            w.newLine();
        }
        System.out.println(Files.readString(p).trim());
        Files.delete(p);
    }
}

import java.nio.file.*;

public class Pfade {
    public static void main(String[] args) {
        Path p = Path.of("ordner", "unter", "bild.png");
        System.out.println(p + " " + p.getFileName() + " " + p.getParent() + " " + p.getNameCount());
        System.out.println(Path.of("/a/b/../c/./d").normalize());
        System.out.println(Path.of("/a/b").relativize(Path.of("/a/b/c/d")));
        System.out.println(p.toString().endsWith(".png") + " " + p.isAbsolute());
    }
}

import java.io.IOException;
import java.nio.file.*;
import java.util.stream.*;

public class Baum {
    public static void main(String[] args) throws IOException {
        Path wurzel = Files.createTempDirectory("baum");
        Files.createDirectories(wurzel.resolve("a/b"));
        Files.writeString(wurzel.resolve("a/eins.txt"), "1");
        Files.writeString(wurzel.resolve("a/b/zwei.txt"), "22");
        try (Stream<Path> s = Files.walk(wurzel)) {
            s.filter(Files::isRegularFile)
             .map(p -> wurzel.relativize(p).toString())
             .sorted()
             .forEach(System.out::println);
        }
        try (Stream<Path> s = Files.walk(wurzel)) {                 // aufräumen: tiefste zuerst
            s.sorted(java.util.Comparator.reverseOrder()).forEach(p -> p.toFile().delete());
        }
        System.out.println(Files.exists(wurzel));
    }
}

import java.util.Scanner;

public class Eingabe {
    public static void main(String[] args) {
        Scanner sc = new Scanner("Mia 17\n3.5\nein ganzer Satz\n");     // statt System.in
        String name = sc.next();
        int alter = sc.nextInt();
        double zahl = sc.nextDouble();
        sc.nextLine();                                     // Rest der Zeile verwerfen
        String satz = sc.nextLine();
        System.out.println(name + " " + alter + " " + zahl + " | " + satz);
        System.out.println(sc.hasNextLine());
    }
}

ObjectMapper mapper = new ObjectMapper();
String json = mapper.writeValueAsString(new Person("Mia", 17));      // {"name":"Mia","alter":17}
Person p = mapper.readValue(json, Person.class);