Main.java

package de.cwansart;

import java.io.IOException;
import java.io.InputStream;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.io.OutputStream;
import java.nio.file.Files;
import java.nio.file.Paths;
import java.util.Arrays;
import java.util.List;

public class Main {
    private static final String HIGHSCORE_FILE = "highscore.class";

    private static List<Integer> highscore;

    public static void main(String[] args) {
        loadHighscore();
        printHighscore();

        // Überschreibe Highscore
        highscore = Arrays.asList(55, 44, 33);

        writeHighscore();
        printHighscore();
    }
    private static void printHighscore() {
        System.out.println("Hightscore");
        for (int i = 0; i < highscore.size(); i++) {
            System.out.println("#" + (i+1) + ": " + highscore.get(i));
        }
    }
    private static void loadHighscore() {
        // Prüfen ob die Highscore-Datei schon existiert. Wenn ja, dann laden, sonst mit einer leeren Liste anfangen.
        if (Files.exists(Paths.get(HIGHSCORE_FILE))) {
            try (InputStream inputStream = Files.newInputStream(Paths.get(HIGHSCORE_FILE));
                 ObjectInputStream objectInputStream = new ObjectInputStream(inputStream)) {
                
                highscore = (List<Integer>) objectInputStream.readObject();
            } catch (IOException | ClassNotFoundException e) {
                e.printStackTrace();
            }
        } else {
            // Highscore Liste mit 3x "0" erzeugen.
            highscore = Arrays.asList(0, 0, 0);
        }
    }
    private static void writeHighscore() {
        try (OutputStream outputStream = Files.newOutputStream(Paths.get(HIGHSCORE_FILE));
             ObjectOutputStream objectOutputStream = new ObjectOutputStream(outputStream)) {
            objectOutputStream.writeObject(highscore);
        } catch (IOException exception) {
            exception.printStackTrace();
        }
    }
}

Output should be something like this:

Hightscore
#1: 333
#2: 222
#3: 111
Hightscore
#1: 55
#2: 44
#3: 33