Чистый код, SOLID и паттерны проектирования

import java.util.HashMap;
import java.util.List;
import java.util.Map;

public class Main {
    public static void main(String[] args) {
        AppConfig.INSTANCE.set("school", "School No. 6, Baku");
        System.out.println(AppConfig.INSTANCE.get("school"));
        System.out.println(AppConfig.INSTANCE == AppConfig.valueOf("INSTANCE"));

        for (String type : List.of("email", "sms")) {
            Notifier notifier = Notifier.of(type);
            notifier.send("Aysel", "Your exam score is 95");
        }
    }
}

enum AppConfig {                                   // Singleton: exactly one instance
    INSTANCE;
    private final Map<String, String> values = new HashMap<>();
    void set(String key, String value) { values.put(key, value); }
    String get(String key) { return values.get(key); }
}

interface Notifier {
    void send(String to, String text);

    static Notifier of(String type) {              // Factory: hides which class is created
        return switch (type) {
            case "email" -> (to, text) -> System.out.println("Email to " + to + ": " + text);
            case "sms" -> (to, text) -> System.out.println("SMS to " + to + ": " + text);
            default -> throw new IllegalArgumentException("Unknown type: " + type);
        };
    }
}

School No. 6, Baku
true
Email to Aysel: Your exam score is 95
SMS to Aysel: Your exam score is 95

public class Main {
    public static void main(String[] args) {
        Report full = new Report.Builder("Aysel")
                .subject("Math")
                .score(95)
                .comment("Excellent work")
                .build();
        Report minimal = new Report.Builder("Murad").build();
        System.out.println(full);
        System.out.println(minimal);
    }
}

record Report(String student, String subject, int score, String comment) {
    static class Builder {                          // Builder: readable step-by-step creation
        private final String student;
        private String subject = "General";
        private int score;
        private String comment = "";

        Builder(String student) { this.student = student; }
        Builder subject(String value) { subject = value; return this; }
        Builder score(int value) { score = value; return this; }
        Builder comment(String value) { comment = value; return this; }
        Report build() { return new Report(student, subject, score, comment); }
    }
}

Report[student=Aysel, subject=Math, score=95, comment=Excellent work]
Report[student=Murad, subject=General, score=0, comment=]

import java.util.ArrayList;
import java.util.List;

public class Main {
    public static void main(String[] args) {
        GradingRule strict = raw -> raw;                          // Strategy 1
        GradingRule withBonus = raw -> Math.min(100, raw + 5);    // Strategy 2

        GradeBoard board = new GradeBoard(withBonus);
        board.subscribe((name, score) -> System.out.println("[parent app] " + name + " got " + score));
        board.subscribe((name, score) -> System.out.println("[teacher log] " + name + ": " + score));

        board.addScore("Leyla", 88);
        board.setRule(strict);
        board.addScore("Elvin", 88);
    }
}

interface GradingRule { int apply(int raw); }
interface ScoreListener { void onScore(String name, int score); }

class GradeBoard {
    private GradingRule rule;
    private final List<ScoreListener> listeners = new ArrayList<>();

    GradeBoard(GradingRule rule) { this.rule = rule; }
    void setRule(GradingRule rule) { this.rule = rule; }
    void subscribe(ScoreListener listener) { listeners.add(listener); }

    void addScore(String name, int raw) {
        int score = rule.apply(raw);
        listeners.forEach(l -> l.onScore(name, score));          // Observer: notify everyone
    }
}

[parent app] Leyla got 93
[teacher log] Leyla: 93
[parent app] Elvin got 88
[teacher log] Elvin: 88