import java.util.Map;
import java.util.function.DoubleUnaryOperator;

public class Main {
    static final Map<String, DoubleUnaryOperator> DISCOUNTS = Map.of(
            "none", price -> price,
            "festive", price -> price * 0.8,
            "flat50", price -> Math.max(0, price - 50));

    static double checkout(double price, String code) {
        return DISCOUNTS.getOrDefault(code, DISCOUNTS.get("none")).applyAsDouble(price);
    }

    public static void main(String[] args) {
        System.out.println(checkout(200, "festive"));
        System.out.println(checkout(200, "flat50"));
        System.out.println(checkout(200, "bogus"));
    }
}

160.0
150.0
200.0

import java.util.ArrayList;
import java.util.List;
import java.util.function.Consumer;

public class Main {
    static final class StockTicker {
        private final List<Consumer<Double>> listeners = new ArrayList<>();

        Runnable subscribe(Consumer<Double> l) {
            listeners.add(l);
            return () -> listeners.remove(l);          // handle to unsubscribe
        }

        void publish(double price) {
            listeners.forEach(l -> l.accept(price));
        }
    }

    public static void main(String[] args) {
        StockTicker ticker = new StockTicker();
        ticker.subscribe(p -> System.out.println("display: " + p));
        Runnable cancel = ticker.subscribe(p -> {
            if (p > 100) System.out.println("alert: above 100");
        });

        ticker.publish(99.5);
        ticker.publish(101.0);
        cancel.run();                                  // stop the alerts
        ticker.publish(120.0);
    }
}

display: 99.5
display: 101.0
alert: above 100
display: 120.0

import java.io.BufferedReader;
import java.io.IOException;
import java.io.StringReader;

public class Main {
    interface Coffee { double cost(); String describe(); }

    record Espresso() implements Coffee {
        public double cost() { return 2.0; }
        public String describe() { return "espresso"; }
    }

    record WithMilk(Coffee inner) implements Coffee {
        public double cost() { return inner.cost() + 0.5; }
        public String describe() { return inner.describe() + " + milk"; }
    }

    record WithSyrup(Coffee inner) implements Coffee {
        public double cost() { return inner.cost() + 0.75; }
        public String describe() { return inner.describe() + " + syrup"; }
    }

    public static void main(String[] args) throws IOException {
        Coffee order = new WithSyrup(new WithMilk(new Espresso()));
        System.out.println(order.describe() + " = " + order.cost());

        // the JDK version: a Reader decorated with buffering and readLine()
        try (BufferedReader r = new BufferedReader(new StringReader("line 1\nline 2"))) {
            System.out.println(r.readLine());
        }
    }
}

espresso + milk + syrup = 3.25
line 1

public class Main {
    static abstract class Report {
        final String render() {                      // the template — not overridable
            return header() + "\n" + body() + "\n-- end --";
        }
        String header() { return "REPORT"; }         // hook with a default
        abstract String body();                      // step subclasses must supply
    }

    static final class SalesReport extends Report {
        @Override String body() { return "sales: 42 units"; }
    }

    public static void main(String[] args) {
        System.out.println(new SalesReport().render());
    }
}

REPORT
sales: 42 units
-- end --

import java.util.function.Supplier;

public class Main {
    static final class LegacyThermometer {                  // can't change this class
        double readFahrenheit() { return 98.6; }
    }

    interface CelsiusSensor { double celsius(); }

    static CelsiusSensor adapt(LegacyThermometer t) {     // the adapter
        return () -> (t.readFahrenheit() - 32) * 5 / 9;
    }

    public static void main(String[] args) {
        CelsiusSensor s = adapt(new LegacyThermometer());
        Supplier<String> fmt = () -> String.format("%.1f C", s.celsius());
        System.out.println(fmt.get());
    }
}

37.0 C