Example2.java

public class Example2 {

    static class NotFoundException extends Exception {}

    static class Database {
        java.util.Map<String,Integer> entries;

        Database() {
            entries = java.util.Collections.synchronizedMap(new java.util.HashMap<String,Integer>());
        }
        int Get(String key) throws NotFoundException{
            Integer value;
            if ((value = entries.get(key)) == null) {
                throw new NotFoundException();
            }
            return value;
        }
        void Set(String key, int value) {
            entries.put(key, value);
        }
    }

    static class TheAPI {
        Database db;

        TheAPI(Database db) {
            this.db = db;
        }
        void Set(String key, int initialValue) {
            db.Set(key, initialValue);
        }
        int Get(String key) throws NotFoundException {
            return db.Get(key);
        }
        int AddTo(String key, int value) throws NotFoundException {
            int newValue = db.Get(key) + value;
            db.Set(key, newValue);
            return newValue;
        }
        int MultiplyBy(String key, int value) throws NotFoundException {
            int newValue = db.Get(key) * value;
            db.Set(key, newValue);
            return newValue;
        }
    }

    /* -------------------------------------------------- */

    static TheAPI api = new TheAPI(new Database());
    static {
        api.Set(new String("fish"), 5);
    }

    public static void main(String args[]) throws NotFoundException {
        Runnable
            adder = () -> {
                try {
                    api.AddTo(new String("fish"), 3);
                } catch (NotFoundException e) {}
            },
                multiplier = () -> {
                try {
                    api.MultiplyBy(new String("fish"), 2);
                } catch (NotFoundException e) {}
            };

        Thread
            t1 = new Thread(adder),
            t2 = new Thread(multiplier);

        t1.start();
        t2.start();
        try {
            t1.join();
            t2.join();
        } catch (InterruptedException e) {}

        System.out.printf("Result: %d\n", api.Get(new String("fish")));
    }
}