import java.util.HashMap;
 import java.util.Map;
 import java.util.Map.Entry;
 
 public enum FRUIT_TYPE {
        NONE,
        APPLE,
        BANANA,
 }
 
 class Program {
 
        /**
         * @param args
         */
        public static void main(String[] args) {
                Fruit.getFruitByTypeName(FRUIT_TYPE.BANANA);
                Fruit.showAll();
                Fruit.getFruitByTypeName(FRUIT_TYPE.APPLE);
                Fruit.showAll();
                Fruit.getFruitByTypeName(FRUIT_TYPE.BANANA);
                Fruit.showAll();
        }
 }
 
 public class Fruit {
        private static Map<FRUIT_TYPE, Fruit> types = new HashMap<FRUIT_TYPE, Fruit>();
 
        /**
         * Using a private constructor to force the use of the factory method.
         * @param type
         */
        private Fruit(FRUIT_TYPE type) {
        }
 
        /**
         * Lazy Factory method, gets the Fruit instance associated with a certain
         * type. Instantiates new ones as needed.
         * @param type Any allowed fruit type, e.g. APPLE
         * @return The Fruit instance associated with that type.
         */
        public static Fruit getFruitByTypeName(FRUIT_TYPE type) {
                Fruit fruit;
 
                if (!types.containsKey(type)) {
                        // Lazy initialisation
                        fruit = new Fruit(type);
                        types.put(type, fruit);
                } else {
                        // OK, it's available currently
                        fruit = types.get(type);
                }
 
                return fruit;
        }
 
        /**
         * Lazy Factory method, gets the Fruit instance associated with a certain
         * type. Instantiates new ones as needed. Uses double-checked locking 
         * pattern for using in highly concurrent environments.
         * @param type Any allowed fruit type, e.g. APPLE
         * @return The Fruit instance associated with that type.
         */
        public static Fruit getFruitByTypeNameHighConcurrentVersion(FRUIT_TYPE type) {
                // This has concurrency issues.  Here the read to types is not synchronized, 
                // so types.put and types.containsKey might be called at the same time.
                // Don't be surprised if the data is corrupted.
                if (!types.containsKey(type)) {
                        synchronized (types) {
                                // Check again, after having acquired the lock to make sure
                                // the instance was not created meanwhile by another thread
                                if (!types.containsKey(type)) {
                                        // Lazy initialisation
                                        types.put(type, new Fruit(type));
                                }
                        }
                }
 
                return types.get(type);
        }
 
        /**
         * Displays all entered fruits.
         */
        public static void showAll() {
                if (types.size() > 0) {
                        System.out.println("Number of instances made = " + types.size());
 
                        for (Entry<FRUIT_TYPE, Fruit> entry : types.entrySet()) {
                                System.out.println(
                                                Constants.firstLetterToUpper(entry.getKey().toString()));
                        }
 
                        System.out.println();
                }
        }
 }

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CategoryCoding