مقداردهی اولیه کاهلانه

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<>();

/**
 * 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;
                // 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)) {
// 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) {
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();
}
}
}