Editing Lazy initialization (section)

===Java===
{{Warning |This example is not thread-safe, see the [[Talk:Lazy initialization#Java code: thread safety flaw and pertinence|talk page]]. Instead see the examples in [[Double-checked locking#Usage in Java]].}}

This example is in [[Java (programming language)|Java]].

<syntaxhighlight lang="java">
import java.util.HashMap;
import java.util.Map;
import java.util.Map.Entry;

public class Program {

    /**
     * @param args
     */
    public static void main(String[] args) {
        Fruit.getFruitByTypeName(FruitType.banana);
        Fruit.showAll();
        Fruit.getFruitByTypeName(FruitType.apple);
        Fruit.showAll();
        Fruit.getFruitByTypeName(FruitType.banana);
        Fruit.showAll();
    }
}

enum FruitType {
    none,
    apple,
    banana,
}

class Fruit {

    private static Map<FruitType, Fruit> types = new HashMap<>();
    
    /**
     * Using a private constructor to force the use of the factory method.
     * @param type
     */
    private Fruit(FruitType 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(FruitType 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(FruitType 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<FruitType, Fruit> entry : types.entrySet()) {
                String fruit = entry.getKey().toString();
                fruit = Character.toUpperCase(fruit.charAt(0)) + fruit.substring(1);
                System.out.println(fruit);
            }
            
            System.out.println();
        }
    }
}
</syntaxhighlight>

'''Output'''
<pre>
Number of instances made = 1
Banana

Number of instances made = 2
Banana
Apple

Number of instances made = 2
Banana
Apple
</pre>