Lazy initialization

In computer programming, lazy initialization is the tactic of delaying the creation of an object, the calculation of a value, or some other expensive process until the first time it is needed.

This is typically accomplished by maintaining a flag indicating whether the process has taken place. Each time the desired object is summoned, the flag is tested. If it is ready, it is returned. If not, it is initialized on the spot.

See lazy evaluation for a general treatment of this idea. In heavily imperative languages this pattern carries hidden dangers, as does any programming habit that relies on shared state.

Contents

The "lazy factory"

In a software design pattern view, lazy initialization is often used together with a factory method pattern. This combines three ideas:

Examples

Java

Here is an example in Java.

import java.util.HashMap;
import java.util.Map;
import java.util.Map.Entry;
 
class Program {
	public static void main(String[] args) {
		Fruit.getFruitByTypeName("Banana");
		Fruit.showAll();
 
		Fruit.getFruitByTypeName("Apple");
		Fruit.showAll();
 
		// returns pre-existing instance from first
		// time Fruit with "Banana" was created
		Fruit.getFruitByTypeName("Banana");
		Fruit.showAll();
	}
}
 
public class Fruit {
	private String typeName;
	private static Map<String, Fruit> types = new HashMap<String, Fruit>();
 
	// using a private constructor to force use of the factory method.
	private Fruit(String typeName) {
		this.typeName = typeName;
	}
 
	/**
	 * Lazy Factory method, gets the Fruit instance associated with a certain
	 * type. Instantiates new ones as needed.
	 * 
	 * @param type
	 *            Any string that describes a fruit type, e.g. "apple"
	 * @return The Fruit instance associated with that type.
	 */
	public static Fruit getFruitByTypeName(String type) {
		Fruit fruit;
 
		if (!types.containsKey(type)) {
			// Lazy initialization
			fruit = new Fruit(type);
 
			types.put(type, fruit);
		} else {
			// Okay, it's available currently
			fruit = types.get(type);
		}
 
		return fruit;
	}
 
	// Same above but used double-checked locking pattern
	// for using in highly concurrent environments
	public static Fruit getFruitByTypeNameHighConcurrentVersion(String type) {
		Fruit fruit;
 
		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 initialization
					types.put(type, new Fruit(type));
				}
			}
		}
 
		fruit = types.get(type);
 
		return fruit;
	}
 
	public static void showAll() {
		if (types.size() > 0) {
			System.out.println("Number of instances made = " + types.size());
 
			for (Entry<String, Fruit> entry : types.entrySet()) {
				System.out.println(entry.getKey());
			}
 
			System.out.println();
		}
	}
}

Output

Number of instances made = 1
Banana

Number of instances made = 2
Apple
Banana

Number of instances made = 2
Apple
Banana

C#

In .NET 4.0 Microsoft has included a Lazy class which can be used to do lazy loading. Below is a dummy code which does lazy loading of Class Fruit

Lazy<Fruit> lazyFruit = new Lazy<Fruit>();
Fruit fruit = lazyFruit.Value;

Here is a dummy example in C#.

The Fruit class itself doesn't do anything here, The class variable _typesDictionary is a Dictionary/Map used to store Fruit instances by typeName.

using System;
using System.Collections;
using System.Collections.Generic;
 
public class Fruit
{
    private string _typeName;
    private static Dictionary<string, Fruit> _typesDictionary = new Dictionary<string, Fruit>();
 
    private Fruit(String typeName)
    {
        this._typeName = typeName;
    }
 
    public static Fruit GetFruitByTypeName(string type)
    {
        Fruit fruit;
 
        if (!_typesDictionary.TryGetValue(type, out fruit))
        {
            // Lazy initialization
            fruit = new Fruit(type);
 
            _typesDictionary.Add(type, fruit);
        }
        return fruit;
    }
 
    public static void ShowAll()
    {
        if (_typesDictionary.Count > 0)
        {
            Console.WriteLine("Number of instances made = {0}", _typesDictionary.Count);
 
            foreach (KeyValuePair<string, Fruit> kvp in _typesDictionary)
            {
                Console.WriteLine(kvp.Key);
            }
 
            Console.WriteLine();
        }
    }
 
    public Fruit()
    {
        // required so the sample compiles
    }
}
 
class Program
{
    static void Main(string[] args)
    {
        Fruit.GetFruitByTypeName("Banana");
        Fruit.ShowAll();
 
        Fruit.GetFruitByTypeName("Apple");
        Fruit.ShowAll();
 
        // returns pre-existing instance from first 
        // time Fruit with "Banana" was created
        Fruit.GetFruitByTypeName("Banana");
        Fruit.ShowAll();
 
        Console.ReadLine();
    }
}

Actionscript 3

The following is an example of a class with Lazy initialization implemented in Actionscript:

package examples.lazyinstantiation
{
	public class Fruit 
	{
		private var _typeName:String;
		private static var instancesByTypeName:Dictionary = new Dictionary();
 
		public function Fruit(typeName:String):void
		{
			this._typeName = typeName;
		}
 
		public function get typeName():String 
		{
			return _typeName;
		}
 
		public static function getFruitByTypeName(typeName:String):Fruit
		{
			return instancesByTypeName[typeName] ||= new Fruit(typeName);
		}
 
		public static function printCurrentTypes():void
		{
			for each (var fruit:Fruit in instancesByTypeName) 
			{
				// iterates through each value
				trace(fruit.typeName);
			}
		}
	}
}

Basic Usage:

package
{
	import examples.lazyinstantiation;
 
	public class Main 
	{
		public function Main():void
		{
			Fruit.getFruitByTypeName("Banana");
			Fruit.printCurrentTypes();
 
			Fruit.getFruitByTypeName("Apple");
			Fruit.printCurrentTypes();
 
			Fruit.getFruitByTypeName("Banana");
			Fruit.printCurrentTypes();
		}
	}
}

JavaScript

Here is an example in JavaScript.

var Fruit = (function() {
  var types = {};
  function Fruit() {};
 
  // count own properties in object
  function count(obj) {
    var i = 0;
    for (var key in obj) {
      if (obj.hasOwnProperty(key)) {
        ++i;
      }
    }
    return i;
  }
 
  var _static = {
    getFruit: function(type) {
      if (typeof types[type] == 'undefined') {
        types[type] = new Fruit;
      }
      return types[type];
    },
    printCurrentTypes: function () {
      console.log('Number of instances made: ' + count(types));
      for (var type in types) {
        console.log(type);
      }
    }
  };
 
  return _static;
 
})();
 
Fruit.getFruit('Apple');
Fruit.printCurrentTypes();
Fruit.getFruit('Banana');
Fruit.printCurrentTypes();
Fruit.getFruit('Apple');
Fruit.printCurrentTypes();

Output

Number of instances made: 1
Apple

Number of instances made: 2
Apple
Banana

Number of instances made: 2
Apple
Banana

C++

Here is an example in C++.

#include <iostream>
#include <string>
#include <map>
 
using namespace std;
 
class Fruit {
    public:
        static Fruit* getFruit(const string& type);
        static void printCurrentTypes();
 
    private:
        static map<string,Fruit*> types;
        string type;
 
        // note: constructor private forcing one to use static getFruit()
        Fruit(const string& t) : type( t ) {}
};
 
//definition needed for using any static member variable
map<string,Fruit*> Fruit::types;        
 
/*
 * Lazy Factory method, gets the Fruit instance associated with a
 * certain type. Instantiates new ones as needed.
 * precondition: type. Any string that describes a fruit type, e.g. "apple"
 * postcondition: The Fruit instance associated with that type.
 */
Fruit* Fruit::getFruit(const string& type) {
    map<string,Fruit*>::iterator it = types.find(type);  // try to find an existing instance; if not found std::map will return types.end()
 
    Fruit *f;
    if (it == types.end()) { // if no instance with the proper type was found, make one
        f = new Fruit(type); // lazy initialization part
        types[type] = f;     // adding the newly created Fruit to the types map for later lookup
    } else { //if already had an instance
        f = it->second; //The return value will be the found fruit
    }
    return f;
}
 
/*
 * For example purposes to see pattern in action
 */
void Fruit::printCurrentTypes() {
    if (!types.empty()) {
        cout << "Number of instances made = " << types.size() << endl;
        for (map<string,Fruit*>::iterator iter = types.begin(); iter != types.end(); ++iter) {
            cout << (*iter).first << endl;
        }
        cout << endl;
    }
}
 
int main(void) {
    Fruit::getFruit("Banana");
    Fruit::printCurrentTypes();
 
    Fruit::getFruit("Apple");
    Fruit::printCurrentTypes();
 
    // returns pre-existing instance from first 
    // time Fruit with "Banana" was created
    Fruit::getFruit("Banana");
    Fruit::printCurrentTypes();
 
    return 0;
}
 
/*
OUTPUT:
Number of instances made = 1
Banana
 
Number of instances made = 2
Apple
Banana
 
Number of instances made = 2
Apple
Banana
*/

Smalltalk

Here is an example in Smalltalk, of a typical accessor method to return the value of a variable using lazy initialization.

    height
        ^height ifNil: [height := 2.0].

The 'non-lazy' alternative is to use an initialization method that is run when the object is created and then use a simpler accessor method to fetch the value.

    initialize
        height := 2.0
 
    height
        ^height

Note that lazy initialization can also be used in non-object-oriented languages.

Ruby

Here is an example in Ruby, of lazily initializing an authentication token from a remote service like Google. The way that @auth_token is cached is also an example of memoization.

require 'net/http'
class Blogger
  def auth_token
    @auth_token ||=
      (res = Net::HTTP.post_form(uri, params)) &&
      get_token_from_http_response(res)
  end
 
  # get_token_from_http_response, uri and params are defined later in the class
end
 
b = Blogger.new
b.instance_variable_get(:@auth_token) # returns nil
b.auth_token # returns token
b.instance_variable_get(:@auth_token) # returns token

Python

Here is an example in Python.

class Fruit:
    def __init__(self, sort):
        self.sort = sort
 
class Fruits:
    def __init__(self):
        self.sorts = {}
 
    def get_fruit(self, sort):
        if sort not in self.sorts:
            self.sorts[sort] = Fruit(sort)
 
        return self.sorts[sort]
 
if __name__ == '__main__':
    fruits = Fruits()
    print fruits.get_fruit('Apple')
    print fruits.get_fruit('Lime')

PHP

Here is an example of lazy initialization in PHP 5:

<?php
header('Content-type:text/plain; charset=utf-8');
 
class Fruit {
    private $type;
    private static $types = array();
 
    private function __construct($type) {
        $this->type = $type;
    }
 
    public static function getFruit($type) {
        // Lazy initialization takes place here
        if (!array_key_exists($type, self::$types)) {
            self::$types[$type] = new Fruit($type);
        }
 
        return self::$types[$type];
    }
 
    public static function printCurrentTypes() {
        echo 'Number of instances made: ' . count(self::$types) . "\n";
        foreach (array_keys(self::$types) as $key) {
            echo "$key\n";
        }
        echo "\n";
    }
}
 
Fruit::getFruit('Apple');
Fruit::printCurrentTypes();
 
Fruit::getFruit('Banana');
Fruit::printCurrentTypes();
 
Fruit::getFruit('Apple');
Fruit::printCurrentTypes();
 
/*
OUTPUT:
 
Number of instances made: 1
Apple
 
Number of instances made: 2
Apple
Banana
 
Number of instances made: 2
Apple
Banana
*/
 
?>

See also

External links