Lazy initialization

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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:

  • using a factory method to get instances of a class (factory method pattern)
  • storing the instances in a map, so you get the same instance the next time you ask for an instance with same parameter (compare with a singleton pattern)
  • using lazy initialization to instantiate the object the first time it is requested (lazy initialization pattern).

Examples

C#

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();
        }
    }
}
 
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();
    }
}

[edit] Java

Here is an example in Java.

public class Fruit {
 
    private static final Map<String,Fruit> types = new HashMap<String, Fruit>();
    private final String type;
 
    // using a private constructor to force use of the factory method.
    private Fruit(String type) {
        this.type = type;
    }
 
    /**
    * 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 synchronized Fruit getFruit(String type) {
        if(!types.containsKey(type)) {
            types.put(type, new Fruit(type)); // Lazy initialization
        }
        return types.get(type);
    }
 
}

[edit] JavaScript

Here is an example in JavaScript.

var Fruit = (function () {
 
  var types = {};
 
  function Fruit() {};
 
 
  // counts 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 (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

[edit] 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) {
    Fruit *f = types[type];   // try to find a pre-existing instance, or std::map'll create one if not found
 
    if (!f) {                   // if it was created by map automatically, it'll be pointing to NULL
        // couldn't find one, so make a new instance
        f = new Fruit(type); // lazy initialization part
        types[type] = f;    // Registering the newly created Fruit in the types' map for later use.
    }
    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
*/

[edit] 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].
        ^height

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.

[edit] 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

[edit] 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')

[edit] PHP

Here is an example of lazy initialization in PHP 5:

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