Editing Lazy initialization (section)

==Examples==

===ActionScript 3===
The following is an example of a class with lazy initialization implemented in [[ActionScript]]:
<syntaxhighlight lang="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);
			}
		}
	}
}
</syntaxhighlight>
Basic use:
<syntaxhighlight lang="actionscript">
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();
		}
	}
}
</syntaxhighlight>

===C===
In [[C (programming language)|C]], lazy evaluation would normally be implemented inside one function, or one source file, using [[static variable]]s.

In a function:
<syntaxhighlight lang="C">
#include <string.h>
#include <stdlib.h>
#include <stddef.h>
#include <stdio.h>

struct fruit {
    char *name;
    struct fruit *next;
    int number;
    /* Other members */
};

struct fruit *get_fruit(char *name) {
    static struct fruit *fruit_list;
    static int seq;
    struct fruit *f;
    for (f = fruit_list; f; f = f->next)
        if (0 == strcmp(name, f->name))
            return f;
    if (!(f = malloc(sizeof(struct fruit))))
        return NULL;
    if (!(f->name = strdup(name))) {
        free(f);
        return NULL;
    }
    f->number = ++seq;
    f->next = fruit_list;
    fruit_list = f;
    return f;
}

/* Example code */

int main(int argc, char *argv[]) {
    int i;
    struct fruit *f;
    if (argc < 2) {
        fprintf(stderr, "Usage: fruits fruit-name [...]\n");
        exit(1);
    }
    for (i = 1; i < argc; i++) {
        if ((f = get_fruit(argv[i]))) {
            printf("Fruit %s: number %d\n", argv[i], f->number);
        }
    }
    return 0;
}
</syntaxhighlight>
Using one source file instead allows the state to be shared between multiple functions, while still hiding it from non-related functions.

fruit.h:
<syntaxhighlight lang="C">

#ifndef _FRUIT_INCLUDED_
#define _FRUIT_INCLUDED_

struct fruit {
    char *name;
    struct fruit *next;
    int number;
    /* Other members */
};

struct fruit *get_fruit(char *name);
void print_fruit_list(FILE *file);

#endif /* _FRUIT_INCLUDED_ */
</syntaxhighlight>

fruit.c:
<syntaxhighlight lang="C">
#include <string.h>
#include <stdlib.h>
#include <stddef.h>
#include <stdio.h>
#include "fruit.h"

static struct fruit *fruit_list;
static int seq;

struct fruit *get_fruit(char *name) {
    struct fruit *f;
    for (f = fruit_list; f; f = f->next)
        if (0 == strcmp(name, f->name))
            return f;
    if (!(f = malloc(sizeof(struct fruit))))
        return NULL;
    if (!(f->name = strdup(name))) {
        free(f);
        return NULL;
    }
    f->number = ++seq;
    f->next = fruit_list;
    fruit_list = f;
    return f;
}

void print_fruit_list(FILE *file) {
    struct fruit *f;
    for (f = fruit_list; f; f = f->next)
        fprintf(file, "%4d  %s\n", f->number, f->name);
}
</syntaxhighlight>

main.c:
<syntaxhighlight lang="C">

#include <stdlib.h>
#include <stdio.h>
#include "fruit.h"

int main(int argc, char *argv[]) {
    int i;
    struct fruit *f;
    if (argc < 2) {
        fprintf(stderr, "Usage: fruits fruit-name [...]\n");
        exit(1);
    }
    for (i = 1; i < argc; i++) {
        if ((f = get_fruit(argv[i]))) {
            printf("Fruit %s: number %d\n", argv[i], f->number);
        }
    }
    printf("The following fruits have been generated:\n");
    print_fruit_list(stdout);
    return 0;
}
</syntaxhighlight>

===C#===
In .NET Framework 4.0 Microsoft has included a <code>Lazy</code> class that can be used to do lazy loading.
Below is some dummy code that does lazy loading of Class <code>Fruit</code>
<syntaxhighlight lang="csharp">
var lazyFruit = new Lazy<Fruit>();
Fruit fruit = lazyFruit.Value;
</syntaxhighlight>

Here is a dummy example in [[C Sharp (programming language)|C#]].

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

<syntaxhighlight lang="csharp">
using System;
using System.Collections;
using System.Collections.Generic;

public class Fruit
{
    private string _typeName;
    private static IDictionary<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();
    }
}
</syntaxhighlight>
A fairly straightforward 'fill-in-the-blanks' example of a Lazy Initialization design pattern, except that this uses an [[Enumerated type|enumeration]] for the type
<syntaxhighlight lang="csharp">
namespace DesignPatterns.LazyInitialization;

public class LazyFactoryObject
{
    // internal collection of items
    // IDictionary makes sure they are unique
    private IDictionary<LazyObjectSize, LazyObject> _LazyObjectList =
        new Dictionary<LazyObjectSize, LazyObject>();

    // enum for passing name of size required
    // avoids passing strings and is part of LazyObject ahead
    public enum LazyObjectSize
    {
        None,
        Small,
        Big,
        Bigger,
        Huge
    }

    // standard type of object that will be constructed
    public struct LazyObject
    {
        public LazyObjectSize Size;
        public IList<int> Result;
    }

    // takes size and create 'expensive' list
    private IList<int> Result(LazyObjectSize size)
    {
        IList<int> result = null;

        switch (size)
        {
            case LazyObjectSize.Small:
                result = CreateSomeExpensiveList(1, 100);
                break;
            case LazyObjectSize.Big:
                result = CreateSomeExpensiveList(1, 1000);
                break;
            case LazyObjectSize.Bigger:
                result = CreateSomeExpensiveList(1, 10000);
                break;
            case LazyObjectSize.Huge:
                result = CreateSomeExpensiveList(1, 100000);
                break;
            case LazyObjectSize.None:
                result = null;
                break;
            default:
                result = null;
                break;
        }

        return result;
    }

    // not an expensive item to create, but you get the point
    // delays creation of some expensive object until needed
    private IList<int> CreateSomeExpensiveList(int start, int end)
    {
        IList<int> result = new List<int>();

        for (int counter = 0; counter < (end - start); counter++)
        {
            result.Add(start + counter);
        }

        return result;
    }

    public LazyFactoryObject()
    {
        // empty constructor
    }

    public LazyObject GetLazyFactoryObject(LazyObjectSize size)
    {
        // yes, i know it is illiterate and inaccurate
        LazyObject noGoodSomeOne;

        // retrieves LazyObjectSize from list via out, else creates one and adds it to list
        if (!_LazyObjectList.TryGetValue(size, out noGoodSomeOne))
        {
            noGoodSomeOne = new LazyObject();
            noGoodSomeOne.Size = size;
            noGoodSomeOne.Result = this.Result(size);

            _LazyObjectList.Add(size, noGoodSomeOne);
        }

        return noGoodSomeOne;
    }
}
</syntaxhighlight>

===C++===
This example is in [[C++]].

<syntaxhighlight lang="cpp">
import std;

class Fruit {
 public:
  static Fruit* GetFruit(const std::string& type);
  static void PrintCurrentTypes();

 private:
  // Note: constructor private forcing one to use static |GetFruit|.
  Fruit(const std::string& type) : type_(type) {}

  static std::map<std::string, Fruit*> types;

  std::string type_;
};

// static
std::map<std::string, Fruit*> Fruit::types;

// Lazy Factory method, gets the |Fruit| instance associated with a certain
// |type|.  Creates new ones as needed.
Fruit* Fruit::GetFruit(const std::string& type) {
  auto [it, inserted] = types.emplace(type, nullptr);
  if (inserted) {
    it->second = new Fruit(type);
  }
  return it->second;
}

// For example purposes to see pattern in action.
void Fruit::PrintCurrentTypes() {
  std::println("Number of instances made = {}", types.size());
  for (const auto& [type, fruit] : types) {
    std::println({}, type);
  }
  std::println();
}

int main() {
  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();
}

// OUTPUT:
//
// Number of instances made = 1
// Banana
//
// Number of instances made = 2
// Apple
// Banana
//
// Number of instances made = 2
// Apple
// Banana
//
</syntaxhighlight>

=== Crystal ===
<syntaxhighlight lang="ruby">
class Fruit
  private getter type : String
  @@types = {} of String => Fruit

  def initialize(@type)
  end

  def self.get_fruit_by_type(type : String)
    @@types[type] ||= Fruit.new(type)
  end

  def self.show_all
    puts "Number of instances made: #{@@types.size}"
    @@types.each do |type, fruit|
      puts "#{type}"
    end
    puts
  end

  def self.size
    @@types.size
  end
end

Fruit.get_fruit_by_type("Banana")
Fruit.show_all

Fruit.get_fruit_by_type("Apple")
Fruit.show_all

Fruit.get_fruit_by_type("Banana")
Fruit.show_all
</syntaxhighlight>

Output:
<pre>
Number of instances made: 1
Banana

Number of instances made: 2
Banana
Apple

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

=== Haxe ===
This example is in [[Haxe]].<ref>{{Cite news|url=https://code.haxe.org/category/design-patterns/lazy-initialization.html|title=Lazy initialization - Design patterns - Haxe programming language cookbook|date=2018-01-11|access-date=2018-11-09|language=en}}</ref>
<syntaxhighlight lang="haxe">
class Fruit {
  private static var _instances = new Map<String, Fruit>();

  public var name(default, null):String;

  public function new(name:String) {
    this.name = name;
  }

  public static function getFruitByName(name:String):Fruit {
    if (!_instances.exists(name)) {
      _instances.set(name, new Fruit(name));
    }
    return _instances.get(name);
  }

  public static function printAllTypes() {
    trace([for(key in _instances.keys()) key]);
  }
}
</syntaxhighlight>Usage<syntaxhighlight lang="haxe">
class Test {
  public static function main () {
    var banana = Fruit.getFruitByName("Banana");
    var apple = Fruit.getFruitByName("Apple");
    var banana2 = Fruit.getFruitByName("Banana");
    
    trace(banana == banana2); // true. same banana
    
    Fruit.printAllTypes(); // ["Banana","Apple"]
  }
}
</syntaxhighlight>

===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>

===JavaScript===
This example is in [[JavaScript]].

<syntaxhighlight lang="javascript">
var Fruit = (function() {
  var types = {};
  function Fruit() {};

  // count own properties in object
  function count(obj) {
    return Object.keys(obj).length;
  }

  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();

</syntaxhighlight>
'''Output'''
<pre>
Number of instances made: 1
Apple

Number of instances made: 2
Apple
Banana

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

===PHP===
Here is an example of lazy initialization in [[PHP]] 7.4:
<syntaxhighlight lang="php">
<?php
header('Content-Type: text/plain; charset=utf-8');

class Fruit
{
    private string $type;
    private static array $types = array();

    private function __construct(string $type)
    {
        $this->type = $type;
    }

    public static function getFruit(string $type)
    {
        // Lazy initialization takes place here
        if (!isset(self::$types[$type])) {
            self::$types[$type] = new Fruit($type);
        }

        return self::$types[$type];
    }

    public static function printCurrentTypes(): void
    {
        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
*/
</syntaxhighlight>

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

<syntaxhighlight lang="python">
class Fruit:
    def __init__(self, item: str):
        self.item = item
    
class FruitCollection:
    def __init__(self):
        self.items = {}
    
    def get_fruit(self, item: str) -> Fruit:
        if item not in self.items:
            self.items[item] = Fruit(item)
        
        return self.items[item]

if __name__ == "__main__":
    fruits = FruitCollection()
    print(fruits.get_fruit("Apple"))
    print(fruits.get_fruit("Lime"))
</syntaxhighlight>

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

<syntaxhighlight lang="ruby">
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
</syntaxhighlight>

===Rust===
[[Rust (programming language)|Rust]] have {{Code|std::cell::LazyCell}}.<ref>{{cite web |title=LazyCell in std::cell - Rust |url=https://doc.rust-lang.org/std/cell/struct.LazyCell.html |website=doc.rust-lang.org |access-date=18 January 2025}}</ref>

<syntaxhighlight lang="rust">
use std::cell::LazyCell;

let lazy = LazyCell::new(|| 42);
</syntaxhighlight>

===Scala===
[[Scala (programming language)|Scala]] has built-in support for lazy variable initiation.<ref>{{Cite book |last1=Pollak |first1=David |date=2009-05-25 |url=https://books.google.com/books?id=Qt-bRFetWw0C&q=scala+lazy+variables&pg=PA30 |title=Beginning Scala |publisher=Apress |isbn=9781430219897}}</ref>
<syntaxhighlight lang="scala" highlight="1,4,6,9">
 scala> val x = { println("Hello"); 99 }
 Hello
 x: Int = 99
 scala> lazy val y = { println("Hello!!"); 31 }
 y: Int = <lazy>
 scala> y
 Hello!!
 res2: Int = 31
 scala> y
 res3: Int = 31
</syntaxhighlight>

===Smalltalk===
This example is in [[Smalltalk]], of a typical [[accessor method]] to return the value of a variable using lazy initialization.

<syntaxhighlight lang="smalltalk">
    height
        ^height ifNil: [height := 2.0].
</syntaxhighlight>
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.

<syntaxhighlight lang="smalltalk">
    initialize
        height := 2.0

    height
        ^height
</syntaxhighlight>

Note that lazy initialization can also be used in non-[[object-oriented language]]s.