惰性初始模式

在程式设计中, 惰性初始是一种拖延战术。在第一次需求出现以前，先延迟创建物件、计算值或其它昂贵程序。
这通常是以一个旗号来实现，用旗号来标示是否完成其程式。每次请求对象时，会先测试此旗号。如果已完成，直接传回，否则当场执行。
对于此想法更一般的论述，可见惰性求值。
对指令式语言，这个模式可能潜藏着危险，尤其是使用共享状态的程式习惯。

C#

Fruit 类别本身在这里不做任合事。_typesDictionary 变数则是一个存Fruit 实例的Dictionary 或 Map ，其透过typeName来存取。
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.ContainsKey(type))
        {
            // 惰性初始
            fruit = new Fruit(type);

            _typesDictionary.Add(type, fruit);
        }
        else
            fruit = _typesDictionary[type];

        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();
    }}

Java

import java.util.*;
  
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);
    }}

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 variablemap<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
    }
    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;}/*輸出：Number of instances made = 1Banana Number of instances made = 2AppleBanana Number of instances made = 2AppleBanana*/

Smalltalk

The following 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.

Ruby

The following 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 classendb = Blogger.newb.instance_variable_get(:@auth_token) # returns nilb.auth_token # returns tokenb.instance_variable_get(:@auth_token) # returns token

Python

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

PHP

<?phpheader('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: 1AppleNumber of instances made: 2AppleBananaNumber of instances made: 2AppleBanana*/?>