Късна инициализация () е създаващ шаблон за дизайн, който се използва в обектно-ориентираното програмиране.
Целта на късната инициализация е да се отложи във времето създаването на обект, изчисляването на стойност или на някакъв друг отнемащ ресурси процес, до момента в който не ни е нужен за първи път.
Примери 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# Същия пример за C# using System; using System.Collections.Generic;
public class Fruit { private static Dictionary<string,Fruit> types = new Dictionary<string,Fruit>(); private string type;
/// <summary> /// using a private constructor to force use of the factory method. /// </summary> /// <param name="type">Type of fruit</param> private Fruit(String type) { this.type = type;
/// </summary> /// <param name="type">Any string that describes a fruit type, e.g. "apple"</param> /// <returns>The Fruit instance associated with that type.</returns> public static Fruit getFruit(string type) { Fruit f;
if (! types.TryGetValue(type, out f))
f = new Fruit(type); // lazy initialization types.Add(type,f);
return f;
public static void printCurrentTypes() { if (types.Count > 0) { Console.WriteLine("Number of instances made = {0}",types.Count); foreach (KeyValuePair<string,Fruit> kvp in types)
Console.WriteLine(kvp.Key);
Console.WriteLine();
class Program { static void Main(string[] args)
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();
Console.ReadLine();
C++ #include <iostream> #include <string> #include <map>
using namespace std;
class Fruit { private: static map<string,Fruit*> types; string type;
// note: constructor private forcing one to use static getFruit() Fruit(const string& t) : type(t) {}
public: static Fruit* getFruit(const string& type); static void printCurrentTypes();
//declaration 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
if (!f) { // 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.size() > 0) { 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 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 require 'net/http' class Blogger def auth_token return @auth_token if @auth_token
res = Net::HTTP.post_form(uri, params) @auth_token = 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
Вижте също Шаблони за дизайн
Външни препратки Article "Java Tip 67: Lazy instantiation - Balancing performance and resource usage" by Philip Bishop and Nigel Warren Java code examples Use Lazy Initialization to Conserve Resources Description from the Portland Pattern Repository Lazy Initialization of Application Server Services Lazy Inheritance in JavaScript
Категория:Шаблони за софтуерен дизайн