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 thing is summoned, the flag is tested. If it's ready, it is returned. If not, it is initialized on the spot.

[edit] 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 some parameter (compare with a singleton pattern)
using lazy initialization to instantiate the object the first time it is requested (lazy initialization pattern)

Here is a dummy example (in Java). The Fruit class itself doesn't do anything here, this is just an example to show the architecture. The class variable types is a map used to store Fruit instances by type.

 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){
     final Fruit f;
 
     if(types.containsKey(type)){
       f= types.get(type); // get the instance for that type
     }
     else {
       f= new Fruit(type); // lazy initialization
       types.put(type,f);
    }
 
    return f;
   } 
 }

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.