Flyweight Pattern

Flyweight Pattern Reuse without multiple instances to be memory friendly

This page describes the following ・ Basic configuration of Flyweight ·Implementation

Flyweight basic configuration

class	Explanation
factory.class	sam.Do not duplicate instances of class
sam.class	Target class for which user instantiates
user(Main.class)	factory.sam via class.Create a class instance

User Includes the meaning that other developers use this pattern

Basic structure of factory.class and sam.class

`factory.class`


class factory{
  Map map;
  sam getSam(){}
}

In factory, the generated sam () instance is held in Map Manage to avoid duplication

`sam.class`


class sam{
  String s;
  sam(){}
}

sam () registers String s in the constructor

Implement the basic structure below

`factory.class`


class factory{
  Map map = new HashMap();        // sam()Store the instance
  sam getSam(String key){         
      sam s = (sam) map.get(key); //Get an instance from map
      if(s != null){return s;}    //If there is an instance on the map, pass it to user
      sam sm= new sam(key);       //If there is no instance in map, create an instance
      map.put(key,sm);            //After creating a new instance, register it in map
      return sm;                  //Publish an instance to user
  }
}

`sam.class`


class sam{
  String s;
  sam(String s){}
}

Implementation

There is a problem with the above code. The factory has a Map and manages the sam () instance. When the factory itself is made into multiple interfaces, different maps will be created by that number.

If that happens, you will not be able to manage the sam () instance properly. So let factory.class be a singleton (It is not possible to prevent the duplication of the map by itself, but as a temporary process)

Also implement a print () method that prints the Map contents for further verification

`factory.class`


class factory{Map map = new HashMap();
  private static final factory fac = new factory();

  private factory(){}
  static  factory getInstance(){
          return fac;
  }

  sam getSam(String key){
      sam s = (sam) map.get(key);
      if( s != null){return s;}
      sam sm = new sam(key);
      map.put(key,sm);
      return sm;
  }

  void print(){
    Iterator it = map.keySet().iterator();
    while(it.hasNext()){
      sam s = (sam) map.get(it.next());
      System.out.println(s+":"+s.get());
    }
  }
}

`sam.class`


class sam{
  private String s;
  sam(String s){this.s=s;}
  String get(){return this.s;}
}

`user(Main.class)`


public static void main(String[] args){
  factory f = factory.getInstance();
  f.getSam("a");
  f.getSam("b");
  f.getSam("c");
  f.getSam("a");
  f.print();
}}