Effective Java--1)Create/Destroy Object: static factory method

Static factory method

Static factory is another way to create a class instance other than constructor. [1] shows an example:

 public static Boolean valueOf(boolean b){  
   return b? Boolean.TRUE:Boolean.FALSE;  
 }  

Static factory is a special kind of static method. Static factory returns a class instance.
Pros:

• Static factory has name. Constructor doesn't have name, sometimes may call the wrong constructor.
• Do not require to create a new object each time they are invoked.
• Like Flyweight technique, greatly improve performance if equivalent objects are requested often, especially when they are expensive to create.
• Can return any subtype of their return type.
• Like EnumSet returns a single long if no more than 65 elements; otherwise backs a long array.
• Static factory reduces the verbosity of creating parameterized type instance.
• Constructor flavor:

   public static Boolean valueOf(boolean b){  
     return b? Boolean.TRUE:Boolean.FALSE;  
   }  
  

• Static factory method flavor:

  
  Map<String,List<String>> m = HashMap.newInstance();
  public static <K,V> HashMap<K,V> newInstance(){ return new HashMap<K,V>;}
  

Cons:

• Class without public/protected constructor cannot be subclassed.
• Not really distinguishable from other static method.