Why is it necessary to create an instance via the static method?

Question

Hi,
For some classes (such as java.util.regex.Pattern), we should call the class method (static method) in order to create an instance (object).
For example, in order to conduct the pattern matching, we should use the java.util.regex.Pattern class as follows:
Pattern p = Pattern.compile ("X[0-9]+X") ;
  // An instance of the Pattern class is created.
Matcher m = p.matcher ("abcX1XYX23Xz") ;
while ( m.find() ){
  System.out.println ( m.start() ) ;
}where the compile static method in the Pattern class creates an instance and returns the pointer (reference) of it. We should NOT call
the constructor of the Pattern class as follows:
Pattern p = new Pattern ("X[0-9]+X") ;    // ERRORThe question is the following:
(1) In what scenes, do we develop the classes that force users to call the static method for creating an instance of it?
(2) Why do the java.util.regex.Pattern class have such a specification?
Thanks in advance.

Answer 1

(1) In what scenes, do we develop the classes that force users to call the static method for creating an instance of it?1. As the other author mentioned, caching is one reason.
2. With such caching, you don't need to take trouble in passing the reference of a cached object(s) to many places in your code. From anywhere in your code base, you can simply invoke the method, the object will come. In essence, the static method provides a global point of access to one or more pre-created (or cached) objects. Hence, the static method simplifies access to the object.
3. Sometimes, the actual class instantiated is not the same as the one with the static method. This allows abstraction of underlying variations. For example, when you say Pattern.compile ("X[0-9]+X") , the returned object type can be different in Windows and Linux (Most probably Pattern class doesn't work like that, but I am showing you a use case. May be Runtime.getRuntime() does actually work like that.). You find this abstraction of variations in many places. Take for example, FacesContext.getExternalContext() method (this is from JSF API). ExternalContext documentation says this:
"This class allows the Faces API to be unaware of the nature of its containing application environment. In particular, this class allows JavaServer Faces based appications to run in either a Servlet or a Portlet environment."
Edited by: Kamal Wickramanayake on Oct 24, 2012 8:04 AM