Category Archives: Java

Singleton pattern revisited

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Singleton pattern

Definition: At any point of time we ensure that there is only object created for the class

Example:
participant classes:

  • Singleton.java
  • SingletonClient.java(client)

Diagram:

Singleton.java

  • This class includes a static member variable singleton with private scope
  • the constructor is private so that no one can instantiate it
  • we have a getCurrentInstance() method to instantiate the class. If the instance is already available then the instance is returned to the caller otherwise it is newly created
public class Singleton {

 private static Singleton singleton = null;

 private Singleton(){

 }

 public static synchronized Singleton getCurrentInstance(){
 if(singleton == null){
 return new Singleton();
 }
 else{
 return singleton;
 }
 }

 public void sayHello(){
 System.out.println("Hello");
 }
}

SingletonClient.java

use getCurrentInstance() method to get the instance of Singleton class and use it.

 public class SingletonClient {

 public static void main(String[] args) {

 Singleton singleton =  Singleton.getCurrentInstance();
 singleton.sayHello();
 }
}

contract:

cannot instatiate more than once.
getCurrentInstance() return only one instance at any point of time

Implementing Prototype pattern

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Prototype pattern

Definition: the pattern is used to create a clone avoiding another creation. We clone the object if the cost of creating the object is expensive than cloning it.

Example:
participant classes:

  • Vinod.java
  • Rinku.java
  • Prototype.java
  • PrototpyeClient.java (client)

Diagram:


Prototype.java

This is an interface class defining signature for doClone() method. The return type is of Prototype type. This throws CloneNotSupportedException.

 public interface Prototype {
 public Prototype doClone() throws CloneNotSupportedException;
}

Vinod.java & Rinku.java

These are simple classes describing Vinod’s and Rinku’s characteristics. They have their attributes and their respective accessors defined in the class

These two classes implement the Prototype interface
doClone() method is implemented returning a new clone of the respective class

//Vinod.java
 public class Vinod implements Prototype,Cloneable{
 String name;
 String age;
 String sex;

 public String getName() {
 return "Vinod";
 }
 public String getAge() {
 return "25";
 }
 public String getSex() {
 return "Male";
 }
 @Override
 public Prototype doClone() throws CloneNotSupportedException {
 return (Prototype) this.clone();

 }

}

//Rinku.java

public class Rinku implements Prototype,Cloneable{
 String name;
 String age;
 String sex;

 public String getName() {
 return "Rinku";
 }
 public String getAge() {
 return "22";
 }
 public String getSex() {
 return "Female";
 }
 @Override
 public Prototype doClone() throws CloneNotSupportedException {
 return (Prototype) this.clone();

 }
}

PrototypeClient.java

Instantiating the objects and cloning the same

public class PrototypeClient {

 public static void main(String[] args) throws CloneNotSupportedException {

 Vinod vinod = new Vinod();
 Vinod vinod2 = (Vinod)vinod.doClone();

 System.out.println(vinod.hashCode() != vinod2.hashCode());
 System.out.println(vinod.getClass() == vinod2.getClass());

 }
}

contract:

x.clone() != x
x.clone().getClass() == x.getClass()

How to hide the viewcriteria from the af:query panel

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If you want to hide the view criteria, then here is the easy way
The code is written in the setter of the af:query binding

public void setQuery_binding(RichQuery query_binding) {

 this.query_binding = query_binding;

 DCBindingContainer bindings = (DCBindingContainer)this.getBindings();
 DCIteratorBinding iter =
 bindings.findIteratorBinding("<TableIterator>");

 ViewObjectImpl voimpl = (ViewObjectImpl)iter.getViewObject();
 ViewCriteria vc =  voimpl.getViewCriteriaManager().getViewCriteria("");
 vc.setProperty(ViewCriteriaHints.CRITERIA_SHOW_IN_LIST, ViewCriteriaHints.CRITERIA_HINT_FALSE);

 }
  • getting the handle to the iterator
  • getting handle to the ViewCriteriaManager, and then getting handle to the specific view criteria that we wanted to hide
  • modifying the ViewCriteriaHints.CRITERIA_SHOW_IN_LIST property to false

note: helper method – getBindings()

 private BindingContainer getBindings() {
 if (this.bindings == null) {
 FacesContext fc = FacesContext.getCurrentInstance();
 this.bindings =
 (BindingContainer)fc.getApplication().evaluateExpressionGet(fc,
 "#{bindings}",
 BindingContainer.class);
 }
 return this.bindings;
 }