Immutable and Singelton

Make a class immutable by following these guidelines :

* ensure the class cannot be overridden - make the class final, or use static factories and keep constructors private
* make fields private and final
* force callers to construct an object completely in a single step, instead of using a no-argument constructor combined with subsequent calls to setXXX methods (that is, avoid the Java Beans convention)
* do not provide any methods which can change the state of the object in any way - not just setXXX methods, but any method which can change state
* if the class has any mutable object fields, then they must be defensively copied when passed between the class and its caller


How can we make a class Singleton

1) make the constructor private.
2)Override clone method and throw clone supported exception.
3) Have a public method that creates an new instance when the instance is null or returns the new one.
A) If the class is Serializable
class Singleton implements Serializable
{
private static Singleton instance;
private Singleton() { }
public static synchronized Singleton getInstance()
{
if (instance == null)
instance = new Singleton();
return instance;
}
/**
If the singleton implements Serializable, then this
* method must be supplied.
*/
protected Object readResolve() {
return instance;
}
/**
This method avoids the object fro being cloned
*/
public Object clone() {
throws CloneNotSupportedException ;
//return instance;
}
}
B) If the class is NOT Serializable
class Singleton
{
private static Singleton instance;
private Singleton() { }

public static synchronized Singleton getInstance()
{
if (instance == null)
instance = new Singleton();
return instance;
}
/**
This method avoids the object from being cloned
**/
public Object clone() {
throws CloneNotSupportedException ;
//return instance;
}
}