Often, there is a requirement that you would want to create only one instance of the class. To put it in other words, you do not want any one to create instance of the class if already created. This is a common requirement in many application designs. So how do we achieve this ?

This can be achieved using the singleton pattern. In this design pattern, we will declare the constructor as a 'Private' one. Private constructor !! Is that possible?

Go to the properties tab of class builder (SE24) and change the instantiation type to private. Then you can create a private constructor. The advantage of having a private constructor is that now no one can create an object of your class.. not even you

So how do we create the instance of this class ? We create a static method GET_INSTANCE that will be used to create the instance. A static method because this method can be called directly with the class reference (without an object). Inside this method, we can create the object (because the constructor is a private method, we can call it within class). 

Now, how to make sure that only one instance is created? Simple.

Store the own reference as an attribute of the class. In the method GET_INSTANCE check if this attribute is populated. If it populated then return the same instance. If not, create a new one, store it in the own attribute and return the instance.

An example implementation of GET_INSTANCE is shown here. Lets say the name of the attribute is GREF_OBJ.

IF gref_obj IS INITIAL.

CREATE OBJECT gref_obj TYPE (class name).

ENDIF.

RETURN gref_obj.

An example how to create the instance of the class is shown here:

lref_obj = (class_name)=>get_instance_of().

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archana  - need answer-doubt   |address59.93.29.142 |2009-01-24 12:46:16 can you please help me in finding the answer of this question?it would be a great help for me... SE24 If you have a "Singleton Pattern", you must ensure that only one object can be created from a cl_singleton class. What mechanisms must you avail of here? -The singleton class must have a class method implemented in which the CREATE OBJECT call is programmed for this one object. -The singleton class must have the addition CREATE PRIVATE in the definition part. -In the singleton class, there must be an even defined that is triggered when the  first and only object is created and also prevents further objects of this class from being created. -The singleton class must have an instance method implemented in which the CREATE OBJECT call is programmed for this one object. -The CREATE OBJECT call for this one object can take place in the class constructor of the single... Reply | Quote 1   0

mastram  - Singleton Class   |Publisher |2009-01-24 16:17:27 Hello Archana, The simplest and most commonly used mechanism is: a) Make the constructor private. To do this go to the properties tab of your class in SE24 and set instantiation type as Private. b) Declare a class attribute called GREF_OBJ TYPE REF TO CL_SINGLETON_CLASS b) Define a public static method named GET_INSTANCE( ) - this will have a returning parameter rref_object of TYPE your CL_SINGLETON_CLASS In this method, check if GREF_OBJ is initial, then create an object of the class and store it in GREF_OBJ. Then return the RREF_OBJ = GREF_OBJ. Points to note: a) Since constructor is private, no one from outside the class can instantiate this class. b) By providing a public static method GET_INSTANCE you provide the outside world to get the instance of this class c) Since before returning the object from GET_INSTANCE you always check if the object is initial, you ensure that only one object is created I hope it was Reply | Quote 0   0

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