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Introduction Did you ever encounter that even after the statement DEFINITION DEFERRED you still need to move the definition of your class? Or have you ever wondered that if a singleton-class has several subclasses, why all those subclasses will be singletons, or that all classes in the tree will be one singleton? Those experiences were new to me, but I encountered them and I wanted to share my amazement. This blog is a mixture of a how-to create a factory-class for singletons with inheritance, some surprises I found during coding and a request for advice. So please, please comment and ask questions. I would love to learn from my mistakes and wrong assumptions. I chose to not use test-driven approach to keep this blog more readable. The road to a solution to test several different solutions for the same problems using inheritance and a factory class was not a straight road – not at all. It was such a bumpy ride that I thought this was worthy of its own blog. This blog first appeared at our own website. The challenge We want to perform an operation on something. In my case, I wanted to test several solutions for a certain operation on an object. So I need: ◉ Uniformity in calling the solution. ◉ Different reactions for the operation. ◉ Managed instantiation. An additional “restraint” is that this is a proof of concept. I didn’t want to clog the system with dozens of artifacts, so I created it in one program. This led to one interesting issue… Interface and inheritance – Creating the Singleton(s) First, I want to have uniformity in calling the solution. So I define an interface that states what operation should be used: INTERFACE: zlif_interface. METHODS: do_something RETURNING VALUE(zrv_text) TYPE string. ENDINTERFACE. This is the most important part of the whole solution, and now it’s done. Almost finished! Let us think now about the other parts. It would be nice if we can re-use some standard code, and only have few differences. That can be solved by creating a superclass, with several subclasses. The polymorphism of the subclasses will take care of the different behaviour. CLASS zlcl_super DEFINITION ABSTRACT CREATE PROTECTED. PUBLIC SECTION. INTERFACES zlif_interface. ALIASES: do_something FOR zlif_interface~do_something. ENDCLASS. CLASS zlcl_subclass_one DEFINITION INHERITING FROM zlcl_super CREATE PROTECTED. PUBLIC SECTION. METHODS: do_something REDEFINITION. ENDCLASS. CLASS zlcl_subclass_two DEFINITION INHERITING FROM zlcl_super CREATE PROTECTED. PUBLIC SECTION. METHODS: do_something REDEFINITION. ENDCLASS. I decided to omit the IMPLEMENTATION part here for better readability. The UML diagram looks like this: UML: Singleton with Inheritance As we will use these classes for operations on other objects, I want to use them as a singleton, adding a static method get_instance( ) and define a static singleton reference in the interface. I mark the classes as CREATE PROTECTED. INTERFACE: zlif_interface. METHODS: do_something RETURNING VALUE(zrv_text) TYPE string. CLASS-DATA: zgr_instance TYPE REF TO zlif_interface. CLASS-METHODS: get_instance RETURNING VALUE(zrv_instance) TYPE REF TO zlif_interface. E
NDINTERFACE. CLASS zlcl_super DEFINITION ABSTRACT CREATE PROTECTED. PUBLIC SECTION. INTERFACES zlif_interface. ENDCLASS. CLASS zlcl_super IMPLEMENTATION. METHOD do_something. … ENDMETHOD. METHOD get_instance. IF zgr_instance IS INITIAL. zgr_instance = NEW #( ). ENDIF. zrv_instance = zgr_instance. ENDMETHOD. ENDCLASS. So far so good, right? Not really! Issue #01: I would like to implement the functionality for a singleton in my superclass, but I would like to keep it abstract as well. What could I do? I tried creating a reference to my interface. Silly me, that’s not allowed of course. I really don’t want to give up that the superclass is abstract, so the only way to go is to define the singleton-method get_instance in every subclass. . But wait! The get_instance( ) method should be a static (class) method. And static methods cannot be redefined. This is a Catch-22, isn’t it? Luckily, I am not the first one who got into this trouble. “Former member” solved this issue before me by suggesting to change the parameters of the get_instance( ) from a returning to a changing parameter in an answer to this question. Hurrah! I can keep the superclass abstract, I can even define all subclasses as CREATE PRIVATE, although I have to define all subclasses as a friend of the superclass. So the code now looks like this: INTERFACE: zlif_interface. METHODS: do_something RETURNING VALUE(zrv_text) TYPE string. CLASS-DATA: zgr_instance TYPE REF TO zlif_interface. CLASS-METHODS: get_instance CHANGING zcv_instance TYPE any. ENDINTERFACE. CLASS zlcl_super DEFINITION ABSTRACT CREATE PROTECTED. PUBLIC SECTION. INTERFACES zlif_interface. ALIASES: zgr_instance FOR zlif_interface~zgr_instance, do_something FOR zlif_interface~do_something, get_instance FOR zlif_interface~get_instance. ENDCLASS. CLASS zlcl_super IMPLEMENTATION. METHOD get_instance. IF zgr_instance IS INITIAL AND cl_abap_refdescr=\u003edescribe_by_data( zcv_instance )-\u003ekind = cl_abap_typedescr=\u003ekind_ref. DATA(lo_ref_descr) = CAST cl_abap_refdescr( cl_abap_refdescr=\u003edescribe_by_data( zcv_instance ) ). DATA(zlv_classname) = lo_ref_descr-\u003eget_referenced_type( )-\u003eget_relative_name( ). DATA: zlr_instance TYPE REF TO zlif_interface. CREATE OBJECT zlr_instance TYPE (zlv_classname). zgr_instance ?= zlr_instance. ENDIF. zcv_instance ?= zgr_instance. ENDMETHOD. ENDCLASS. Nice! Now we have a singleton with inheritance. I did encounter some frustrations though. I love chaining methods, so actually I would have liked this statement: CREATE OBJECT DATA(zlr_instance) TYPE ( CAST cl_abap_refdescr( cl_abap_refdescr=\u003edescribe_by_data( zcv_instance ) )-\u003eget_referenced_type( )-\u003eget_relative_name( ) ). That I can’t use the CAST keyword in the type-declaration I can understand. But I find it quite annoying that I can’t use a method to retrieve the type in the type-declaration. This yields a syntax error: CREATE OBJECT DATA(zlr_instance) TYPE \
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Hướng dẫn sử dụng Singleton pattern để hiện thực một đối tượng tồn tại duy nhất, có thể truy xuất mọi lúc mọi nơi trong chương trình.
Java Design Pattern singleton
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Singleton design pattern, singleton pattern, singleton vs static class, why singleton is an anti-pattern, why singleton sealed, singleton vs transient
