ABAP 类

类是我们需要理解和掌握的基本结构，以便为实体建模。

一个类可以代表任何东西；一个文件、一辆车、动物，或者任何有特征和行为的东西。

ABAP 中的类的编码分为两步：首先，你编码定义部分，然后是实现部分。定义部分是你定义所有数据和方法的地方，这些方法将在类中使用。

在这里，你必须指定 public、private 和 protected 部分，而且它们必须按照你在代码中看到的顺序放置，否则你会出现语法错误。

下面的代码显示了一个类的结构：

CLASS <Name of the class> DEFINITION.
  PUBLIC SECTION.
  PROTECTED SECTION.
  PRIVATE SECTION.
ENDCLASS.

CLASS <Name of the class> IMPLEMENTATION.
ENDCLASS.

请创建一个新程序，命名为 ZCLASS 并写入以下代码：

report  zclass.

class cl_animal definition abstract.
  public section.
    methods: constructor importing i_name type string, " Visible to everyone
             make_a_sound,
             my_name_is,
             get_type,
             introduce_me.

  protected section. " Visible only in child classes
    data p_class type string.

  private section. " Visible only internally
    data p_name type string.
endclass.

class cl_animal implementation.
  method constructor.
    p_name = i_name.     " p_name was defined already in the definition part of the class as private
    p_class = 'Unknown'. " p_class was defined already in the definition part of the class as protected
  endmethod.

  method make_a_sound.
    write 'Nothing'.
  endmethod.

  method my_name_is.
    write: / 'My name is: ', p_name.
  endmethod.

  method get_type.
    write: / 'I''m type of: ', p_class.
  endmethod.

  method introduce_me.
    me->my_name_is( ). " The keyword 'me' is used to specify class member. Is the equivalent of the keyword 'this' in C#
    make_a_sound( ).
    get_type( ).
  endmethod.
endclass.

class cl_dog definition inheriting from cl_animal.
  public section.
    methods: constructor importing i_dog_name type string,
             make_a_sound redefinition. " Change the behaviour of the method. Reimplement the code.
endclass.

class cl_dog implementation.
  method constructor.
    super->constructor( i_dog_name ). " Initialize the constructor and internally pass the parameter to the abstract class
    p_class = '"Dog"'.                " This is the protected member which is visible only in child classes
  endmethod.

  method make_a_sound.
    write: / 'My sound is:', 'Woof, woof'.
  endmethod.
endclass.

class cl_cat definition inheriting from cl_animal.
  public section.
    methods: constructor importing i_cat_name type string,
             make_a_sound redefinition.
endclass.

class cl_cat implementation.
  method constructor.
    super->constructor( i_cat_name ).
    p_class = '"Cat"'.
  endmethod.

  method make_a_sound.
    write: / 'My sound is:', 'Meow, meow'.
  endmethod.
endclass.

class cl_animal_factory definition.
  public section.
    class-methods create_animal importing i_animal type i returning value(r_animal) type ref to cl_animal. " Class method, in C# this is called a static method
endclass.

class cl_animal_factory implementation. " Factory pattern
  method create_animal.
    case i_animal.
      when 1.
        data dog type ref to cl_dog.
        create object dog exporting i_dog_name = 'Sparky'.
        r_animal = dog. " It is returned a cl_dog instance.
      when 2.
        data cat type ref to cl_cat.
        create object cat exporting i_cat_name = 'Fluffy'.
        r_animal = cat. " It is returned a cl_cat instance.
      when others.
    endcase.
  endmethod.
endclass.

class cl_introducer definition.
  public section.
    class-methods introduce importing i_animal type ref to cl_animal. " Here the method receives a cl_animal type parameter
endclass.

class cl_introducer implementation.
  method introduce.
    if i_animal is not initial.
      i_animal->introduce_me( ).
    else.
      write / 'I''m nothing'.
    endif.
  endmethod.
endclass.


start-of-selection.
  data wa_animal type ref to cl_animal.

  wa_animal = cl_animal_factory=>create_animal( 1 ).
  cl_introducer=>introduce( wa_animal ). " The i_animal parameter is implicitly specified. Useful when is only one parameter.
  write /.

  wa_animal = cl_animal_factory=>create_animal( 2 ).
  cl_introducer=>introduce( i_animal = wa_animal ). "  The i_animal parameter is explicitly specified and is necessary its use when is more than one paramter.
  write /.

  wa_animal = cl_animal_factory=>create_animal( 3 ).
  cl_introducer=>introduce( wa_animal ).

根据定义，类可以被创建为普通的、抽象的和继承的。

• 公共部分是你声明在类外可见信息的地方。

• 私有部分有内部成员，即使在子类中也不能暴露在类外。

• 受保护的部分有只在内部公开或可见的信息，并与所有子类一起。

在每个部分，你都可以指定实例数据和方法，也可以指定类的数据和方法。

实例成员和类成员的区别在于，对于前者来说，必须创建对象（我们很快就会知道）才能使用这些成员，而对于后者来说，不需要实例化，我们可以直接从类中使用它们，并使用一个特殊的符号（ =>），对于其他实例数据和方法，则使用（->）符号。

class cl_animal definition abstract.
  public section.
    methods: constructor importing i_name type string, " Visible to everyone
             make_a_sound,
             my_name_is,
             get_type.

  protected section. " Visible only here and in child classes
    data p_class type string.

  private section. " Visible only internally
    data p_name type string.
endclass.

实现部分是用来编码方法的。在这里你对类的行为进行编码，你可以看到包括参数在内的所有成员都没有出现，这是因为它们已经在类的定义部分被定义了。

class cl_animal implementation.
  method constructor.
    p_name = i_name.     " p_name was defined already in the definition part of the class as private
    p_class = 'Unknown'. " p_class was defined already in the definition part of the class as protected
  endmethod.

  method make_a_sound.
    write 'Nothing'.
  endmethod.

  method my_name_is.
    write: / 'My name is: ', p_name.
  endmethod.

  method get_type.
    write: / 'I''m type of: ', p_class.
  endmethod.
endclass.