Prerequisite

You should be familiar with how to work with ABAP unit test framework.
Requirement is to write unit test for this method below:

I copy the standard class CL_PRDTXT_TEXTCUCO_CN02 into a local class ZCL_PRDTXT_TEXTCUCO_CN02 and generate test class based on the latter. The same approach could be applied to the standard class for sure.
Create local unit test class for it via wizard:

Here below is automatically generated code:

Since usually we will manipulate private attributes of CUT ( class under test ) so insert the following code into header part of local test class, so that local test class can modify private attributes of CUT.

First attempt

When performing unit test for the first time, the statement coverage is extremely low:

Check the source code of tested method, the reason is in our local test class, we didn't pass a valid value for parameter FOCUS_BO, so the method directly return when its first line is executed.

Solution

I plan to pass a fake bol entity for parameter FOCUS_BO. Then I construct it in method class_setup:

METHOD class_setup.
    lo_prod = zcl_prod_unit_test_tool=>get_fake_bol_entity(
       iv_bol_name = 'Product'
       is_data = get_sample_data( )
       iv_key = get_sample_data( )-product_guid ).

  ENDMETHOD.

METHOD get_sample_data.
    rs_data = VALUE #( product_guid = '0123456789123456' product_id = 'I042416' product_type = '01' ).
  ENDMETHOD.
Now the local test class method is changed as below:
METHOD on_new_focus.
    DATA focus_bo TYPE REF TO if_bol_bo_property_access.

    focus_bo ?= lo_prod.
    f_cut->on_new_focus( focus_bo ).
  ENDMETHOD.

Second attempt

When I run unit test, it fails this time with following error message:

The reason is in line 42, the method being tested tries to read product with a valid guid, unfortunately in my test code I have passed a fake guid '0123456789123456', thus not_found exception is raised.

Solution

Change get_sample_data in local test class as below, which can ensure the product_guid is always valid since it is read from DB table.

METHOD get_sample_data.
    DATA:ls_prod TYPE comm_product.
    SELECT SINGLE * INTO ls_prod FROM comm_product WHERE product_type = '01'.
    rs_data = VALUE #( product_guid = ls_prod-product_guid product_id = ls_prod-product_id product_type = '01' ).
  ENDMETHOD.

Third attempt

Now the unit test could pass successfully, however still some statement is not executed at all.

Those unreached statements are marked as red. The reason is there is no entity contained in collection wrapper.

Solution

Create a new method in test class:

METHOD create_wrapper.
    DATA: lr_attr TYPE REF TO crmst_uiu_text_attr.

    CREATE DATA lr_attr.

    DATA(lr_value) = NEW cl_bsp_wd_value_node( lr_attr ).
    CREATE OBJECT f_cut->collection_wrapper.

    f_cut->collection_wrapper->add( lr_value ).
  ENDMETHOD.

And call it in set_up method:

METHOD setup.
    CREATE OBJECT f_cut.
    create_wrapper( ).
  ENDMETHOD.

Finally, the unit test is finished successfully and all executable statements have been covered:

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