调用ABAP类方法后返回值RV_BOOKS为空的问题排查

我编写了全局类ZCL_CHE_UT_BOOK用于单元测试。调用GET_BOOKS方法时，返回值RV_BOOKS能正常获取数据，但调用其他方法后RV_BOOKS变为空，无任何输出。以下是我的代码：

class ZCL_CHE_UT_BOOK definition
  public
  final
  create public .

public section.

  types:
    BEGIN OF TY_BOOK,
             CLIENT     TYPE ZCHE_BOOKS-CLIENT,
             BOOKID     TYPE ZCHE_BOOKS-BOOKID,
             LIBRARYID  TYPE ZCHE_BOOKS-LIBRARYID,
             BOOKTITLE  TYPE ZCHE_BOOKS-BOOKTITLE,
             AUTHOR     TYPE ZCHE_BOOKS-AUTHOR,
             GENRE      TYPE ZCHE_BOOKS-GENRE,
             QUANTITY   TYPE ZCHE_BOOKS-QUANTITY,
             CUKY_FIELD TYPE ZCHE_BOOKS-CUKY_FIELD,
             PRICE      TYPE ZCHE_BOOKS-PRICE,
             P_DATA     TYPE ZCHE_BOOKS-P_DATA,
           END OF TY_BOOK .
  types:
    TT_BOOK TYPE STANDARD TABLE OF TY_BOOK WITH DEFAULT KEY .

  data:
    LT_BOOKS TYPE TABLE OF ZCHE_BOOKS .

  methods GET_BOOKS
    returning
      value(RV_BOOKS) type TT_BOOK .
  methods OUTPUT_BOOKS
    importing
      !RV_BOOKS type TT_BOOK .
  methods FIND_RARE_BOOKS
    importing
      !RV_BOOKS type TT_BOOK
    returning
      value(RV_RARE_BOOKS) type TT_BOOK .
  methods GET_BOOKS_BY_GENRE
    importing
      !RV_BOOKS type TT_BOOK
      !IV_GENRE type ZCHE_BOOKS-GENRE
    returning
      value(RV_TOTAL) type INT4 .
  PROTECTED SECTION.
  PRIVATE SECTION.
ENDCLASS.

CLASS ZCL_CHE_UT_BOOK IMPLEMENTATION.

  METHOD FIND_RARE_BOOKS.
    DATA: LT_RARE_BOOKS TYPE TABLE OF ZCHE_BOOKS,   " For storing special books
          LS_RARE_BOOK  TYPE ZCHE_BOOKS.           " Work work with special books
    LOOP AT RV_BOOKS INTO DATA(LS_BOOK).
      IF LS_BOOK-QUANTITY < 5 AND LS_BOOK-PRICE > 20.
        " Additional activity with some books
        LS_RARE_BOOK = LS_BOOK.
        APPEND LS_RARE_BOOK TO LT_RARE_BOOKS.
      ENDIF.
    ENDLOOP.
    RV_RARE_BOOKS = LT_RARE_BOOKS.
    IF RV_RARE_BOOKS IS NOT INITIAL.
      WRITE: / 'List of rare books:'.
      LOOP AT RV_RARE_BOOKS INTO LS_RARE_BOOK.
        WRITE: / 'Book:', LS_RARE_BOOK-BOOKTITLE,
      'Author:', LS_RARE_BOOK-AUTHOR,
      'Genre:', LS_RARE_BOOK-GENRE.
      ENDLOOP.
    ELSE.
      WRITE: / 'Rare books not found.'.
    ENDIF.
  ENDMETHOD.

  METHOD GET_BOOKS.
    SELECT *
      FROM ZCHE_BOOKS
      INTO TABLE @RV_BOOKS.
    SORT RV_BOOKS ASCENDING.
  ENDMETHOD.

  METHOD GET_BOOKS_BY_GENRE.
    DATA(LV_TOTAL) = REDUCE INT4(
    INIT COUNT = 0 FOR C IN RV_BOOKS
    WHERE ( QUANTITY > 0 AND GENRE = IV_GENRE ) NEXT COUNT = COUNT + C-QUANTITY ).
    RV_TOTAL = LV_TOTAL.
  ENDMETHOD.

  METHOD OUTPUT_BOOKS.
    WRITE: / 'List of all books:'.
    LOOP AT RV_BOOKS INTO DATA(LS_BOOK).
      " Output data by parameters into console
      WRITE: / 'Book:', LS_BOOK-BOOKTITLE,
      'Author:', LS_BOOK-AUTHOR,
      'Genre:', LS_BOOK-GENRE.
    ENDLOOP.
  ENDMETHOD.
ENDCLASS.

核心问题是表类型不匹配导致的隐式转换失败：

• 类中定义的TT_BOOK是基于自定义结构TY_BOOK的内表类型，但在FIND_RARE_BOOKS方法里，LT_RARE_BOOKS和LS_RARE_BOOK用的是数据库表ZCHE_BOOKS的类型。当执行RV_RARE_BOOKS = LT_RARE_BOOKS时，两种不同结构的内表直接赋值会触发隐式转换，ABAP中这种跨类型赋值会导致目标表被初始化为空，最终出现无输出的情况。
• 虽然GET_BOOKS方法中把数据库表数据查入RV_BOOKS（TT_BOOK类型）时能正常工作，但后续方法中混用不同类型的内表，会破坏数据一致性，引发空表问题。

方案1：统一内表类型 #

把所有操作内表的变量都改成自定义的TT_BOOK和TY_BOOK类型，彻底消除类型不匹配：

METHOD FIND_RARE_BOOKS.
  DATA: LT_RARE_BOOKS TYPE TT_BOOK,   " 替换为TT_BOOK类型
        LS_RARE_BOOK  TYPE TY_BOOK.   " 替换为TY_BOOK类型
  LOOP AT RV_BOOKS INTO DATA(LS_BOOK).
    IF LS_BOOK-QUANTITY < 5 AND LS_BOOK-PRICE > 20.
      LS_RARE_BOOK = LS_BOOK.
      APPEND LS_RARE_BOOK TO LT_RARE_BOOKS.
    ENDIF.
  ENDLOOP.
  RV_RARE_BOOKS = LT_RARE_BOOKS.
  IF RV_RARE_BOOKS IS NOT INITIAL.
    WRITE: / '稀有书籍列表:'.
    LOOP AT RV_RARE_BOOKS INTO LS_RARE_BOOK.
      WRITE: / '书籍:', LS_RARE_BOOK-BOOKTITLE,
      '作者:', LS_RARE_BOOK-AUTHOR,
      '分类:', LS_RARE_BOOK-GENRE.
    ENDLOOP.
  ELSE.
    WRITE: / '未找到稀有书籍.'.
  ENDIF.
ENDMETHOD.

方案2：复用数据库表类型（可选） #

如果自定义结构TY_BOOK和数据库表ZCHE_BOOKS的结构完全一致，可以直接复用数据库表的类型，省去重复定义：

types:
  TT_BOOK TYPE STANDARD TABLE OF ZCHE_BOOKS WITH DEFAULT KEY .

这样后续所有内表都使用TT_BOOK和ZCHE_BOOKS类型，从根源避免类型不匹配问题。

额外优化建议 #

• 尽量不要在类方法里直接用WRITE输出，单元测试类应专注于数据处理，输出逻辑可以放到调用该类的测试程序中。
• 在GET_BOOKS方法中添加空表判断，避免后续方法处理空数据时出现异常。

内容的提问来源于stack exchange，提问作者Alexander Chebotarev