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  1. JDK
  2. JDK-4301188

implementation of abstract method which is not inherited by subclass

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      Description



      Name: laC46010 Date: 12/21/99



      The JLS, p. 8.4.3.1 requires to report compile-time error for the
      following example:
      ----------------------clss02802_b.java----------------------
       package clss02802_b;
       abstract class clss02802_a { abstract void m(); }
       public class clss02802_b extends clss02802_a { void m(){} }
      ----------------------clss02802.java------------------------
       package clss02802;
       import clss02802_b.clss02802_b;
       class clss02802_c extends clss02802_b {}
      ------------------------------------------------------------

      However javac (all versions) compiles this program without complaints.
      I believe it's indeed a valid Java program and p. 8.4.3.1 doesn't take
      into account the case when abstract method in not inherited by
      subclass.


      p. 8.4.3.1 (p.158 of JLS 1st edition) reads :
          "Every subclass of A that is not abstract must provide an
          implementation for m, or a compile-time error occurs. More
          precisely, for every subclass C of the abstract class A,
          if C is not abstract, then there must be some class B such
          that all of the following are true:
        
             1) B is a superclass of C or is C itself.
             2) B is a subclass of A.
             3) B provides a declaration of the method m that is not
                abstract, and this declaration is inherited by C,
                thereby providing an implementation of method m that
                is visible to C.
          If there is no such class B, then a compile-time error
          occurs."

      At the same time JLS, p. 8.2 reads :
          "Only members of a class that are declared protected or
          public are inherited by subclasses declared in a package
          other than the one in which the class is declared."
         
      But in present test, the class "clss02802_c" does not inherit the
      method "m" because this method belongs to a class from an another
      package and has default access. So the class "clss02802_c" is a
      subclass of the abstract class "clss02802_a" and there is no some class
      B that "B provides a declaration of the method m that is not abstract,
      and this declaration is inherited by C". Therefore compile-time error
                           ^^^^^^^^^^^^
      must occur. However javac (all versions) compiles this program without
      complaints.

      Note that above assertion is reduced in the draft of
      the JLS 2nd edition to a single sentence:
          "Every subclass of A that is not abstract must provide an
          implementation for m, or compile-time error occur."
      but it still doesn't explain the discussed case.

      ======================================================================

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            • Assignee:
              gbrachasunw Gilad Bracha (Inactive)
              Reporter:
              leosunw Leo Leo (Inactive)
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