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  1. JDK
  2. JDK-8299861

Inconsistent handling of effectively final and blank final variables

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    • Bug
    • Resolution: Unresolved
    • P3
    • 23
    • 20
    • specification

    Description

      The handling of effectively final and blank final variables in the JLS differs slightly. In particular, for blank final variables, there's no assertion about definite assignment, only definite unassignment:
      JLS 16:
      For every assignment to a blank final variable, the variable must be definitely unassigned before the assignment, or a compile-time error occurs.

      But, for effectively final variables, both definite assignment and unassignment are considered:
      JLS 4.12.4.:
      A local variable declared by a statement and whose declarator lacks an initializer is effectively final if all of the following are true:
      ...
      Whenever it occurs as the left hand side in an assignment expression, it is definitely unassigned and not definitely assigned before the assignment; that is, it is definitely unassigned and not definitely assigned after the right hand side of the assignment expression (§16 (Definite Assignment)).
      ...

      This leads to cases like:
      int i; //note the variable can be marked final
      if (false) {
          i = 0; //this assignment makes the variable not effectively final, as "i" is definitely assigned and unassigned at the point when the assignment happens
      } else {
          i = 1;
      }

      Where a variable is not effectively final, but can be marked final, which is contrary to:
      JLS 4.12.4.:
      If a variable is effectively final, adding the final modifier to its declaration will not introduce any compile-time errors. Conversely, a local variable or parameter that is declared final in a valid program becomes effectively final if the final modifier is removed.

      It would be good to evaluate if the JLS can be made more consistent.

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              gbierman Gavin Bierman
              jlahoda Jan Lahoda
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                Created:
                Updated: