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  1. JDK
  2. JDK-7043414

incompatible types error with generic static factory method

    Details

    • Type: Bug
    • Status: Closed
    • Priority: P4
    • Resolution: Duplicate
    • Affects Version/s: 6u25
    • Fix Version/s: None
    • Component/s: tools
    • Labels:
    • Subcomponent:
    • CPU:
      x86
    • OS:
      windows_xp

      Description

      FULL PRODUCT VERSION :
      java version "1.6.0_25"
      Java(TM) SE Runtime Environment (build 1.6.0_25-b06)
      Java HotSpot(TM) Client VM (build 20.0-b11, mixed mode, sharing)

      ADDITIONAL OS VERSION INFORMATION :
      Microsoft Windows XP [Version 5.1.2600]

      A DESCRIPTION OF THE PROBLEM :
      Using a static factory method of a class with a constrained type parameter gives a compiler error: incompatible types.

      The class compiles without error when using jdk7 preview or ecj.



      STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
      Compile the class listed under source code:

      javac Foo.java


      EXPECTED VERSUS ACTUAL BEHAVIOR :
      EXPECTED -
      The class should compile without errors.
      ACTUAL -
      The compiler reports a type error.

      ERROR MESSAGES/STACK TRACES THAT OCCUR :
      Foo.java:8: incompatible types; no instance(s) of type variable(s) T exist so that Foo<T> conforms to Foo<java.lang.Integer>
      found : <T>Foo<T>
      required: Foo<java.lang.Integer>
              Foo<Integer> obj = emptyFoo();

      REPRODUCIBILITY :
      This bug can be reproduced always.

      ---------- BEGIN SOURCE ----------
      public class Foo<T extends Comparable<? super T>> {
          
          public static <T extends Comparable<? super T>> Foo<T> emptyFoo() {
              return new Foo<T>();
          }

          public static void testFact() {
              Foo<Integer> obj = emptyFoo();
          }
          
      }

      ---------- END SOURCE ----------

      CUSTOMER SUBMITTED WORKAROUND :
      Several:

      1. The factory method is only used to reduce repetition. Code works just fine if we construct the object directly:

       Foo<Integer> obj = new Foo<Integer>();

      2. Use jdk7. Not yet practical for a production release.

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              • Assignee:
                Unassigned
                Reporter:
                webbuggrp Webbug Group
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                • Created:
                  Updated:
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