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  1. JDK
  2. JDK-8066974

Compiler doesn't infer method's generic type information in lambda body

    Details

    • Type: Bug
    • Status: Closed
    • Priority: P4
    • Resolution: Fixed
    • Affects Version/s: 8u11
    • Fix Version/s: 9
    • Component/s: tools
    • Subcomponent:
    • Resolved In Build:
      b44
    • CPU:
      x86
    • OS:
      os_x
    • Verification:
      Verified

      Backports

        Description

        FULL PRODUCT VERSION :
        java version "1.8.0_05"
        Java(TM) SE Runtime Environment (build 1.8.0_05-b13)
        Java HotSpot(TM) 64-Bit Server VM (build 25.5-b02, mixed mode)

        ADDITIONAL OS VERSION INFORMATION :
        Windows x64
        OSX 10.9.4

        A DESCRIPTION OF THE PROBLEM :
        The compiler does not infer the correct type arguments for a method invocation that appears in a lambda body. The full example is on stackoverflow here: http://stackoverflow.com/questions/24986279/java-8-generics-exceptions-compile-time-error-when-using-a-lambda-expression/25101439#25101439



        STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
        Compile the following class with javac

        import java.util.ArrayList;
        import java.util.List;


        public class JavaBugTest
        {
            interface AbleToThrowException<E extends Exception>
            {
            }

            interface Parameter {
                public <E extends Exception> Object execute(AbleToThrowException<E> algo) throws E;
            }

            interface ThrowsRuntimeException extends AbleToThrowException<RuntimeException>
            {
            }

            static ThrowsRuntimeException foo;


            public static Object manualLambda(Parameter p)
            {
                return p.execute(foo);
            }

            public static void main(String[] args)
            {
                List<Parameter> params = new ArrayList<>();
                params.stream().map(p -> p.execute(foo)); // Gives a compile time error.
                params.stream().map(JavaBugTest::manualLambda); // Works fine.
            }

        }

        EXPECTED VERSUS ACTUAL BEHAVIOR :
        EXPECTED -
        The class should compile with 'p.execute(foo)` inferring 'RuntimeException' as its type argument.
        ACTUAL -
        We get a compilation error that is the result of the compiler failing to infer the correct type information.

        ERROR MESSAGES/STACK TRACES THAT OCCUR :
        [~/test]$ javac JavaBugTest.java
        JavaBugTest.java:30: error: unreported exception E; must be caught or declared to be thrown
                params.stream().map((Parameter p) -> p.execute(foo)); // Gives a compile time error.
                                                              ^
          where E is a type-variable:
            E extends Exception declared in method <E>execute(AbleToThrowException<E>)
        1 error

        REPRODUCIBILITY :
        This bug can be reproduced always.

        ---------- BEGIN SOURCE ----------
        import java.util.ArrayList;
        import java.util.List;


        public class JavaBugTest
        {
            interface AbleToThrowException<E extends Exception>
            {
            }

            interface Parameter {
                public <E extends Exception> Object execute(AbleToThrowException<E> algo) throws E;
            }

            interface ThrowsRuntimeException extends AbleToThrowException<RuntimeException>
            {
            }

            static ThrowsRuntimeException foo;


            public static Object manualLambda(Parameter p)
            {
                return p.execute(foo);
            }

            public static void main(String[] args)
            {
                List<Parameter> params = new ArrayList<>();
                params.stream().map(p -> p.execute(foo)); // Gives a compile time error.
                params.stream().map(JavaBugTest::manualLambda); // Works fine.
            }

        }
        ---------- END SOURCE ----------

        CUSTOMER SUBMITTED WORKAROUND :
        You can explicitly provide the type argument

        params.stream().map(p -> p.<RuntimeException>execute(foo)); // Gives a compile time error.

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                • Assignee:
                  mcimadamore Maurizio Cimadamore
                  Reporter:
                  webbuggrp Webbug Group
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                  • Created:
                    Updated:
                    Resolved: