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  1. JDK
  2. JDK-8064803

Javac erroneously uses instantiated signatures when merging abstract most-specific methods

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    • Subcomponent:
    • Resolved In Build:
      b42
    • Verification:
      Verified

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        Test includes 3 interfaces, 1 implementation and 1 test class.
        public interface ParentA<T> {
            T process() throws Exception;
        }
        public interface ParentB<T> {
            T process() throws Exception;
        }
        public interface Child<T> extends ParentA<T>, ParentB<T> {
           // everything works fine when this line is uncommented
           // T process() throws Exception;
        }
        public class ChildImpl<T> implements Child<T> {
            @Override
            public T process() {
                return null;
            }
        }
        public class Test {
            public static void main(String[] args) throws Exception {
                Child<String> child = new ChildImpl<String>();
                String result = child.process();
                System.err.println(result);
            }
        }
        1 - When I compile these with javac 8 and run the main method in Test class, it fails with the following exception:
        Exception in thread "main" java.lang.NoSuchMethodError: Child.process()Ljava/lang/String;
        at Test.main(Test.java:5)
        2 - When I uncomment overriding method definition in `Child` interface, then it works fine.
        public interface Child<T> extends ParentA<T>, ParentB<T> {
           T process() throws Exception;
        }
        3 - When I remove `throws Exception` declaration from `process` method, then it works fine again.
        Then I looked at the disassembled bytecode of the Test.class and saw in Java 6 and 7 versions there is an additional `checkcast` instruction after `Child.process()` method call. See;
        - With Java8:
        public class Test {
          public Test();
            Code:
               0: aload_0
               1: invokespecial #1 // Method java/lang/Object."<init>":()V
               4: return
          public static void main(java.lang.String[]) throws java.lang.Exception;
            Code:
               0: new #2 // class ChildImpl
               3: dup
               4: invokespecial #3 // Method ChildImpl."<init>":()V
               7: astore_1
               8: aload_1
               9: invokeinterface #4, 1 // InterfaceMethod Child.process:()Ljava/lang/String;
              14: astore_2
              15: getstatic #5 // Field java/lang/System.err:Ljava/io/PrintStream;
              18: aload_2
              19: invokevirtual #6 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
              22: return
        }
        - With Java7:
        Compiled from "Test.java"
        public class Test {
          public Test();
            Code:
               0: aload_0
               1: invokespecial #1 // Method java/lang/Object."<init>":()V
               4: return
          public static void main(java.lang.String[]) throws java.lang.Exception;
            Code:
               0: new #2 // class ChildImpl
               3: dup
               4: invokespecial #3 // Method ChildImpl."<init>":()V
               7: astore_1
               8: aload_1
               9: invokeinterface #4, 1 // InterfaceMethod Child.process:()Ljava/lang/Object;
              14: checkcast #5 // class java/lang/String
              17: astore_2
              18: getstatic #6 // Field java/lang/System.err:Ljava/io/PrintStream;
              21: aload_2
              22: invokevirtual #7 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
              25: return
        }
        Javac 8 is expecting to call a method with return type of String (InterfaceMethod Child.process:()Ljava/lang/String), but on Java 7 version is expecting a method with return type of Object (InterfaceMethod Child.process:()Ljava/lang/Object) which is expected because of type-erasure then it's checking type of returning value with `checkcast` instruction.

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                • Assignee:
                  mcimadamore Maurizio Cimadamore
                  Reporter:
                  mcimadamore Maurizio Cimadamore
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                    Updated:
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