In Scala, given a subclass, how to check if it overrides the default method of its parent interface (defined in Java) using reflection?

user_550324 0
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(defined in Java)
interface Foo {
  default int bar() {
    throw new RuntimeException("not implemented");
  }
}

(defined in Scala)
class FooImpl extends Foo {
  override int bar() {
    return 42;
  }
}

In the above code snippet, I have an interface defined in Java, with a default method bar which just throws exception. The question is how to check if a subclass FooImpl overrides the default method or not. It seems in Java we can check the declaring class on the Method instance from reflection: if FooImpl overrides the method, the declaring class will be itself, and otherwise will be the interface Foo.

However above seems not true in Scala (2.12): in both situations the declaring class will be FooImpl.

reflection scala java
2 Answers

For example if I have something like this:

interface Test {
    default void go() {

    }
}

static class Impl implements Test, AutoCloseable {

    @Override
    public void go() {

    }

    @Override
    public void close() throws Exception {

    }
}

I can find out if Method m is the one you are looking for by:

 private static boolean isDefaultAndOverridden(Method m, Class<?> cls) {

    Class<?>[] interfaces = ClassUtils.getAllInterfacesForClass(cls);
    System.out.println(Arrays.toString(interfaces));


    return Arrays.stream(interfaces)
                 .flatMap(in -> Arrays.stream(in.getDeclaredMethods()))
                 .filter(Method::isDefault)
                 .filter(method -> method.getName().equals(m.getName()))
                 .filter(method -> method.getReturnType() == m.getReturnType())
                 .filter(method -> Arrays.equals(method.getParameterTypes(), m.getParameterTypes()))
                 .count() == 1;

}

I am using ClassUtils from Spring to get all the interfaces that a class implements, direct or not. There are many libraries that exposes such functionality also (but it's not that complicated to write it yourself)

user_10593720

.getDeclarignClass seems to work fine for me.

Here is what I did. In java:

interface Foo {
   default String bar() {
      return "bar";
   }
   default String baz() {
     return "baz";
   }
}

In scala:

class Bar extends Foo {
    override def bar() = "baz"
}

object Bar {
    def main(argv: Array[String]) = classOf[Bar]
      .getMethods.filter(_.getName.startsWith("ba"))
      .map { m => m.getName -> m.getDeclaringClass.getName }
      .foreach(println)
}

This prints:

(bar,Bar)
(baz,Foo)

So, something like this should work:

    def isOverridden(mtd: Method, clazz: Class) = 
       mtd.getDeclaringClass == clazz
user_42545170
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