Java OCA/OCP 8 Programmer Practice Tests: OCP Practice Exam, Q #84

[Juan Pablo Barale]

84. What is the output of the following application?

package music;
interface DoubleBass {
  void strum();
  default int getVolume() {return 5;}
}
interface BassGuitar {
  void strum();
  default int getVolume() {return 10;}
}
class ElectricBass implements DoubleBass, BassGuitar {
  @Override public void strum() {System.out.print("A");}
}
public class RockBand {
  public static void main(String[] strings) {
    final class MyElectricBass extends ElectricBass {
      public void strum() {System.out.print("E");}
    }
  }
}

Answer:
   D. One line of code does not compile.

But the answer should be:
   E. Two lines of code do not compile.
MyElectricBass class is also inheriting the problem from the ElectricBass class.
By compiling this code we can see the 2 compilation errors showing up:
$ java -version java version "1.8.0_181" Java(TM) SE Runtime Environment (build 1.8.0_181-b13) Java HotSpot(TM) 64-Bit Server VM (build 25.181-b13, mixed mode) $ javac RockBand.java RockBand.java:10: error: class ElectricBass inherits unrelated defaults for getVolume() from types DoubleBass and BassGuitar class ElectricBass implements DoubleBass, BassGuitar { ^ RockBand.java:15: error: class MyElectricBass inherits unrelated defaults for getVolume() from types DoubleBass and BassGuitar    final class MyElectricBass extends ElectricBass {          ^ 2 errors
Regards,

[Niall Kev]

Can we not have two of the same default methods in separate interfaces? Only in the case we implement both interfaces? good question. I wasnt aware of this. if someone can elaborate. I will try to compile also

[Juan Pablo Barale]

If a class is implementing two interfaces with the same default method (firm), the class doesn't know which implementation to use, so you are forced to override it in order to compile.
The same problem occurs in the case an interface extends two interfaces with the same default method.

[Jeanne Boyarsky]

Ugh. I tested the example. In Eclipse, it shows one compiler error and at the command line it shows two.  Which means the exam could ask about the first line that gives an error, but now how many (because it is ambiguous).

I added an item in the errata to say consider D or E to be correct.