Java OCA OCP Practice Question 1987

Question

What is the output of the following application?. 

package mypkg; //w w w .j  av  a 2s .com
import java.util.concurrent.*; 
public class Main { 
   public static void completePaperwork() { 
      System.out.print("[Filing]"); 
   } 
   public static double getPi() { 
      return 3.14159; 
   } 
   public static void main(String[] args) throws Exception { 
      ExecutorService x = Executors.newSingleThreadExecutor(); 
      Future<?> f1 = x.submit(() -> completePaperwork()); 
      Future<Object> f2 = x.submit(() -> getPi()); 
      System.out.print(f1.get()+" "+f2.get()); 
      x.shutdown(); 
   } 
}
  • A. [Filing]null 3.14159
  • B. The declaration of f1 does not compile.
  • C. The declaration of f2 does not compile.
  • D. An exception is thrown at runtime.


A.

Note

The code compiles without issue, so Options B and C are incorrect.

The f1 declaration uses the version of submit() in ExecutorService, which takes a Runnable and returns a Future<?>.

The call f1.get() waits until the task is finished and always returns null, since Runnable expressions have a void return type.

The f2 declaration uses an overloaded version of submit(), which takes a Callable expression and returns a generic Future object.

Since the double value can be autoboxed to a Double object, the line compiles without issue with f2.get() returning 3.14159.

Option A is the correct answer.

Option D is incorrect because no exception is expected to be thrown at runtime.




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