examlab(question-53)-unreachable statement NAO?

[Arjun Srivastava]

public class Main{
     public static void main(String args[]) throws Throwable {
          try{
               getMe();
          }catch(ArithmeticException ae){
               System.out.print(1);
          }catch(ArrayIndexOutOfBoundsException ae){
               System.out.print(2);
          }catch(NullPointerException ae){
               System.out.print(3);
          }catch(RuntimeException ae){
               System.out.print(4);
          }catch(Exception ae){
               System.out.print(5);
          }
     }
     public static void getMe()throws ArithmeticException{
          if(true) throw new NullPointerException(); //line1
          throw new ArrayIndexOutOfBoundsException();  //line2
               //System.out.print(5);//line3
     }
}

output:3
yes this is because line 1 throw nullpointerexception,so line 2 will no get executed. ok fine
So line 2 should be called a unreachable statement.isn't it?
Also but if we commented out line 3,it says line3 is a unreachable statement gives compiler error.

[Christophe Verré]

So line 2 should be called a unreachable statement.isn't it?

No, because it's in a conditional block.

Also but if we commented out line 3,it says line3 is a unreachable statement gives compiler error.

Did you leave throw new ArrayIndexOutOfBoundsException(); uncommented ? If you don't comment it, or if you don't put it in a conditional block like the NPE, the compiler will complain.

[Wouter Oet]

Line 18 is conditional. So it can (and in this case always does) throw an Exception. But because it's conditional lines after that are reachable.
Line 19 is unconditional so the compiler knows it will always throw an Exception so lines after that are unreachable.

[Arjun Srivastava]

Christophe Verré wrote:

So line 2 should be called a unreachable statement.isn't it?

No, because it's in a conditional block.

Wouter Oet wrote:
Line 19 is unconditional so the compiler knows it will always throw an Exception so lines after that are unreachable.

line 19 and line 2 are same.
can you guys compare your answers,is line 19(aka line2) is in conditional block(as Christophe Verré says) or is unconditional(as wouter says).

[Christophe Verré]

Try this :

public static void getMe()throws ArithmeticException{  
           if(true) throw new NullPointerException();
           throw new ArrayIndexOutOfBoundsException();
           System.out.print(5);//line3  
      }

and this :

public static void getMe()throws ArithmeticException{  
           if(true) throw new NullPointerException();
           //throw new ArrayIndexOutOfBoundsException();
           System.out.print(5);//line3  
      }

and this :

public static void getMe()throws ArithmeticException{  
           if(true) throw new NullPointerException();
           if(true) throw new ArrayIndexOutOfBoundsException();
           System.out.print(5);//line3  
      }

Do you see the difference ?

[Arjun Srivastava]

got the losing concept.
thanks Sheriff and Mr. Rancher for your reply that can be digested easily.

[Saibabaa Pragada]

Hi, What is this logic/concept/feature called here ? The first statement after if condition is "dead Code". Second statement maybe "Dead Code" or "Unreachable code" as given below..What is happening here ?

    public static void getMe1() throws ArithmeticException {
	if (true)
	    throw new NullPointerException();
	throw new ArrayIndexOutOfBoundsException();	//Dead code
	System.out.print(5);//Error : Unreachable code
    }

    public static void getMe3() throws ArithmeticException {
	if (true)
	    throw new NullPointerException();
	if (true)	//Dead Code
	    throw new ArrayIndexOutOfBoundsException();
	System.out.print(5);
    }    

[Henry Wong]

Saibabaa Pragada wrote:Hi, What is this logic/concept/feature called here ? The first statement after if condition is "dead Code". Second statement maybe "Dead Code" or "Unreachable code" as given below..What is happening here ?

I am pretty sure that if you follow the links in this topic, you'll get an explanation.... but here it is anyway.

The specification actually has relaxed rules for the "if" statement, in regards to the check for unreachability. The relaxed rules states that the compiler should not assume that the path is taken or not taken, even though it can be determined at compile time. The reason for this is to allow for debugging / test flags.

    
final static boolean testing = true;

public static void getMe1() throws ArithmeticException {
	if (testing)
	      throw new NullPointerException();
	// Actual body if not testing mode
}
 

Henry

[Saibabaa Pragada]

This is the reason I am looking for. Thanks Henry.

Henry Wong wrote:The specification actually has relaxed rules for the "if" statement, in regards to the check for unreachability. The relaxed rules states that the compiler should not assume that the path is taken or not taken, even though it can be determined at compile time. The reason for this is to allow for debugging / test flags.