Exception Doubt

[Abhi vijay]

Source: Examlab

  public class FCilin{

     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(); // Line 2 
     }

}  

Here if Line 1 and Line 2 would have been in the Try block, we would have got Unreachable statement, Compiler Error, why not in this case?

[Sandeep Bhandari]

Can you post the code with try blocks added?

[Wouter Oet]

With ^^, because this compiles just fine

public static void getMe() throws ArithmeticException
	{
		try
		{
			if(true)
			{
				throw new NullPointerException(); // Line1
			}
			throw new ArrayIndexOutOfBoundsException(); // Line 2
		} catch (Exception e)
		{
			System.out.println("Error");
		}
	}

[James Tharakan]

@Abhi vijay,
I dont think such a question would turnup in the real exam even though its in Examlab.( this is my personal option ! !)
The unreachability with respect to if is a bit complex.Check out the JLS 14.21. Even they have discussed it separately.
BElow is what they say.
Hope it helps you ....

[Treimin Clark]

:| Are you sure about this? I checked the both programs, neither I got a compile error!

[Abhi vijay]

 public class FCilin{

     public static void main(String args[]) throws Throwable {
          try{
               throw new NullPointerException();
          throw new ArrayIndexOutOfBoundsException();
          
          
          }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);
          }



     }

   }   

Now I get a Compiler Error, saying UNREACHABLE statement. But when it is in the if block, there is no such error.

[Treimin Clark]

Aha it is very simple problem. For the exam, you should keep in mind that the if expression will not evaluate by the the compiler, for the purpose of finding unreachable statements.

For an example:

if(true) throw new Exception();
System.out.println("e");

This is ok, because the compiler don't evaluate the if condition, so it don't know whether the exception will throw or not.


Warning: But you should note that if expression will be evaluate, for the purpose of checking for local variable initialization. For an example:

int x;
if(true) x=1;
System.out.println(x);

The above code compiles, but the code below will not:

int x;
if(false) x=1;
System.out.println(x);

Keep in mind the above two concepts for the exam. I remembered that I got a question on the exam, that uses the if(true) to check unreachable statements.

[Wouter Oet]

Treimin Clark wrote:
Warning: But you should note that if expression will be evaluate, for the purpose of checking for local variable initialization. For an example:

int x;
if(true) x=1;
System.out.println(x);

The above code compiles, but the code below will not:

int x;
if(false) x=1;
System.out.println(x);

Only if the statement uses final variables otherwise compilation will fail because the variable might not have been initialized.

[Abhi vijay]

  if(true) throw new Exception();
System.out.println("e");  

In this case when throw new Exception(); then an Exception is thrown and the associated catch block executes, then the control never comes back to the method, so System.out.println("e"); is an Unreachable statement??? Whys is the compiler not detecting it?

[Punit Singh]

  final int constz=5;//compile time constant
	int z;
	if(constz>1){
		z=1;
	}
	System.out.println(z);//ok valid
	
	final int constk;//runtime constant
	constk=5;
	int k;
	if(constk>1){
		k=1;
	}
	System.out.println(k);//compiler error
	  

Look for these types of questions also.

[Ankit Garg]

Abhi vijay wrote:

if(true) throw new Exception();
System.out.println("e");  

In this case when throw new Exception(); then an Exception is thrown and the associated catch block executes, then the control never comes back to the method, so System.out.println("e"); is an Unreachable statement??? Whys is the compiler not detecting it?

Well I raised such a question a long time ago. It is a strange kind of behavior of the compiler. If you initialize a field in a if(true){} block, then the compiler knows that the field will be initialized, but if you throw an exception in the if(true){} block, then the compiler doesn't flags the next statement of the if block as unreachable. That's the way it is. You will have to memorize it. Bert also confirmed that this type of behavior will not be tested on the exam so don't worry...

[Treimin Clark]

But I clearly remembered that I got a question like this on the real exam too. They used if(true) condition before throwing an exception.

[Abhi vijay]

Thanks a lot Guys!

Punit, the example was very Good.

[Ruben Soto]

I think the reason the compiler doesn't take into account if statements for the purpose of unreachable statements is to support "conditional compilation." The whole purpose of conditional compilation is to make specific statements unreachable. However, if the unreachable statements involve necessary local or final variable initialization code, then the compiler will complain.