About Question enthuware.ocajp.i.v7.2.1048 :

[ETS User]

Why throw NullPointerException whe RUn?

Tank you

[admin]

NullPointerException is thrown by the statement throw re; because re is null. This will be caught by the catch block, which prints the exception.

HTH,
Paul.

[adrian110288]

NullPointerException is thrown by the statement throw re; because re is null. This will be caught by the catch block, which prints the exception.

HTH,
Paul.

Still dnt quite understand it :/ Is NullPointerException caught, because the try block throws null value? I thought the NullPointerException is thrown when we try to access an objec which is null, but here we dnt try to access the value thrown by try block, do we? im confused ...

[admin]

Yes, the NPE is thrown because of the statement throw re, where re is null.

HTH,
Paul.

[Ad9999]

throw null; = throw new NullPointerException();

Cool. Null is so versatile.

[gianmarcoelg]

You could found inside the NullPointerException class the following:
*Throwing {@code null} as if it were a {@code Throwable} value.

Regards.
gmeg

[__JJ__]

Hi

A NullPointerException will be thrown if the expression given to the throw statement results in a null pointer.

This is very interesting. I had thought the throw would throw a null object which would nonetheless be caught by the catch but that the attempt to print it in the SOP (calling e.toString() maybe) would cause an NPE.
I had to verify it for myself. To drive the point home:

Code: Select all

        try{
            IOException e = null;
            throw e;
        }
        catch(IOException e){
            System.out.println("UCK IOE");
            System.out.println(e);
        }
        catch(NullPointerException e){
            System.out.println("UCK NPE");
            System.out.println(e);
        }
        catch(IllegalArgumentException e){
            System.out.println("UCK IAE");
            System.out.println(e);
        }
        catch(RuntimeException e){
            System.out.println("UCK RTE");
            System.out.println(e);
        }

prints

Code: Select all

UCK NPE
java.lang.NullPointerException

[MariaRoxana]

Hy,

Can someone, please, explain to me, why System.out.println(e) outputs java.lang.RuntimeException? I remember that println(Object o) will do String.valueOf(x) to get the printed object's string value but x is null, so it prints null because of print(String x). I tried to read the documentation but I didn't understand very well the idea.

Thx

[admin]

You remember correctly that if the argument to SOP is null, then null is printed. However, in the given code, e is not null. The statement "throw re;" in the try block does not send a null reference to the catch block. Instead, this statement actually causes a NullPointerException. That is why, the e in catch(Exception e) does not get a null but a reference to a valid NullPointerException object, which is what is printed by the SOP statement.