🚀 go-pugleaf

Vladimir Olenin <keenonjava@collegemail.com> wrote:

> i have a class A that extends Thread. Then, i have class B
> that extends A. The first problem was that i didn't compile
> printing out the error that "class A doesn't have A()
> constructor" To tell the truth it really hadn't have such.
> When i added an empty one, this error disappeared.

Since B is a subclass of A, each constructor for B must either 
call a constructor for A, or another constructor for B (which 
must eventually call an A constructor).  If you do not specify a 
constructor call in the B constructor, such as one of the 
following:

   super(...);    // Calls the superclass constructor
   this(...);     // Calls another B constructor

then the compiler will generate a call to the superclass 
constructor with no arguments.  So the compiler is generating a 
call to A() in one of your B constructors.

If you do not specify any constructors in a class, then the 
compiler will automatically generate one for you.  In other 
words, if you did not define any constructors for A, it would 
have generated the A() constructor that is missing.  But since 
you did define an A constructor with arguments, it does not do 
this.

> So, the question is can i do this (call A constructor
> from try catch blok) or not? 

No.  The first thing done in a constructor must always be to call 
the superclass constructor.  And calling a constructor from any 
other location is not allowed.

-- 
Regards,

John P. McGrath
<mcgrath@enter.net>
<jpm@nscs.fast.net>