exception from Constructor

[Vijay Kumar]

Hi all,
Is it good practice to throw exceptions from constuctors.

Regards
Vijay

[Jesper de Jong]

My opinion: A constructor should always initialize the object so that it is in a well-defined state.

If an error occurs in the constructor which prevents the object from being initialized properly, then the constructor should throw an exception.

[Jan van Mansum]

I think so. It is the best way I know to write a constructor that can fail. The other way would be to set some error field on the object and tell the client of the class to first check if construction was successful, which I think is ugly.

Compare:

 
// Ugly
 
class A
{
  private boolean initSuccess = true;
 
  A()
  { 
    try
    {
       // Execute some code that might fail
    }
    catch(Exception e)
    {
       initSuccess= false;
    }
  }
 
  boolean isInitSuccess()
  {
     return isInitSuccess;
  }
}
 
// use
A a = new A();
 
if(a.isInitSucess())
{
 // use a
}
else
{
 // plan b
}
 
 

and

 
// Not so ugly
 
class SomeException extends Exception
{
  SomeException(Exception aOrginalException)
  {
    super(aOrginalException);
  }
}
 
 
class A
{
  A() throws SomeExcepcion
  {
    try
    {
      // Execute some code that might fail
    }
    catch(Exception e)
    {
      // Wrap in our own exception type SomeException
      throw new SomeException(e);
    }
  }
}
 
// Use
A a = new A();
 

The Java compiler will oblige you to either handle SomeException yourself or add it to the throws declaration of your function, which will require the callers of your function to handle it (or either ... etc). There is no need for the user of your class to remember to check whether construction was successful.

OK, if have still not convinced you, look at the Java API. Here are some examples where API constructors that throw exceptions.
java.net.URL
java.net.URI

A lot of Java API constructors also throw IllegalArgumentException. This is a descendent of RunTimeException and therefore you are not required to handle it.

HTH,

Jan van Mansum.
[ October 25, 2007: Message edited by: Jan van Mansum ]

[Vijay Kumar]

Hi,
I have abstract class constructor which is initializing the state of an protected member of this abstract class which will be available to all sub class.

This constructor UnMarshalling a xml file which throws an JAXBException.
Is it right way to do it. If I don't provide unmarshalling in constructor then every sub class will call unMarshalling explicitly.

Regards
Vijay

[Jan van Mansum]

OK, if I understand you correctly, you want the abstract base class to do the unmarshalling of the XML so that all the derived classes don't have to do that. That is possible.

You can have the base class constructor and all the derived class constructors throw a JAXBException or - better - some application specific exception with the JAXBException wrapped in it (see my previous post).

[Vijay Kumar]

Hi Jan van Mansum,

you understnad correctly what I want to say.Some time its require to throw an exception from the constructor So that sub class can through the same error or it can wrap that exception.
My idea was that without unmarshalling the xml concrete class object should not be created.


Thanks
Vijay Kumar
SCJP 1.4