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Constructing java objects

Ad Criag
Greenhorn

Joined: Jun 27, 2007
Posts: 1
Why are some java objects constructed in this manner:

objectype objectname = new objecttype

but others like this :

objectname = new objecttype

ie the object type is not specified on the left hand side of the =
Srikanth Ramu
Ranch Hand

Joined: Feb 20, 2007
Posts: 76
Your second option is not a valid one and you need to specify the type either in new objecttype() statement or separtely. C
heck your code you should find statments like:

Pradeep Kadambar
Ranch Hand

Joined: Oct 18, 2004
Posts: 148
Hey Criag,

First things first. All objects are basically created in new <Class_Name>() fashion.

Now coming to your point it is the initialization. In case


You create a new object and then store the reference to the newly created object, there by initializing var_name.

But in cases where the initialization may happen based on certain runtime conditions.



Hope this makes your understanding clear.


[ June 27, 2007: Message edited by: Pradeep Kadambar ]
Katrina Owen
Sheriff

Joined: Nov 03, 2006
Posts: 1362
    
  17
Originally posted by Ad Criag:
objectname = new objecttype


Sometimes you know that you need a variable for an object of a specific type, but you don't know exactly what that object will be. You might then see code like this:

Bob Ruth
Ranch Hand

Joined: Jun 04, 2007
Posts: 320
Well, what you have there is two different coding techniques to work with:

--------------------------
One, declare the class reference variable ahead of time, then instantiate it later, like this


public static void main( String [] args) {

ClassName myClass; // declares the variable myClass of type ClassName

// other lines....

myClass = new ClassName();

// the above line uses new to allocate a new class of type ClassName,
// and assign it's reference to the variable, myClass, whose type is
// already known to the compiler because it was declared earlier

---------------------------

Two, use one line to declare the variable AND instantiate the object:

public static void main(String [] args) {

ClassName myClass = new ClassName();

// This performs the same work but does it all in one source line.
// The first ClassName tells the compiler the type of the variable
// (myClass) the second ClassName tells the compiler what type of
// object to instantiate.

--------------------------

I think more a matter of style but then, certain coding standards may specify a preference.

(Edited to add) I had not caught Katrina's post and that is a good example. The dynamics of a particular design may well have you instantiating objects "on the fly" and you may not know what type until that point. So the one liner is a little more "source efficient". So in that case you might choose the one-liner. If you are initializing an environment to a known condition, you may want to declare variables up front and instantiate later.
[ June 27, 2007: Message edited by: Bob Ruth ]

------------------------
Bob
SCJP - 86% - June 11, 2009
Remko Strating
Ranch Hand

Joined: Dec 28, 2006
Posts: 893
I would prefer:



even if I would know the real initialization would be later.

By doing this I will not have the famous NullPointerException at runtime if I forget the initialization later.

Also if I declare an object within a method I must initialize it, before using.


Remko (My website)
SCJP 1.5, SCWCD 1.4, SCDJWS 1.4, SCBCD 1.5, ITIL(Manager), Prince2(Practitioner), Reading/ gaining experience for SCEA,
Ulf Dittmer
Marshal

Joined: Mar 22, 2005
Posts: 41877
    
  63
Originally posted by Remko Strating:
I would prefer:

even if I would know the real initialization would be later.


There are cases where this is not possible, though. E.g., the way to get and use a connection in JDBC is like this:

The getConnection call must be inside the "try" block -because it can throw an exception- but "con" must be declared outside of it, because it's needed in the "finally" block.

This is just an example; you'd encounter this whenever an object is needed in the finally block, but must be created or retrieved in the try block.


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