Discuss- When to use local variable and instance variable?

[Lim Cody]

Hi,
I've a DAO class (not a singleton) like below
code 1( instance variable):

public class UserDaoImpl implements UserDao{
private String findUserName="select ... from .....";
private ArrayList parametes;

 public ArrayList findUserNameByXXX(String xxx){
 parametes=new ArrayList ();
 parametes.add(xxx);
 ArrayList result = query.run(findUserName,parametes);
 return result ;
 }
}

ArrayList "parametes" would be reused for more than one method if there are more methods are added in the future



code 2(local variable):

public class UserDaoImpl implements UserDao{

 public ArrayList findUserNameByXXX(String xxx){
 String findUserName="select ... from .....";
 ArrayList parametes=new ArrayList ();
 ArrayList  result = query.run(findUserName);
 return result ;
 }
}

What is your view of the performance, best practice and etc on both sample code?

[William Brogden]

In the code you show, parametes is created new (line 6) for every query so why not keep it local.

In any case, since a database query is involved, the query takes almost all the time and there will be no measurable difference.

Just go for clarity of expression and security rather then trying to shave microseconds.

Bill

[Saifuddin Merchant]

For one I'll keep the query as a final static string of the class. Its more from the maintenance/readability and style benefit rather than performance.

As for the ArrayList its better to have it as a local variable inside the method. One reason is if the Object is involved in multi-threaded invocations it would be thread safe. Having the ArrarList as a member variable gives no benefit.

Also in simple O-O terms, parametes does not seem to be a 'property' of UserDaoImpl.

[Poo Kan]

i would prefer to keep the parameters list inside the method as it would be thread safe in a multi thread environment. And i would suggest to load the quarries form a properties file,cache it and use it, so that you will not need to change the source code for any changes in query

Apart from 'parameters' list and query, i would also suggest to change the method signature to return List than ArrayList. Always better to have the return type as an interface unless you have any special reasons to return the concrete class.it hides the internal implementation and also flexible for any internal changes.