Generalized casting to a given class

[Joe Simone]

I have some code that looks like this ...

 
public static boolean findProductIdInList(List list, String id) {
Iterator i = list.iterator();
boolean found = false;
while (i.hasNext()) {
Product o = (Product)i.next();
if (o.getId().equals(id)) {
found = true;
break;
}
}
return found;
}  

public static boolean findCustomerIdInList(List list, String id) {
Iterator i = list.iterator();
boolean found = false;
while (i.hasNext()) {
Customer o = (Customer)i.next();
if (o.getId().equals(id)) {
found = true;
break;
}
}
return found;
}  

public static boolean findReferralIdInList(List list, String id) {
Iterator i = list.iterator();
boolean found = false;
while (i.hasNext()) {
Referral o = (Referral)i.next();
if (o.getId().equals(id)) {
found = true;
break;
}
}
return found;
}    

It seems kind of silly to write the same method over and over again, each time casting the iterators next() to a different class. Note: the only thing different in each of the three methods are the lines

 
Product o = (Product)i.next();
...
Customer o = (Customer)i.next();
...
Referral o = (Referral)i.next();

How can I take these methods and roll them into one general method that casts to a given class?

Many thanks,
Joe

[Stan James]

You can easily cast instances of all three concrete classes to some common abstraction - superclass or interface.

public static boolean findThingIdInList(List list, String id) {
Iterator i = list.iterator();
boolean found = false;
while (i.hasNext()) {
Findable o = (Findable)i.next();
if (o.getId().equals(id)) {
found = true;
break;
}
}
return found;
}  

Now you can pass a list of anything that implements or extends Findable. Looking at Product, Customer, and Referral, an "is a" (inheritance) relationship seems unlikely. But completely unrelated classes might all implement a common interface, maybe "HasProjectStandardIdentity" or something meaningful to you.

[Layne Lund]

If you are using Java 5.0, you can use generics to reduce the amount of code you have to type in situations like this. I'm not too familiar with this new feature, so I won't elaborate any further. Besides, you can google for more information if you want.

Pre-Java 5.0 requires you to use some inheritence scheme like that mentioned above, either with a base class or an interface.

Layne
[ February 16, 2005: Message edited by: Layne Lund ]

[Joe Simone]

Awesome!

Thanks Stan and Layne. I implemented an interface and the code works perfectly.

I will investigate the features of 5.0 to investigate further.

Thanks for your very prompt reply.

Kind regards,
Joe

[Ernest Friedman-Hill]

Generics would let you dispense with the casting by creating a "List<Findable>" -- A List that held Findable objects only. But you'd still have to do the same thing with the Findable interface.

[Layne Lund]

template <class T>
bool findReferralIdInList(List list, String id) {
Iterator i = list.iterator();
boolean found = false;
while (i.hasNext()) {
[B]T[/B] o = ([B]T[/B])i.next();
if (o.getId().equals(id)) {
found = true;
break;
}
}
return found;
}  

Is something similar possible with generics? I really need to find some time to play around with all the new 5.0 features myself. At the moment, all my knowledge of them is purely academic.

Layne
[ February 16, 2005: Message edited by: Layne Lund ]

[Ernest Friedman-Hill]

Originally posted by Layne Lund:

Is something similar possible with generics? I really need to find some time to play around with all the new 5.0 features myself.

No, generics don't work that way. You could do something like this (perturbing your example as little as possible

        boolean findReferralIdInList(List<Findable> list, String id) {
Iterator<Findable> i = list.iterator();
boolean found = false;
while (i.hasNext()) {
Findable o = i.next();
if (o.getId().equals(id)) {
found = true;
break;
}
}
return found;
}  

It is indeed possible to parameterize classes and methods by free type parameters like "T", but under those circumstances you either can only assume Object's methods for T, or you have to explicitly restrict T:

class Foo<T implements Findable> { ... }

In the body of Foo<T>, you can now use Findable's methods on T, but specific instantiations of Foo<T> would be created for concrete Findable-implementing classes.

The relevant conceptual difference here is that everything you do with the parameter T is type checked in Java when the template class is itself separately compiled, while in C++ it's not looked at until the template is instantiated for a specific T.

[Layne Lund]

So it looks like in this case, generics would only save you from making the cast, but not much else. Hmm...

I think I also made another mistake. In C++, you can templatize functions (methods) and classes individiually. IIRC, generics in Java can only be applied to classes. Is that correct?

Layne

[Ernest Friedman-Hill]

Originally posted by Layne Lund:
IIRC, generics in Java can only be applied to classes. Is that correct?

Nope, actually you can have individual generic methods, too, like C++ function templates. From my little pile of learning examples:

public class Foo {
    public <A extends Number> double sum(A a1, A a2) {
        return a1.doubleValue() + a2.doubleValue();
    }
    
    public static void main(String[] argv) {
        Foo f = new Foo();
        double d1 = f.sum(new Integer(3), new Integer(5));
        double d2 = f.sum(new Double(3), new Double(5));
    }
}

Although in this example, again, you don't gain much -- sum() could merely take two Number objects as arguments!

[Nischal Tanna]

 
public static boolean findProductIdInList(List list, String id) {
Iterator i = list.iterator();
boolean found = false;
while (i.hasNext()) {
Product o = (Product)i.next();
if (o.getId().equals(id)) {
found = true;
break;
}
}
return found;
}  

public static boolean findCustomerIdInList(List list, String id) {
Iterator i = list.iterator();
boolean found = false;
while (i.hasNext()) {
Customer o = (Customer)i.next();
if (o.getId().equals(id)) {
found = true;
break;
}
}
return found;
}  

public static boolean findReferralIdInList(List list, String id) {
Iterator i = list.iterator();
boolean found = false;
while (i.hasNext()) {
Referral o = (Referral)i.next();
if (o.getId().equals(id)) {
found = true;
break;
}
}
return found;
}    

It seems kind of silly to write the same method over and over again, each time casting the iterators next() to a different class. Note: the only thing different in each of the three methods are the lines

 
Product o = (Product)i.next();
...
Customer o = (Customer)i.next();
...
Referral o = (Referral)i.next();

How can I take these methods and roll them into one general method that casts to a given class?

Many thanks,
Joe[/qb]<hr></blockquote>



Hi Joe
I Think After listening to all this people u may be confused as the solutions procided by them are R&D and not practical Neways i guess u can do the following way. U can use the instanceOf operator to decide to which Class the instance belongs to.

  
 
public static boolean findReferralIdInList(List list, String id) {
Iterator i = list.iterator();
boolean found = false;
while (i.hasNext()) {
                       if(i.hasNext() instanceOf Referral){// check the syntax and complete the code for the Referral case as well as for the rest cases
Referral o = (Referral)i.next();
if (o.getId().equals(id)) {
found = true;
break;
}
}
return found;
}    
 
 

[ February 17, 2005: Message edited by: Nischal Tanna ]