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Usage of generics

 
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Hi,
I have just started peeping into jdk 1.5.

Generics were introduced so as to avoid the type casting for objects when retrieved from a collection. This feature is quite helpfull but i need some clarifications.
I have written a class as :

public class Test {
public static void main(String args[]) {
Test t = new Test();
ArrayList<String> a = new ArrayList();
a.add("a");
a.add("b");
a.add("c");
a.add("d");
a.add("e");
t.arrayListTest(a);
}
//******* generics in Collections **********//
public void arrayListTest(ArrayList a) {
for (int i=0; i < a.size();i++) {
String str1 = a.get(i);
System.out.println(str1);
}
}
}

When the java class is compiled,
I recieve an error saying "Cannot convert from Object to String"
at
String str1 = a.get(i);

I would like to know why is the generic type set for ArrayList a in main() is not applicable inside arrayListTest() method.

Regards
P. Vamsi Mohan
 
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Originally posted by Vamsi Mohan Pemmaraju:
...I would like to know why is the generic type set for ArrayList a in main() is not applicable inside arrayListTest() method.


It's because the method arrayListTest takes an argument of type ArrayList. If you change the argument type to ArrayList<String>, then it will work.
 
Vamsi Mohan Pemmaraju
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What is the diference between Arraylist<String> and ArrayList>?

Of course I understand that ArrayList<String> specifies that it holds objects of type String.

Any Implementation differences when using Generics??

Regards
P.Vamsi Mohan
 
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Hi,
Arraylist without any type parameter acts as an plain old Collection's class,in which we can store different kind of objects.However to retrieve object from Arraylist you need to cast the object returned by get() function.This casting get resolved only at run time.
Try to run by casting the following snippets of code given above:
replace : -> String str1 = a.get(i);
with :-> String str1 = (String) a.get(i);

Now using this,we can get ClassCastException at run time(if user has kept different objects also in Arraylist and trying to get those objects).

To aviod runtime check ,we use Generics;
It allows us to declare the kind of objects we are going to store in Arraylist and thus ensures that compiler checks for any bugs at compile time only.
Try to parametrized
arrayListTest(ArrayList a) --> arrayListTest(ArrayList<String> a)
This restrict us to add only object of type String in Arraylist.

This will keep us out from casting the get() function and thus runtime checking.

Thanks
Mittu
 
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