Generic Declarations

[Faraah Dabhoiwala]

Please explain me how to make our own Generic Class and Generic Methods?

its quite confusing

[Deepak Bala]

Let us know what you find confusing and we can narrow down on the problem.

Have you used some of the generic collections ?

[Faraah Dabhoiwala]

1) Regarding generic class
Ok,,

So for example,,


here is a class

 
public class rentalGeneric<T>{
//some code here

}

 

now here i understood that <T> is type it can be anything say,Integer,Long or say , Animal (provided Animal is a class)

so we can write

 
public class rentalGeneric<Integer>{
//some code here

}

 

but how to use this class ..
why at all we define generic type after class name???

also there is another example where there are two generic types declared after class name

 
public class useTwo<T,X>{
//some code here

}

 

1) Regarding generic method

public class CreateAnArrayList{

     public <T> void makeArrayList(T t)
     {
             list.add(t);
     }


}


    

how to use this method?

[Henry Wong]

Shadab Wadiwala wrote:1) Regarding generic class
Ok,,

So for example,,

here is a class

 
public class rentalGeneric<T>{
//some code here

}

 

now here i understood that <T> is type it can be anything say,Integer,Long or say , Animal (provided Animal is a class)

so we can write

 
public class rentalGeneric<Integer>{
//some code here

}

 

but how to use this class ..

The second case is not doing what you think it does. The first case declares a type T, which can be any type. The second case declares a type Integer, which can be any type. "Integer" is a type variable, and not refering to the Integer class.

why at all we define generic type after class name???

In these two examples, there is no reason to -- as you don't use the types anywhere.

Henry

[Henry Wong]

BTW, the Sun tutorial on Generics may be a good place to start ...

http://java.sun.com/docs/books/tutorial/java/generics/index.html

Henry

[Ankit Garg]

When you declare your class as rentalGeneric<Integer>, this doesn't mean that your class rentalGeneric has type java.lang.Integer. Try this

public class rentalGeneric<Integer>{
    public static void main(String[] args) {
        rentalGeneric<String> s = new rentalGeneric<String>();   //ok
    }
}

Basically you are just using Integer instead of T but that doesn't make your Integer the java.lang.Integer class. So don't use anything like Integer or String with your class declaration as it doesn't mean what you think it means.

As far as methods go, your code doesn't use generic methods very well. You've shown list.add as a statement in your method but where is this list declared. If a generic method is used in conjunction with generic class, then it works like this

public class RentalGeneric<T>{
    public void display(List<T> list) {
        //iterate over the list and display the elements.
    }
}

Now if you want to use this class, then here's how you can use it

    public static void main(String[] args) {
        RentalGenerics<Integer> rg = new RentalGenerics<Integer>();
        List<Integer> list = new ArrayList<Integer>();
        //add elements to the list
        rg.display(list);
    }

So here you declare your class to have Integer as the type of T, so you pass the display method a List of Integers. If you want to use a generic method which declares its own type, then here's how you can do it.

    public <T> LinkedList<T> convert(ArrayList<T> al) {
        LinkedList<T> ll = new LinkedList<T>();
        ll.addAll(al);
        return ll;
    }

This method will accept an array list of a specific type and return a LinkedList of the same type. This is how you can call it

    public static void main(String[] args) {
        ArrayList<Integer> list = new ArrayList<Integer>();
        //add elements to the list
        LinkedList<Integer> ll = obj.convert(list);
    }