Allow only String to List

Question

Hello,
I have a ArrayList.
ArrayList as = new ArrayList();Now i want to allow only string to be added in this arraylist how can i achieve this? Don't want to use generic.
Please guide.

Answer 1

Ankur Raiyani wrote:
i have been asked to do not use generic.
In given example where you have seen generics. It is clearly said if you don't want to use generics then use instanceof keyword.
I want such a way that List only allows me to add String if i add other object then it should throw an error.Exactly , i've gave an idea how to use instanceof and allow only string objects in arraylist.
If not clear, please go through the below program:
import java.util.*;
import java.lang.*;
class ArrayListForOnlyString
     public static void main(String st[])
          List stringArrayList = new ArrayList(){
               public ArrayList ar = new ArrayList();
               public boolean add(Object ob) throws UnsupportedOperationException
                    if(ob instanceof String)
                         ar.add(ob);
                         return true;
                    else
                    throw new UnsupportedOperationException("Only String Object are accepted");
               public void add(int index, Object ob)
                    if(ob instanceof String)
                         ar.add(index,ob);
                    else                    
                    throw new UnsupportedOperationException("Only String Object are accepted");
               public Object[] toArray()
                    return ar.toArray();
               public String toString()
                    return ar.toString();
          try
          stringArrayList.add(new String("10"));
          stringArrayList.add(1,new String("10"));
                // Uncomment the below line and run, it 'll give error , as you are trying to add Integer Object to array list.
          //stringArrayList.add(new Integer("10"));
          System.out.println("The objects of arrayList are : "+stringArrayList);
         }catch(Exception e)
                    e.printStackTrace();
}Edited by: 833545 on Aug 31, 2011 3:53 PM