Map Question

[Akhil Maharaj]

Hello ,

Why I am getting Runtime exception at Line 1 in the follwing code .

  
 
import java.util.*;
 
class AskMapQuestion {
   public static void main (  String [] args ) {
      Map<Integer, String> map = new LinkedHashMap<Integer, String> ();
 
      Map  nap = new HashMap();
 
      populate( map );
      populate( nap );
 

     System.out.println(map.get(1));  // Line 1    ----  Run time exception at Line 1
     System.out.println(nap.get(1));
 
   }
 
   static void populate ( Map  m ) {
      for ( int i = 0 ; i < 10 ; i++ ) {
         m.put(i,i);
      }
   }
}
 
  

[ahmed yehia]

map.get(1)
Returns String but contains Integer, ClassCastException occur when you try to treat the returned value as String.

while nap.get(1)
Returns Object and contains Integer, passing it to the print statement will invoke object's toString().

[Akhil Maharaj]

Ahmed ,

map.get(1)
Returns String but contains Integer

Agreed

, ClassCastException occur when you try to treat the returned value as String.

Agreed

Question : All I am doing here is trying to print the Integer Object that I get out of map.get(1) . I did not treat it as String Object

Any more comments will be of great help

[Henry Wong]

Question : All I am doing here is trying to print the Integer Object that I get out of map.get(1) . I did not treat it as String Object

Actually, you did. You declared the map as having strings as values (generics) -- and then you placed integers into them, by not using generics. So you effectively tricked the compiler into thinking that you were working with strings, and to use the println() method that took a string, instead of one that took an object.

Henry