Object Vs wild card (?)

Unbounded wild card is used for unknown types, can able to accept any kind of types. Will see it by an Example.

/* Class demonstrates the use of Unbounded Wild Card */
import java.util.*;
 
class ObjectVsWildCard{
 public static void printList(List<?> data){
  System.out.println("Elements in the List are");
  for(Object obj: data)
   System.out.print(obj +" ");
  System.out.println();
 }
   
 public static void main(String args[]){
  List<Integer> myList1 = new ArrayList<Integer> ();
  List<Double> myList2 = new ArrayList<Double> ();
      
  myList1.add(10);
  myList1.add(20);
  myList1.add(30);
      
  myList2.add(10.01);
  myList2.add(10.02);
  myList2.add(10.03);
      
  printList(myList1);
  printList(myList2);        
 }
}

Output

Elements in the List are
10 20 30
Elements in the List are
10.01 10.02 10.03

As you see the prototype of the printList

printList(List<?> data)

'?' represents unknown type, So printList able to accepts any type of list.

/* Class demonstrates the use of Unbounded Wild Card */
import java.util.*;
 
class ObjectVsWildCard1{
 
 public static void printList(List<Object> data){
  System.out.println("Elements in the List are");
  for(Object obj: data)
   System.out.print(obj +" ");
  System.out.println();
 }
   
 public static void main(String args[]){
  List<Integer> myList1 = new ArrayList<Integer> ();
  List<Double> myList2 = new ArrayList<Double> ();
      
  myList1.add(10);
  myList1.add(20);
  myList1.add(30);
      
  myList2.add(10.01);
  myList2.add(10.02);
  myList2.add(10.03);
      
  printList(myList1);
  printList(myList2);        
 }
}

When you tries tries to compile the above program, compiler throws the below error. Since List<Object> can't take List<Integer>, List<Double>. Where as List<?> takes any type of List.

ObjectVsWildCard1.java:25: error: method printList in class ObjectVsWildCard1 cannot be applied to given types;
                printList(myList1);
                ^
  required: List<Object>
  found: List<Integer>
  reason: actual argument List<Integer> cannot be converted to List<Object> by method invocation conversion
ObjectVsWildCard1.java:26: error: method printList in class ObjectVsWildCard1 cannot be applied to given types;
                printList(myList2);
                ^
  required: List<Object>
  found: List<Double>
  reason: actual argument List<Double> cannot be converted to List<Object> by method invocation conversion
2 errors
 

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