Why Generic types in Java are invariant?

Generic types in Java are invariant. That means, if T2 is a subtype of T1, List<T2> is not a sub type of List<T1>.

Ex

Integer is a subtype of Number.

But Stack<Integer> is not a sub type of Stack<Number>.

Why java makes generics as invariant?

Let me explain with an example. 

  /* Define a character list */
  List<Character> chars = new ArrayList<Character>();

  /* Java do not allow this */
  List<Object> objs = chars;

  /* Adding double to character list */
  objs.add(1.23);

  /* ClassCastException: Cannot cast double to Character */
  Character s = chars.get(0);

As you notify above snippet. I defined a character list ‘chars’ and cast it to list of objects ‘objs’. I added a double to ‘objs’. Last statement cast the double to character, which is not possible. To prohibit these kind of things and ensure run time safety of the application, java generics are invariant.

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