Generics and parameterized types in Java
java
Solution
I recommend reading this article, http://java.dzone.com/articles/covariance-and-contravariance. (Italic additions are mine)
In summary, we use covariance `<? extends Object>` when we only intend to take generic values out of a structure. We use contravariance `<? super Object>` when we only intend to put generic values into a structure and we use an invariant `<Object>` when we intend to do both.
You defined your list as holding a covariant Object type, which means you can write `Object o = a1.get(1)` but you can't write `a1.add("foo")`. If you want to be able to add Objects but not get them back, then you would need to define your list like this:
List<? super Object> a1 = new ArrayList<>();
It's a bit unfortunate, in my opinion, language authors used the terms `extend` and `super` to indicate covariance and contravariance, especially in the above example since there is no super type of `Object`.
Problem
The below code causes a compile time error : The method add(capture#1-of ? extends Object) in the type List is not applicable for the arguments (String) code : ``` List<? extends Object> a1 = new ArrayList(); a1.add("string"); ``` Error is at line : ``` a1.add("string"); ``` Since class String extends from Object why ref a1 not accept a String type ?