Beginner's mistake of using Object as parameter in Generics

Imagine you have a method that processes the elements of a List, e.g. print out all elements in the List. Here is how such a method could look:

public void printElements(List<Object> elements){
   for(Object o : elements){
      System.out.println(o);
   }
}

Could you call this method with a List<String> instance?
The answer is no. A List<String> is not a subtype of List<Object>. The reason is this:
Imagine you could upcast a List<String> to a List<Object>. It would not cause any problems to iterate each String instance in the List as an Object. After all, a String is a subtype of Object.

The problem arises when you insert elements into the List. In a List<Object> instance you can insert any Object instance, also non-String elements. Thus, if you could upcast a List<String> to a List<Object>, you could now insert non-String objects into the List, meaning the type safety would be broken.
This is true for any kind of collection, not just List's.

So what's the solution for it. Its solution is wildcards (click on link to read on wildcards).