Question

Although String implements CharSequence, Java does not allow this. What is the reason for this design decision?

Answer 1

The decision to disallow that was made because it’s not type-safe:

public class MyEvilCharSequence implements CharSequence
{
    // Code here
}

HashMap<CharSequence, CharSequence> map = new HashMap<String, String>();
map.put(new MyEvilCharSequence(), new MyEvilCharSequence()); 

And now I’ve tried to put a MyEvilCharSequence into a String map. Big problem, since MyEvilCharSequence is most definitely not a String.

However, if you say:

HashMap<? extends CharSequence, ? extends CharSequence> map = new HashMap<String, String>();

Then that works, because the compiler will prevent you from adding non-null items to the map. This line will produce a compile-time error:

// Won't compile with the "? extends" map.
map.put(new MyEvilCharSequence(), new MyEvilCharSequence());

See here for more details on generic wildcards.