Why can't I set the HashMap <CharSequence, CharSequence> to HashMap <String, String>

Question

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

Answer 1

The decision to ban was made because it is not 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 tried to put MyEvilCharSequence in a String map. A big problem, since MyEvilCharSequence definitely not a String .

However, if you say:

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

Then it works, because the compiler will stop you from adding non- null elements to the map. This line will result in a compile-time error:

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

See here for general wildcards for more details .

Answer 2

Should it be HashMap<? extends CharSequence, ? extends CharSequence> HashMap<? extends CharSequence, ? extends CharSequence>