Why can't I set HashMap to HashMap

Question

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

http://stackoverflow.com/questions/7098402/implementations-and-collections/7098461#7098461 — zw324, Aug 18 '11 at 18:20
possible duplicate of [Any simple way to explain why I cannot do List animals = new ArrayList()?](http://stackoverflow.com/questions/2346763/any-simple-way-to-explain-why-i-cannot-do-listanimal-animals-new-arraylistdo) — Daniel Pryden, Aug 18 '11 at 20:14

score 6 · Accepted Answer · edited Feb 16 '20 at 14:42

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.

score 3 · Answer 2 · answered Aug 18 '11 at 18:21

3

It should be HashMap<? extends CharSequence, ? extends CharSequence>

answered Aug 18 '11 at 18:21

Reverend Gonzo

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Why can't I set HashMap to HashMap

2 Answers2