A ConsString Gotcha

[Tal Liron]

ConsString can cause some hard-to-find bugs when interacting with Java 
classes.

Consider this code:

   var map = new java.util.HashMap

   var prefix = 'a'
   var key = prefix + 'b'

   map.put(key, 'hello')

   print(map.get(key) + '\n')
   print(map.get('ab') + '\n')

It will output 'hello' and then 'null'. The reason is that key is a 
ConsString. See for yourself:

   for (var i = map.keySet().iterator(); i.hasNext(); ) {
     print(i.next().getClass() + '\n')
   }

This can cause some very hard-to-find bugs. I know too well. :). I'm not 
sure how best to deal with it, but here are some suggestions.

1. Document it in the user guide, possibly with the simple example I 
gave above. I expect that many of these bugs will happen when dealing 
with generic classes, such as those in java.util. The workaround for the 
example above:

   var key = String(prefix + 'b')

But that's hardly intuitively necessary.

2. It may be a good idea for Nashorn to always coerce ConsString into 
String when interacting with Java, even if the method signature expects 
an Object.

The one downside I can think of is that it will make it impossible to 
offer special handling for ConsString in Java classes. This would seem, 
however, to be quite a rare usage and may not be worth the potential for 
awful bugs.

What do you think?