I have written this lambda that is getting values from a map
<Code, Message>contained inside an enum
MessageKeyRegistry and creating adding to another map with Code as the key and the
MessageKeyRegistry as the value.
Map<MessageKey, MessageKeyRegistry> keyMap = new HashMap<>(); EnumSet.allOf(MessageKeyRegistry.class) .forEach(messageKeyRegistry -> messageKeyRegistry.getCodeToMessage().keySet() .forEach(code -> keyMap.put(code, messageKeyRegistry)));
Now I want to satisfy the immutability concept of functional programming and write a lambda that returns an immutable map directly.