Why isn't a Map<String,List<SomeBean>> castable to Map<String,List<?>>?
What I'm doing now is this:
Map<String, List<SomeBean>> fromMap = new LinkedHashMap<String, List<SomeBean>>();
/* filling in data to fromMap here */
Map<String,List<?>> toMap = new LinkedHashMap<String, List<?>>();
for (String key : fromMap.keySet()) {
toMap.put(key, fromMap.get(key));
}
In my opinion there should be a way around this manual transformation, but I can't figure out how. Any Ideas?
The cast is invalid because in Map<String,List<?>> you can put List<String> and List<WhatEver>, but not in Map<String, List<SomeBean>>.
For instance:
//List<SomeBean> are ok in both lists
fromMap.put("key", new ArrayList<SomeBean>());
toMap.put("key", new ArrayList<SomeBean>());
//List<String> are ok in Map<String,List<?>>, not in Map<String, List<SomeBean>>
fromMap.put("key", new ArrayList<String>()); //DOES NOT COMPILE
toMap.put("key", new ArrayList<String>());
To simplify your code, you may use the appropriate constructor to simplify your code:
Map<String, List<SomeBean>> fromMap = new LinkedHashMap<String, List<SomeBean>>();
Map<String,List<?>> toMap = new LinkedHashMap<String, List<?>>(fromMap);
Not realy an answer to your question, but as an extra: I would not use keyset here... If you want to iterate through all the elements of a map, use the entrySet() method. Its faster because it does not require the key-value lookup for each element.
for (Map.Entry<String, List<SomeBean>> entry : fromMap.entrySet()) {
toMap.put(entry.getKey(), entry.getValue());
}
If you really want to, you could cast to a raw Map (but what you want is not type safe):
Map<String,List<?>> toMap = (Map) new LinkedHashMap<String, List<String>>();
When assigning to a Map, where K and V are not wildcard parameters, the Map being assigned must have exactly the same K and V. In your case, V must be exactly List<?>.
The workaround it to use a wildcard V.
Map<String, ? extends List<?>> map = new LinkedHashMap<String, List<String>>();
Because the V you are assigning to is a wildcard, the V being assigned must only be assignable to V (rather than being exactly V).
Map<String,List<?>> toMap = (Map<String,List<?>>) new Map<String, List<SomeBean>>(). ~Chris