How to copy HashMap (not shallow copy) in Java
You're right that a shallow copy won't meet your requirements. It will have copies of the List
s from your original map, but those List
s will refer to the same List
objects, so that a modification to a List
from one HashMap
will appear in the corresponding List
from the other HashMap
.
There is no deep copying supplied for a HashMap
in Java, so you will still have to loop through all of the entries and put
them in the new HashMap
. But you should also make a copy of the List
each time also. Something like this:
public static HashMap<Integer, List<MySpecialClass>> copy( HashMap<Integer, List<MySpecialClass>> original){ HashMap<Integer, List<MySpecialClass>> copy = new HashMap<Integer, List<MySpecialClass>>(); for (Map.Entry<Integer, List<MySpecialClass>> entry : original.entrySet()) { copy.put(entry.getKey(), // Or whatever List implementation you'd like here. new ArrayList<MySpecialClass>(entry.getValue())); } return copy;}
If you want to modify your individual MySpecialClass
objects, and have the changes not be reflected in the List
s of your copied HashMap
, then you will need to make new copies of them too.
Serialize to json and deserialize afterwards:
Map<String, Object> originalMap = new HashMap<>();String json = new Gson().toJson(originalMap);Map<String, Object> mapCopy = new Gson().fromJson( json, new TypeToken<Map<String, Object>>() {}.getType());
For special classes you might need to write a custom deserializer.