Issue
I need to make a copy of HashMap<Integer, List<MySpecialClass> >
but when I change something in the copy I want the original to stay the same. i.e when I remove something from the List<MySpecialClass>
from the copy it stays in the List<MySpecialClass>
in the original.
If I understand it correctly, these two methods create just shallow copy which is not what I want:
mapCopy = new HashMap<>(originalMap);
mapCopy = (HashMap) originalMap.clone();
Am I right?
Is there a better way to do it than just iterate through all the keys and all the list items and copy it manually?
Solution
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.
Answered By - rgettman Answer Checked By - Willingham (PHPFixing Volunteer)
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