Creating a copy of an ArrayList of HashMaps

Question

I have some code which creates an ArrayList of HashMaps, I then need a copy of that ArrayList of HashMaps for performing calculations, but I want the original to stay the same.

I have tried all sorts, from people's SO answers, here's my current code:

List<Map> counts = new ArrayList<>();

The counts list is filled with HashMaps.

I need a copy of that but I don't want the calculations I perform on the copy to affect the original.

I have tried:

List<Map> copyCounts = new ArrayList<Map>(counts);

But whenever I perform changes it alters the original

[Clone it](http://stackoverflow.com/questions/4592478/clone-arraylist-clone-i-thought-does-a-shallow-copy) — Alfabravo, Apr 11 '17 at 15:51
You can use java-8 stream for calculations, it will not change the state of the original list — Anton Balaniuc, Apr 12 '17 at 08:55

score 2 · Accepted Answer · answered Apr 11 '17 at 15:54

2

Here is how to create a deep copy:

List<Map> counts = ...

List<Map> copy = new ArrayList<>();
for(Map m : counts){
  copy.add(new HashMap(m));
}

answered Apr 11 '17 at 15:54

satnam

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For future reference, use generics! – Jacob G. Apr 11 '17 at 16:03
How would you use generics for this @JacobG. – user3667111 Apr 11 '17 at 16:26
@user3667111 I can't give an example without knowing the key/value types of the `Map`. – Jacob G. Apr 11 '17 at 16:27
Both Doubles @JacobG. – user3667111 Apr 11 '17 at 16:31
@user3667111 Then it would be along the lines of replacing `Map` with `Map` – Jacob G. Apr 11 '17 at 16:32
@user3667111 A compilation error will be thrown if you attempt to add anything other than a `Double` to the `Map`, rather than an exception thrown at runtime. – Jacob G. Apr 11 '17 at 16:34
Okay thank you, I'm now refreshing my memory on java generics :) thanks for the pointers @JacobG. – user3667111 Apr 11 '17 at 16:34

score 1 · Answer 2 · answered Apr 11 '17 at 15:51

1

That's because all the maps you're adding to your new array list are still by reference. You also need to copy the individual maps as well.

answered Apr 11 '17 at 15:51

Charles Durham

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Creating a copy of an ArrayList of HashMaps

2 Answers2