Assigning a map to a new map in java, and then changing the new map also changes original map

Question

I have a use case which looks something like this :

    Map<String,String> pr = new HashMap<>();
    pr.put("ab","ab");
    pr.put("xy","xy");
    Map<String,String> prNew = new HashMap<>();
    prNew = pr;
    prNew.remove("ab");
    System.out.println(pr);
    System.out.println(prNew);

This is the output i got :

{xy=xy}
{xy=xy}

But I was expecting 'pr' to remain as :

{ab=ab, xy=xy}

It looks like prNew took a reference of pr, and any modifications to prNew will modify pr as well.

So how do i get around this? In my use case I need to take a copy of pr and make some changes but it should not affect the original pr.

Any help would be greatly appreciated. Thanks.

You want to copy your map, not just assign it. HashMap already has a copy constructor that you can use: `prNew = new HashMap<>(pr);` — OH GOD SPIDERS, Mar 27 '18 at 12:04
Read this for reference: [Is Java "pass-by-reference" or "pass-by-value"?](https://stackoverflow.com/questions/40480/is-java-pass-by-reference-or-pass-by-value). Also: [Copying a HashMap in Java](https://stackoverflow.com/questions/10079266/copying-a-hashmap-in-java) — Malte Hartwig, Mar 27 '18 at 12:07

score 2 · Accepted Answer · answered Mar 27 '18 at 12:08

Why the code doesn't work

You create the following in memory:

So when you change 1 map, you change both, because they are the same map.

Solution

You need to make a copy of the map if you want to change 1 while keeping the other the same.

You can do this using the constructor which takes a map in.

score 1 · Answer 2 · answered Mar 27 '18 at 12:07

for the moment pr and prNew reference the same object, a modification on pr or prNew is the same, they are just aliases for the same Map
when you create prNew you have to use the constructor that takes another Map, to make a copy of, all the mappings will be copied into the new one

//...

Map<String, String> prNew = new HashMap<>(pr); // <--
prNew.remove("ab");
System.out.println(pr);     // {ab=ab, xy=xy}
System.out.println(prNew);  // {xy=xy}

Assigning a map to a new map in java, and then changing the new map also changes original map

2 Answers2

Why the code doesn't work

Solution