Computing the union of the keySets of two HashMaps in Java

Question

I want to compute the union of the keys of two hashmaps. I wrote the following code (MWE below), but I get UnsupportedOperationException. What would be a good of accomplishing this? ``` import java.util.HashMap; import java.util.Map; import java.util.Set; public class AddAll { public static void main(String args[]){ Map first = new HashMap(); Map second = new HashMap(); first.put("First", 1); second.put("Second", 2); Set one = first.keySet(); Set two = second.keySet(); Set union = one; union.addAll(two); System.out.println(union); } } ```

Accepted Answer

So, `union` is not a copy of `one`, it is `one`. It is `first.keySet()`. And `first.keySet()` isn't a copy of the keys of `first`, it's a view, and won't support adds, as documented in `Map.keySet()`.

So you need to actually do a copy. The simplest way is probably to write

```
 one = new HashSet<String>(first);
```

which uses the "copy constructor" of `HashSet` to do an actual copy, instead of just referring to the same object.

Computing the union of the keySets of two HashMaps in Java

Solution

Problem