Java.lang.UnsupportedOperationException when combining two Sets

collectionsjavaset

I have 2 different instances of HashMap

I want to merge the keysets of both HashMaps;

Code:

Set<String> mySet = hashMap1.keySet();
mySet.addAll(hashMap2.keySet());

Exception:

java.lang.UnsupportedOperationException
    at java.util.AbstractCollection.add(AbstractCollection.java:238)
    at java.util.AbstractCollection.addAll(AbstractCollection.java:322)

I don't get a compile warning or error.

From java doc this should work. Even if the added collection is also a set:

boolean addAll(Collection c)

Adds all of the elements in the specified collection to this set if
they're not already present (optional operation). If the specified
collection is also a set, the addAll operation effectively modifies
this set so that its value is the union of the two sets. The behavior
of this operation is undefined if the specified collection is modified
while the operation is in progress.

Best Answer

If you look at the docs of the HashMap#keySet() method, you'll get your answer(emphasis mine).

Returns a Set view of the keys contained in this map. The set is backed by the map, so changes to the map are reflected in the set, and vice-versa. If the map is modified while an iteration over the set is in progress (except through the iterator's own remove operation), the results of the iteration are undefined. The set supports element removal, which removes the corresponding mapping from the map, via the Iterator.remove, Set.remove, removeAll, retainAll, and clear operations. It does not support the add or addAll operations.

Therefore, you need to create a new set and add all the elements to it, instead of adding the elements to the Set returned by the keySet().

Related Solutions

Java – How to concatenate two arrays in Java

I found a one-line solution from the good old Apache Commons Lang library.
ArrayUtils.addAll(T[], T...)

Code:

String[] both = ArrayUtils.addAll(first, second);

Java – Iterating through a Collection, avoiding ConcurrentModificationException when removing objects in a loop

Iterator.remove() is safe, you can use it like this:

List<String> list = new ArrayList<>();

// This is a clever way to create the iterator and call iterator.hasNext() like
// you would do in a while-loop. It would be the same as doing:
//     Iterator<String> iterator = list.iterator();
//     while (iterator.hasNext()) {
for (Iterator<String> iterator = list.iterator(); iterator.hasNext();) {
    String string = iterator.next();
    if (string.isEmpty()) {
        // Remove the current element from the iterator and the list.
        iterator.remove();
    }
}

Note that Iterator.remove() is the only safe way to modify a collection during iteration; the behavior is unspecified if the underlying collection is modified in any other way while the iteration is in progress.

Source: docs.oracle > The Collection Interface

And similarly, if you have a ListIterator and want to add items, you can use ListIterator#add, for the same reason you can use Iterator#remove — it's designed to allow it.

In your case you tried to remove from a list, but the same restriction applies if trying to put into a Map while iterating its content.

Best Answer

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Java – How to concatenate two arrays in Java

Java – Iterating through a Collection, avoiding ConcurrentModificationException when removing objects in a loop

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