Java - 将 map(key, set) 中的不可修改集转换为可修改集

Question

I implemented a function which return Map<key,Set<objects>> , when I call this function it return the sets with type Unmodifiable.

I need to do some operations to this sets, how to convert them to modifiable sets in best practice? Otherwise I got

Exception in thread "main" java.lang.UnsupportedOperationException

Thanks in advance.

Answer 1

Assuming that the Map itself is mutable, you use something like

map.replaceAll((key, set) -> new HashSet<>(set));

Example:

Map<Integer,Set<Object>> map = new HashMap<>();
map.put(5, Collections.emptySet());
map.put(10, Collections.singleton("foo"));

map.replaceAll((key, set) -> new HashSet<>(set));

map.get(5).add(42);
map.get(10).add("bar");

map.entrySet().forEach(System.out::println);

5=[42]
10=[bar, foo]

Of course, you can also replace new HashSet<>(set) with new TreeSet<>(set) or generally every Set implementation type following the copy constructor convention. When you can't use a copy constructor, you have to resort to addAll , eg

map.replaceAll((key, set) -> {
    TreeSet<Object> newSet = new TreeSet<>(Comparator.comparing(Object::toString));
    newSet.addAll(set);
    return newSet;
});

---

There's another option. Instead of converting all values of the map, only convert the sets on demand, ie when you actually want to modify them and they turn out not to have the intended type:

Map<Integer,Set<Object>> map = new HashMap<>();
map.put(5, Collections.emptySet());
map.put(10, Collections.singleton("foo"));

map.computeIfPresent(5, (key,set)->set instanceof HashSet? set: new HashSet<>()).add(42);
map.computeIfPresent(10, (key,set)->set instanceof HashSet?set:new HashSet<>()).add("bar");

map.entrySet().forEach(System.out::println);

Answer 2

You could copy the original set to another one that can be modified .

Something like this:

Set<YourType> newSet = unmodifiableSet
                        .stream()
                        .collect(Collectors.toSet());
// Or maybe...
Set<YourTYpe> otherSet = new HashSet<>();
otherSet.addAll(unmodifiableSet);

Then, you could modify the new list without any problem, and re-assign it into the map.

Answer 3

Just use the copy constructor

For a HashSet :

Set<Type> modifiable = new HashSet<>(unmodifiable);

For a TreeSet :

SortedSet<Type> modifiable = new TreeSet<>(unmodifiable);

For a LinkedHashSet :

Set<Type> modifiable = new LinkedHashSet<>(unmodifiable);

If you use a fancy Set implementation without such constructor:

Set<Type> modifiable = new MyFancySet<>();
modifiable.addAll(unmodifiable);

Answer 4

You can do that as a one-liner with Streams :

With map being your original map (eg Map<Integer, ImmutableSet<Object>> map; )

Map<Integer, Set<Object>> mutableMap = map.entrySet()
                .stream()
                .collect(Collectors.toMap(Map.Entry::getKey, Sets::newHashSet));