Updating an entry of a HashMap whose values is a List

Question

I have a HashMap<Integer, List<Integer>> "sq_diff" and I am trying to update an entry.

I tried:

List<Integer> values = sq_diff.get(diff);
values.add(c);
values.add(d);
sq_diff.put(diff, values);

and

sq_diff.get(diff).add(c);
sq_diff.get(diff).add(d);

and

sq_diff.computeIfPresent(diff, (k, v) -> v.add(c);
sq_diff.computeIfPresent(diff, (k, v) -> v.add(d);

None of them working:

Exception in thread "main" java.lang.UnsupportedOperationException

The map contains an entry of "diff"

Highly appreciated if you can indicate problems on each of them.

What actual type is the List you are using? It looks very much like you are using some kind of fixed-size list that does not allow inserting new objects. (Like the List you get When you use `Arrays.asList` method) — OH GOD SPIDERS, Feb 21 '20 at 17:08
see: [Java List.add() UnsupportedOperationException](https://stackoverflow.com/questions/5755477/java-list-add-unsupportedoperationexception) — OH GOD SPIDERS, Feb 21 '20 at 17:13
@OH GOD SPIDERS, on creation I am using "sq_diff.putIfAbsent(diff, Arrays.asList(c, d));" — User_67128, Feb 21 '20 at 17:28
@User_67128 have attention to answer at which all cases are working fine. — catch23, Apr 13 '20 at 15:41

catch23 · Answer 1 · 2020-02-26T23:27:18.763

Trouble is when you initialize your list by Arrays.asList(). As mentioned at the documentation:

This method also provides a convenient way to create a fixed-size list initialized to contain several elements

And later when you want to update your fixed-size list with adding new elements you got: UnsupportedOperationException.

As a soltion you can use a quite useful Guava's Lists utilities class:

Lists.newArrayList(c, d);

It also provides Maps utilities as well.

Code for it will look like:

public class MapDemo {
    public static void main(String[] args) {
        final int diff = 42;
        HashMap<Integer, List<Integer>> map = Maps.newHashMap();
        map.putIfAbsent(diff, Lists.newArrayList(1, 2));
        // 1:            
        final List<Integer> integers = map.get(diff);
        integers.add(10);
        integers.add(11);
        // 2: 
        map.get(diff).add(20);
        map.get(diff).add(21);
        // 3:    
        map.computeIfPresent(diff, (k, v) -> {
            v.add(30);
            return v;
        });
        map.computeIfPresent(diff, (k, v) -> {
            v.add(31);
            return v;
        });

        System.out.println(map);
    }
}

Output:

{42=[1, 2, 10, 11, 20, 21, 30, 31]}

As you can see all 3 cases are executed well.

score 2 · Accepted Answer · answered Feb 21 '20 at 17:36

2

Try this,

List<Integer> values = new ArrayList<>(Arrays.asList(c, d));
sq_diff.put(diff, values);

instead of

sq_diff.put(diff, Arrays.asList(c, d));

answered Feb 21 '20 at 17:36

abhimanyue

196
1
12

It works but "sq_diff.computeIfPresent(diff, (k, v) -> v.add(c);" still not working. – User_67128 Feb 21 '20 at 17:42

Updating an entry of a HashMap whose values is a List

2 Answers2