ArrayList retainAll confusing me

Question

I am trying to work with intersections in java. If I have the following, i just want an arraylist with nothing in it because the intersection gives me zero.

List<String> coll1 = Arrays.asList(["A"]);
List<String> coll2 = Arrays.asList(["B","C"]);

coll1.retainAll(coll2);

However retainAll throws this exception :

java.lang.UnsupportedOperationException
    at java.util.AbstractList.remove(AbstractList.java:161)
    at java.util.AbstractList$Itr.remove(AbstractList.java:374)
    at java.util.AbstractCollection.retainAll(AbstractCollection.java:410)

If I convert the lists to sets, then I get a set which is empty if I call retainAll :

Set<String> set1 = new HashSet<String>(coll1);
Set<String> set2 = new HashSet<String>(coll2);

set1.retainAll(set2); //gives me an empty set, which is good!

Why does ArrayList have a problem with empty intersections?

See [remove() on List created by Arrays.asList() throws UnsupportedOperationException](https://stackoverflow.com/q/7885573/4391450) (already flagged differently but this is a duplicate...) — AxelH, Apr 13 '18 at 09:42
Please edit the code . The code given `List coll1 = {"A"}` is errorful. — Number945, Apr 13 '18 at 09:42
When you create a [mcve], please do so accurately. `List coll1 = {"A"};` is not valid Java. Everyone is asking you about that one. — Erwin Bolwidt, Apr 13 '18 at 09:43
Does this really work `List coll1 = {"A"};` ? I had to use `Arrays.asList` to initialise the array — vikingsteve, Apr 13 '18 at 09:45

OldCurmudgeon · Accepted Answer · 2018-04-13T10:36:11.337

6

This is because Arrays.asList creates an immutable list so when you call retainAll which tries to remove entries from the list it complains.

The solution is to create a new ArrayList copy of it.

public void test(String[] args) {
    List<String> coll1 = new ArrayList<>(Arrays.asList("A"));
    List<String> coll2 = new ArrayList<>(Arrays.asList("B","C"));
    coll1.retainAll(coll2);
    System.out.println(coll1);
}

prints:

[]

edited Apr 13 '18 at 10:36

answered Apr 13 '18 at 09:40

OldCurmudgeon

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It complains specifically because the `remove` operation is unsupported in an immutable list, and `retainAll` calls `remove` on the iterator. – vikingsteve Apr 13 '18 at 09:44

score 2 · Answer 2 · answered Apr 13 '18 at 09:41

In your code, and explain in your answer, you are not using explicitly an ArrayList but an AbstractList. And as explain in the doc

Throws: UnsupportedOperationException - if the retainAll operation is not supported by this collection

As you do not declare explicitly an ArrayList, you are not using an ArrayList but an AbstractList that has not implemented retainAll, So it throws you this Exception.

Declare your List as ArrayList and it should be okay

List<String> list1 = new ArrayList<String>();

ArrayList retainAll confusing me

2 Answers2