java.util.Arrays.asList when used with removeIf throws UnsupportedOperationException

Question

I am preparing for an OCPJP 8 exam for the next 2 months and currently I this one got my attention as I dont understand why

public class BiPredicateTest {     public static void main(String[] args) {         BiPredicate<List<Integer>, Integer> containsInt = List::contains;         List<Integer> ints = java.util.Arrays.asList(1,20,20);         ints.add(1);         ints.add(20);         ints.add(20);         System.out.println(containsInt.test(ints, 20));          BiConsumer<List<Integer>, Integer> listInt = BiPredicateTest::consumeMe;         listInt.accept(ints, 15);      }      public static void consumeMe(List<Integer> ints, int num) {         ints.removeIf(i -> i>num);         ints.forEach(System.out::println);     } } 

this clearly is going to compile OK! but when you run it you will see the exception like this

C:\Users\user\Documents>javac BiPredicateTest.java  C:\Users\user\Documents>java BiPredicateTest true Exception in thread "main" java.lang.UnsupportedOperationException         at java.util.AbstractList.remove(AbstractList.java:161)         at java.util.AbstractList$Itr.remove(AbstractList.java:374)         at java.util.Collection.removeIf(Collection.java:415)         at BiPredicateTest.consumeMe(BiPredicateTest.java:22)         at BiPredicateTest.main(BiPredicateTest.java:17) 

I need some help here to understand why the asList method is not working with removeIf? i assume it will return an instance of ArrayList which implements removeIf method!.

Any answer will be appreciated.

cheers!

Answer 1

java.util.Arrays.asList() produces a list from which it is impossible to remove elements, so it throws on a removal attempt.

You could wrap it with ArrayList:

List<Integer> ints = new java.util.ArrayList<>(java.util.Arrays.asList(1,20,20)); 

Update

Arrays.asList() returns return new ArrayList<>(a); where ArrayList is not java.util.ArrayList, but java.util.Arrays.ArrayList (internal class), which does not allow removal.