UnsupportedOperationException at java.util.AbstractList.add

While working on JAVA assignment, discovered strange error while trying to add an element to list. I am using example code to demonstrate and fix this exception.

Note : Here existing array is converted into list using JAVA API.

Code Sample :

    public static void main(String[] args) {     

        Integer [] array = {1,2,3};

        List<Integer> asList = Arrays.asList(array);

        asList.add(0, 100);

        asList.forEach(s-> System.out.println(s+ " "));

    }

Error : Exception in thread "main" java.lang.UnsupportedOperationException

     at java.base/java.util.AbstractList.add(AbstractList.java:153)

     at com.learning.concepts.PlusOne.main(PlusOne.java:13)

Root Cause : Arrays.asList() method returns a non-resizable list backed by the array. From that method's documentation:

Returns a fixed-size list backed by the specified array. (Changes to the returned list "write through" to the array.)

Solution : In order to use a resizable list use the following to convert array into list.

List<Integer> asList = new ArrayList<Integer>(Arrays.asList(array));