Why calling remove on a list throws UnsupportedOperationException

You may face this UnsupportedOperationException when playing with some Java collections from various sources or creation operations.

The root of this exception is that the returned object has a fixed-size and maybe uses one object of java.util.Arrays.

All the methods provided by the Arrays class are static in nature and some of its methods throw the UnsupportedOperationException. add or remove methods are the concerned ones for this type of fixed-size lists.

This is different from what we expected to return, which is the standard java.util.ArrayList.

The solution is to use java.util.ArrayList constructor to create a new list from an object based on java.util.Arrays:

ArrayList<String> newList = ...
// if it throws UnsupportedOperationException
newList = new ArrayList<>(badArrayList);

// if you used `asList` to create a list
ArrayList<String> list = Arrays.asList("a", "b", "c");
// create a new list with ArrayList and adapt your code
ArrayList<String> newList = new ArrayList<>(list);

In conclusion, it’s essential to understand that removing or adding objects of a list can be problematic and how to find a simple and efficient solution.