Why Do I Get an UnsupportedOperationException When Modifying a List?

Unsupported Operation Exception During List Modification: Unveiling the Root Cause

When attempting to modify a List by adding elements, an UnsupportedOperationException may arise. This exception stems from the immutable or restricted nature of certain List implementations.

Identifying the Affected Implementation

One common instance involves the List returned by Arrays.asList(). As documented, this List is fixed-size and prohibits structural modification, including the addition and removal of elements.

Even if you are not directly interacting with the List from Arrays.asList(), the issue may still arise due to the presence of other immutable List implementations or those with limited modification capabilities.

Exploring the Exception's Insight

To grasp the root cause, refer to the documentation for both UnsupportedOperationException and List.add(). The latter specifies that the add() operation is an "(optional operation)", as outlined in the List documentation.

Resolving the Issue: Crafting a Workaround

As a workaround, consider duplicating the immutable List into a modifiable implementation, such as ArrayList:

<code class="java">seeAlso = new ArrayList<>(seeAlso);</code>

This approach allows you to perform modifications such as adding elements to the list without encountering the UnsupportedOperationException.

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