Why Does Java's List.add() Throw an UnsupportedOperationException?

Java List.add() UnsupportedOperationException: Cause and Workaround

Modifying a list can be an essential part of Java development, but what happens when the add() method throws an UnsupportedOperationException? Understanding the cause of this error is crucial to resolving it.

In Java, not all list implementations support the add() method. One common example is the list returned by Arrays.asList(). According to its documentation, this list is fixed-size and does not allow structural modifications, such as adding or removing elements.

Other list implementations may also be immutable or only support certain types of changes. To determine the exact reason for the error, consult the documentation of UnsupportedOperationException and List.add().

Solution:

The most straightforward solution is to use a modifiable list implementation, such as ArrayList. This can be achieved by copying the existing list into a new modifiable list:

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

Once the list is copied into a modifiable implementation, you can safely add elements.

In summary, handling UnsupportedOperationException when using List.add() requires understanding the list's implementation and its supported operations. By switching to a modifiable list or taking other appropriate actions, you can ensure proper modifications to your data structures.

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