Quick Answer

Handle immutable collection modifications.

Understanding the Issue

Thrown when trying to modify immutable collections like those from Arrays.asList() or Collections.unmodifiableList().

The Problem

This code demonstrates the issue:

Java Error 
List<String> fixed = Arrays.asList("a", "b");
fixed.add("c");  // Throws

The Solution

Here's the corrected code:

Java Fixed 
// Solution: Create mutable copy
List<String> mutable = new ArrayList<>(fixed);
mutable.add("c");

// Or using Java 10+ copyOf
List<String> copy = List.copyOf(fixed); // Immutable

Key Takeaways

Be aware of immutable collections returned by convenience methods.