title: Creating a mutable collection from an immutable one
body: <p>Let's say I have the following <code>Map</code> which is created using Guava's library: (<code>List&lt;Integer&gt;</code> is also immutable)</p>
  
  <pre><code>Map&lt;String, List&lt;Integer&gt;&gt; map = ImmutableMap.builder()...
  </code></pre>
  
  <p>I pass this map to a class where I want to create a mutable copy of it and modify it. It is of course possible to do it manually, but is there a way to convert a nested immutable collection back to a mutable one?</p>