title: Java Map implementation that returns a default value instead of null
body: <p>I have a <code>Map&lt;String, List&lt;String&gt;&gt;</code> in my code, where I'd avoid potential null pointers if the map's <code>#get()</code> method returned an empty list instead of null. Is there anything like this in the java API? Should I just extend <code>HashMap</code>?</p>