title: Odd syntax in API "String::concat"
body: <p>I was looking at some of the changes made to the Java SE API with 1.8, and I when looking at the new method  <a href="http://docs.oracle.com/javase/8/docs/api/java/util/Map.html#merge-K-V-java.util.function.BiFunction-" rel="noreferrer">Map.merge</a> it shows an example of how to use it with the line </p>
  
  <pre><code>map.merge(key, msg, String::concat)
  </code></pre>
  
  <p>I understand how to use a lambda expressions to create anonymous functional interfaces, but this seems to use a method as a BiFunction. I like to understand and use obscure java syntaxes, but I can't find any mention of this one anywhere. </p>