title: IntFunction<String> and Function<Integer, String>
body: <p>I have following two simple code:</p>
  
  <pre><code>IntFunction&lt;String&gt; f1 = Integer::toString;
  Function&lt;Integer, String&gt; f2 = Integer::toString;
  </code></pre>
  
  <p>I thought that both definitions are correct and equivalently the same thing, but the second one has compiling errors, complaining that <code>Required Function&lt;Integer, String&gt;,but Method Reference is found.</code></p>