title: Java abstract interface
body: <p>Consider an example (which compiles in java)</p>
  
  <pre><code>public abstract interface Interface {
      public void interfacing();
      public abstract boolean interfacing(boolean really);
  }
  </code></pre>
  
  <p>Why is it necessary for an interface to be "declared" abstract? Is there other rules that applies with an abstract interface?</p>
  
  <hr>
  
  <p>Finally: If <code>abstract</code> is obsolete, why is it included in Java? Is there a history for abstract interface?</p>