title: Java add() method convention
body: <p>So, I'm normally a ruby programmer, so my grasp of Java conventions is shaky at best.  If I have a class A and want to define a method to add two instances of that class, what's the convention on the behaviour and return type?</p>
  
  <pre><code>public class A
  {
      //...
      public NotSureWhatTypeItShouldReturn add (A that) { /* ... */ }
  </code></pre>
  
  <p>Should I </p>
  
  <ul>
  <li>return a boolean indicating success and modify the target, or</li>
  <li>return a modified copy of the target and throw an exception on error</li>
  </ul>
  
  <p>Which fits with the normal Java convention for this kind of method?</p>