title: How to write a matcher that is not equal to something
body: <p>I am trying to create a mock for a call.
  Say I have this method I am trying to stub out:</p>
  
  <pre><code>class ClassA {
    public String getString(String a) {
      return a + "hey";
    }
  }
  </code></pre>
  
  <p>What I am trying to mock out is:
  1st instance is</p>
  
  <pre><code>when(classA.getString(eq("a")).thenReturn(...);`
  </code></pre>
  
  <p>in the same test case</p>
  
  <pre><code>when(classA.getString([anything that is not a])).thenReturn(somethingelse);
  </code></pre>
  
  <p>The 2nd case is my question: How do I match <code>anyString()</code> other than "a"?</p>