How to write a matcher that is not equal to something

I am trying to create a mock for a call. Say I have this method I am trying to stub out:

class ClassA {
  public String getString(String a) {
    return a + "hey";
  }
}

What I am trying to mock out is: 1st instance is

when(classA.getString(eq("a")).thenReturn(...);`

in the same test case

when(classA.getString([anything that is not a])).thenReturn(somethingelse);

The 2nd case is my question: How do I match anyString() other than "a"?

With Mockito framework, you can use [AdditionalMatchers][1]

ClassA classA = Mockito.mock(ClassA.class);
Mockito.when(classA.getString(Matchers.eq("a"))).thenReturn("something"); 
Mockito.when(classA.getString(AdditionalMatchers.not(Matchers.eq("a")))).thenReturn("something else");

Hope it helps. [1]: https://static.javadoc.io/org.mockito/mockito-core/2.23.0/org/mockito/AdditionalMatchers.html

Use argThat with Hamcrest:

when(classA.getString(argThat(CoreMatchers.not(CoreMatchers.equalTo("a")))...

You might also be able to do this via ordering. If you put one when(anyString) and when(eq("a")) in the correct order, Mockito should test them in order and do the "a" logic when appropriate and then "anyString" logic otherwise.

In mockito the last stubbing is the most important. This means that you can simply use the standard matchers for your needs:

// "Default" return values.
when(classA.getString(ArgumentMatchers.anyString())).thenReturn(somethingelse);
// Specific return value for "a"
when(classA.getString(ArgumentMatchers.eq("a"))).thenReturn(something);

Note that you have to use ArgumentMatchers for both since you're mixing them.

I actually took this approach after carefully looking at the suggested answers:

doAnswer(new Answer<String>() {
  public String answer(InvocationOnMock invocation) throws Throwable {
    String originalParam = (String) invocation.getArguments()[0];
    return StringUtils.equalsIgnoreCase(originalParam, "a") ? "Something" : "Something Else";
  }
}).when(classA).getString(anyString());

This allows me to handle more than just two cases by adjusting the return base on the params.