how to test for nested classes?

I just start with this @test, someone could help me with just one example for the next case, because within the coverage also asks me to cover this class, and try several things but not. THANK YOU

public class MappersVars{

 private MappersVars(){}

 public static class A{
    public static final String NUEVO_EJEMPLO="NUEVO_EJEMPLO";
    public static final String HOLA="HOLA";
    ...
 }

  public static class B{
    public static final String SALUDO="SALUDO";
    public static final String VALOR="VALOR";
    ...
 }
}