code: public static Class<?> unwrap(final Class<?> cls) {
          N.checkArgNotNull(cls, "cls");
  
          Class<?> unwrapped = PRIMITIVE_2_WRAPPER.getByValue(cls);
  
          return unwrapped == null ? cls : unwrapped;
      }
docs: Returns the corresponding primitive type of {@code type} if it is a wrapper type; otherwise
  returns {@code type} itself. Idempotent.
  
  <pre>
  unwrap(Integer.class) == int.class
  unwrap(int.class) == int.class
  unwrap(String.class) == String.class
  </pre>