code: protected <T> KeyTypeConfiguration<T> key( Class<T> type ) {
          if ( type.isPrimitive() ) {
              throw new IllegalArgumentException( "Primitive types cannot be used as a map's key" );
          }
          return new KeyTypeConfiguration<T>( type );
      }
docs: Return a {@link KeyTypeConfiguration} to configure key serializer and/or deserializer for the given type.
  
  @param type a {@link java.lang.Class} object.
  @param <T> Type
  @return a {@link com.github.nmorel.gwtjackson.client.AbstractConfiguration.KeyTypeConfiguration} object.