code: private void writeInterface(final GeneratorContext context, final TreeLogger logger,
        final BeanHelper bean) {
      final PrintWriter pw = context.tryCreate(logger, bean.getPackage(), bean.getValidatorName());
      if (pw != null) {
        final TreeLogger interfaceLogger = logger.branch(TreeLogger.TRACE,
            "Creating the interface for " + bean.getFullyQualifiedValidatorName());
  
        final ClassSourceFileComposerFactory factory =
            new ClassSourceFileComposerFactory(bean.getPackage(), bean.getValidatorName());
        factory.addImplementedInterface(
            GwtSpecificValidator.class.getCanonicalName() + " <" + bean.getTypeCanonicalName() + ">");
        factory.addImport(GWT.class.getCanonicalName());
        factory.makeInterface();
        final SourceWriter sw = factory.createSourceWriter(context, pw);
  
        // static MyValidator INSTANCE = GWT.create(MyValidator.class);
        sw.print("static ");
        sw.print(bean.getValidatorName());
        sw.print(" INSTANCE = GWT.create(");
        sw.print(bean.getValidatorName());
        sw.println(".class);");
  
        sw.commit(interfaceLogger);
        pw.close();
      }
    }
docs: Write an Empty Interface implementing
  {@link de.knightsoftnet.validators.client.impl.AbstractGwtSpecificValidator} with Generic
  parameter of the bean type.