There are multiple problems with this App, may be it must be split to multiple issues.

Following sources from info.magnolia.rest.client.app.ui.RestClientAppPresenter class

Problems

• It wont execute requests if there is no parameters - only request body. Exception: wrong argument count. 

    private Object[] parseParameters(Method method, String parametersAsString, String body) throws IOException {
        List<Object> parameters = new ArrayList<Object>();

        if (StringUtils.isNotEmpty(parametersAsString)) {
            ...
        }
    }

• It wont parse request body from json string to desired argument type (info.magnolia.rest.client.app.ui.RestClientAppPresenter#corvertValue)

• If argument is primitive ... Then it will send its hashCode() instead of actual value

    protected Object corvertValue(String input, Class clazz) throws IOException {
        ....
        if (clazz.isPrimitive()) {
            return convertedValue.hashCode(); //hashCode = primitive value
        }
        return convertedValue;
    }