Storing POJO using Offline

SmartClient Version: v10.0p_2015-06-10/PowerEdition Deployment (built 2015-06-10)
Browser Version : IE 11.0.9600.17843


The Offline functionality is not supporting to get the Java object stored in the Offline browser.

TestObject test = new TestObject();

Offline.put("testKey",test );

TestObject testBrow = (TestObject )Offline.get("testKey");
-- This line returning ClassCast Exception..

Is that Offline only support String to store?

Thanks