如何处理实体管理器无法找到对象实例案例？

I have sceneario in which I have to check if an onject instance exist in entity manager. It works fine when the instance eexists but throws null pointer exception when it doesn't. When it doesn't I have to do another thing. so how can catch this state? I already tried try catch, but it doesn't work.I have sceneario in which I have to check if an