You need to deploy your app for eclipse, open it with eclips. Now you can enter your java code. Make sure your code is between //BEGIN USER CODE and //END USER CODE, otherwise it will be overwritten. And super important (I say that because it happens to me all the time) SAVE your changes in eclipse.
Now you can run your application and should see the result.
Try to import your java action in mendix module and then call it in microflow.
rightclick on module[yourmodule]->add->resources->javaaction