Java Service can be accessed from within your app using Variables. These variables allow one to trigger the methods within the Java Service, pass the necessary parameters to the method and access the results from the method.
For each method within the Java Service, a service variable can be generated using the appropriate option from the Operation property; with the input parameters available within the Data tab and the output available as the datavalue for binding.
You can invoke the Java Service by triggering a call to the Service Variable generated. In case you have multiple public class methods in your Java Service, you will see multiple Operation options.