We can connect external java application using SOAP or JMS or IAC.
You have the Integration connectors like Connect-SOAP, Connect-REST, Connect-EJB and many other connectors.
Kindly look into the Integration-Connectors from Records explorer to check the already existing rules for the
Connecting an external Java application to Pega without using the Connect-Java rule can be achieved through various methods such as leveraging REST APIs, SOAP services, JMS (Java Messaging Service), and using Pega’s integration capabilities