@Anonymous Thanks for reaching out. Depending upon your requirement you can create any of supported trigger to trigger your function app and your function app code will make the HTTP calls. You can simply leverage the same code to call any endpoint using REST in Java. In case if there are any dependencies on Third party libraries then you can refer to this section.
You can leverage the HTTPRequest client and build your request as per your need.
Sharing previous discussion on restful call in Java and third-party document for your reference. Please update and make the necessary changes as per your needs.
https://stackoverflow.com/questions/3913502/restful-call-in-java
https://www.ibm.com/docs/en/rtw/9.0.1?topic=services-example-calling-rest-api-java-http