Installing jdk11 and timer app

hi guys,
i installed jdk11 and a timer app and i dont know how to “activate” them. Im new to linux 8)

jdk11-openjdk from the repos or another source?

Which would be…?

the jdk11 file is from the repos and the timer app is called BreakTimer

From where? I found this one but it’s not a Java app: