JDK installation in Apple iPad Air4

Other than limited Swift support, iPadOS is not ideal for code development.

Within the Apple App Store, you may find something that may be suitable for your needs. Here is one example that supports Java - Pico:

https://apps.apple.com/gb/app/pico-compiler-java-ide-api/id992268206

pretty sure you can't

only way to install apps on a non jailbroken ipad is apples app store

if JDK does not show up in there when you search it's a no go