BuildPath Errors in Eclipse

Question

I've been using Eclipse IDE for... nearly a month now and one day all my projects had suddenly an error sign over them. I've already tried to assign in the Library tab from Build Path the JDK I already have (the jdk.11.0.2, or at least that's the one it recognizes) and it still popped up 30 error messages. Mainly these two: The project cannot be built until build path errors are resolved and The project was not built due to "release 15 is not found in the system". Fix the problem, then try refreshing this project and building it since it may be inconsistent.

I'm using the latest version of Eclipse IDE.

I've tried to clean it, it didn't work. I've tried to search on Stack Overflow but I found answers I didn't understand. I've downloaded a JDK package from here (https://jdk.java.net/15/), the Windows version if it matters, but I realized I do not actually know how to actually install it. If someone would be so kind to guide me a little I would be grateful. Thank you for considering my question.

Answer 1

I had this problem with an auto update.

Had to go to my project properties and change the JDK Compliance level.