Eclipse Java 15 support not working, compiler does not show correct version

Question

I have followed the steps in https://marketplace.eclipse.org/content/java-15-support-eclipse-2020-09-417 to install the Java 15 support for Eclipse after previously upgrading my Eclipse to 2020-12 (4.18.0). However it doesn't work (compiler not showing version above 12), and build path does not recognize a JDK 15 when added manually:

I wonder if the small red plus icon next to the Java 15 plugin in the third screenshot means anything.

Any help is appreciated.

Answer 1

In your last screenshot, the two items whose version ends with _JAVA15_PATCH should not be there and the Eclipse Java Development Tools is too old (according to the version number ...v201906... , released June 2019 instead of December 2020) to support Java up to version 15.

To get Java 15 support, update the Java Development Tools (JDT):

1. In the Eclipse IDE Installation Details dialog select the two items whose version ends with _JAVA15_PATCH and click Uninstall... (they might prevent updating JDT)
2. In Window > Preferences: Install/Update > Available Software Sites add the update site http://download.eclipse.org/releases/latest if it not yet exists
3. Run Help > Check for Updates to update JDT