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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:

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

Any help is appreciated.

NightShade
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    2020-12 has built in java 15 support, the marketplace code is only for 2020-09 – greg-449 Feb 02 '21 at 08:49
  • Weird then that it did not recognize Java 15 when I upgraded to 2020-12 – NightShade Feb 02 '21 at 09:15
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    The two items with a version ending with `_JAVA15_PATCH` should not be there (select it and click _Uninstall..._) and the item _Eclipse Java Development Tools_ is too old (June 2019 instead of December 2020) which causes this issue. In _Window > Preferences: Install/Update > Available Software Sites_ add the update site [`http://download.eclipse.org/releases/latest`](http://download.eclipse.org/releases/latest) and then do _Help > Check for Updates_. – howlger Feb 02 '21 at 09:51
  • Looks like you have a mixture of code from different releases - not a good idea. – greg-449 Feb 02 '21 at 10:59
  • @howlger doing as you said made it work! The old Eclipse Java Development Tools was the issue. You may want to add that as answer. Thank you both! – NightShade Feb 02 '21 at 11:40

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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
howlger
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