I had the similar problem. It was a maven project with the following snippet of pom.xml.
<build> <plugins> <plugin> <groupId>org.apache.maven.plugins</groupId> <artifactId>maven-compiler-plugin</artifactId> <version>3.8.0</version> <configuration> <release>9</release> </configuration> </plugin> </plugins> </build>
I had to change the following.
<build> <plugins> <plugin> <groupId>org.apache.maven.plugins</groupId> <artifactId>maven-compiler-plugin</artifactId> <version>3.8.0</version> <configuration> <release>11</release> </configuration> </plugin> </plugins> </build>
If you have already installed JDK 11 and working with java 9 or java 10 as maven compiler, eclipse can not detect. Hence change the release to 11 or the actual installed version of JDK.