Var Cannot Be Resloved To A Type

This is the error thrown at my face by LSP (jdtls) each time I used var to declare a new variable. Even though this keyword is added to Java since JDK10 and my code compiled with no issue, LSP never stooped complaining about it.
I got this error while I was using Java 17 and didn’t have any older JDK installed on my machine so this couldn’t be an issue about JDTLS using the wrong version of compiler.
And more interestingly this problem only exist for Maven projects and projects generated via Gradle were acting OK.

So what was the problem? Not specifying the source and target Java versions.
It doesn’t make total sense because it’s not clear what version is JDTLS assuming we’re using. So Java 8 features are supported but Java 10’s aren’t?

Anyways, the solution is specifying source and target java versions in the POM. For example you can set Java level for maven-compiler-plugin.
For this, just add below properties to your pom.xml:

<properties>
    <maven.compiler.source>17</maven.compiler.source>
    <maven.compiler.target>17</maven.compiler.target>
</properties>