IntelliJ gives Fatal Error: Unable to find package java.lang in classpath or bootclasspath

Somehow the JDK language jars were removed from the classpath. All I had to do was reapply the JSDK home path.

Goto: File -> Project Structure -> Platform Settings -> SDKs Re-apply the JSDK home path.

Doing this added about 15 jars to the classpath. Apparently these are important for compiling.

File -> Project Structure -> Platform Settings -> SDKs, remove existing SDK and add it again.

Works for me in IntelliJ 2017.3.

This error happend to me after i've updated my JDK version to jdk1.8.0_161 -
But the project's sdk was still pointing to the old jdk 1.8.0_131

(Seems that updgrading java causing the previous version to be wiped).

Do this:

1. In the IntelliJ IDE go to File > Project Structure, from there:

2. Within Platform Settings > SDKs:
   Make sure that the JDK Home path field is pointing to a valid JDK folder (add a new entry if needed e.g. jdk1.8.0_161).

3. Within Project Settings > Modules:
   Make sure that the Module SDK is set to the same entry you picked in the Platform Settings > SDKs

4. Save & Build again.