Error:java: invalid source release: 8 in Intellij. What does it mean?

I had the same issue when "downgrading" a project from Java 8 to Java 6. The reason was that it was not changed at all places in IntelliJ.

In IntelliJ 13.1.4 I had to change Java and SDK version on the following places not to get this error:

• File -> Project Structure -> Project Settings
• File -> Project Structure -> Module Settings -> Tab: Sources: Language Level
• File -> Project Structure -> Module Settings -> Tab: Dependencies: Module SDK
• File -> Settings -> Compiler -> Java Compiler -> Target bytecode version

The last bullet was the one that was not updated in my case. Once I changed this, the error disappeared.

Check your pom.xml first (if you have one)
Check your module's JDK dependancy. Make sure that it is 1.8
To do this,go to Project Structure -> SDK's
Add the path to where you have stored 1.8 (jdk1.8.0_45.jdk in my case)
Apply the changes
Now, go to Project Structure ->Modules
Change the Module SDK to 1.8
Apply the changes

Voila! You're done

For Gradle users having this issues, if nothing above helps this is what solved my problem - apply this declarations in your build.gradle files:

targetCompatibility = 1.6 //or 1.7;1.8 and so on
sourceCompatibility = 1.6 //or 1.7;1.8 and so on

Problem solved!