How to Resolve 'Error: Java: Invalid Target Release: 11' in IntelliJ IDEA

How can I fix the 'Error: Java: invalid target release: 11' in IntelliJ IDEA after upgrading from Java 8 to Java 11?

// Not applicable for this issue.

The 'Error: Java: invalid target release: 11' in IntelliJ IDEA typically occurs when the project is configured to use an incompatible Java version. This issue arises especially when a project initially built with Java 8 is upgraded to Java 11, but the environment setup is incorrect or incomplete.

// Setting up Project SDK in IntelliJ IDEA
// Ensure the following step is taken to configure JDK properly in IntelliJ
// 1. Go to File > Project Structure > Project.  
// 2. Choose 'Project SDK' and select JDK 11 from the dropdown list.

Causes

• Mismatched Java SDK configuration in IntelliJ IDEA and system environment variables.
• Incorrect project structure settings, specifically targeting bytecode versions.
• Installation of Java 11 may not have been done properly, leading to IntelliJ not recognizing it.

Solutions

• Verify that the project JDK is set to JDK 11 in IntelliJ IDEA. Go to 'File' > 'Project Structure' > 'Project', and ensure the Project SDK is JDK 11.
• Clear and reset any Java version settings in 'File' > 'Project Structure' > 'SDKs'. Ensure to delete previous SDK entries that may conflict (like JDK 7 or 8).
• Reinstall JDK 11 using the official Oracle installer to ensure the installation is valid and complete. Ensure that both JDK and JRE are installed.
• Check the 'Targets bytecode version' under 'File' > 'Project Structure' > 'Modules'. Ensure that it is set to 11 for all modules.

Mistake: Not setting the correct Project SDK to JDK 11 after upgrade.

Solution: Check and update the Project SDK from 'File' > 'Project Structure' > 'Project'.

Mistake: Having remnants of older JDK versions or incorrect paths.

Solution: Remove older JDK paths from the system environment variables and ensure they point only to JDK 11.

Mistake: Inconsistent module settings related to Java Compiler.

Solution: Go to 'File' > 'Project Structure' > 'Modules' and double-check each module's bytecode version settings.