How to Resolve Maven Compilation Error: Use -source 7 or Higher to Enable Diamond Operator

How can I resolve the Maven compilation error indicating that the diamond operator requires -source 7 or higher in Java?

<properties>
  <maven.compiler.source>1.8</maven.compiler.source>
  <maven.compiler.target>1.8</maven.compiler.target>
</properties>

If you're facing a Maven compilation error in IntelliJ that states, "use -source 7 or higher to enable diamond operator," it indicates that the Java compiler is set to an older version that doesn't support these features, namely Java 1.5. Here's how to resolve this issue.

<project xmlns="http://maven.apache.org/POM/4.0.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
  <modelVersion>4.0.0</modelVersion>
  <groupId>com.example</groupId>
  <artifactId>my-project</artifactId>
  <version>1.0-SNAPSHOT</version>
  <properties>
    <maven.compiler.source>1.8</maven.compiler.source>
    <maven.compiler.target>1.8</maven.compiler.target>
  </properties>
</project>

Causes

• The project is incorrectly configured to use an outdated Java version (1.5).
• The Maven compiler version needs to be explicitly set to match the Java 8 (1.8) requirement.

Solutions

• Ensure that the `maven.compiler.source` and `maven.compiler.target` properties in your `pom.xml` file are set to `1.8` to leverage Java 8 features like the diamond operator and try-with-resources.
• Check your IntelliJ project settings to confirm that the Project SDK and Language Level are set to JDK 1.8 (or higher).
• Run `mvn clean compile` after making changes to clear any cached data that may still reflect previous configurations.

Mistake: Not specifying the `maven.compiler.source` and `maven.compiler.target` properties in `pom.xml`.

Solution: Add the properties section in your POM file to match your desired Java version.

Mistake: Using the wrong project SDK version in IntelliJ.

Solution: Ensure that the project SDK is set to JDK 1.8 or higher in IntelliJ settings.

Mistake: Forgetting to clean the project after changing configurations.

Solution: Run `mvn clean compile` to re-compile the project with updated settings.