Many answers regarding Maven are right but you don't have to configure the plugin directly.
Like described on the wiki page of the Apache Maven Compiler Plugin you can just set the 2 properties used by the plugin.
<project>
[...]
<properties>
<maven.compiler.source>1.8</maven.compiler.source>
<maven.compiler.target>1.8</maven.compiler.target>
</properties>
[...]
</project>