Q:

'Error: java: cannot access java.util.function.Function' - while trying to use WebDriverWait

I'm using Maven and Intellij, project set for jdk-12.0.2, maven in pom 1.8 and when trying to run this code :
WebDriverWait wait = new WebDriverWait(driver, 5);
WebElement el =  wait.until(ExpectedConditions.elementToBeClickable(By.linkText("https://poczta.wp.pl")));

I get :

Error:(37, 30) java: cannot access java.util.function.Function   class
  file for java.util.function.Function not found

Pointing to 'wait'
Tried changing maven in pom from 1.7 to 1.8. > Invalidate cache/Restart

A:

Given you have maven tag my expectation is that your project is being managed by Maven hence built by Maven Compiler Plugin so your IDE language level settings might derive from what is defined in the pom.xml 
I would recommend setting project language level in:

Maven pom.xml file like:
<properties>
    <maven.compiler.source>1.8</maven.compiler.source>
    <maven.compiler.target>1.8</maven.compiler.target>
</properties>    

In Idea on Project Properties page you can choose the Language Level for each module

More information:

Selenium Webdriver Tutorial with JAVA: How to set up a TestNG maven based project from scratch
stop IntelliJ IDEA to switch java language level every time the pom is reloaded (or change the default project language level)

