title: Why are my classes marked in red in IntelliJ IDEA
body: <p>I have some Java classes which are marked in red in project view, but IntelliJ IDEA shows no errors and it can run properly. What can be the problem?</p>
  <p><a href="https://i.stack.imgur.com/D9ryX.png" rel="noreferrer"><img src="https://i.stack.imgur.com/D9ryX.png" alt="screenshot" /></a></p>