How to sanitize HTML code in Java to prevent XSS attacks?

You can try OWASP Java HTML Sanitizer. It is very simple to use.

PolicyFactory policy = new HtmlPolicyBuilder()
    .allowElements("a")
    .allowUrlProtocols("https")
    .allowAttributes("href").onElements("a")
    .requireRelNofollowOnLinks()
    .build();

String safeHTML = policy.sanitize(untrustedHTML);