If you are a developer wanting to see the old J2ME WhatsApp client for research:
For developers interested in building similar applications, here are some technical details about WhatsApp's Java architecture: Whatsapp java j2me
Java version of WhatsApp client is now available for old phones If you are a developer wanting to see
While the official for Java (J2ME) was discontinued years ago, several independent developers have recently released custom J2ME clients and tools to bring basic functionality back to vintage mobile phones. Modern J2ME WhatsApp Clients Whatsapp java j2me