Showing buttons based on a message received by arduino

Indeed the boolean primitive datatype can be either true or false:

boolean received; // defaults to false

For a 3rd state you can use a Boolean type instead, which can be null in addition to true or false:
Docs.Oracle.com/en/java/javase/11/docs/api/java.base/java/lang/Boolean.html
Boolean received; // defaults to null

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