Add FAQ

FAQ.md

@@ -0,0 +1,29 @@
+# FAQ
+
+## What happens if I store a null value?
+
+Null values are stored normally. Listeners will be notified if the null differs from the previous value.
+
+## Can a listener remove itself?
+
+Yes, a listener can call `Ctx.INSTANCE.removeListener(...)` on itself if needed.
+
+## What if a listener throws an exception?
+
+Exceptions thrown in one listener will not stop other listeners from being notified, but you should handle exceptions inside the listener code.
+
+## Can I use primitive types?
+
+Yes, via their boxed equivalents, e.g. `Integer`, `Double`, etc.
+
+## How about arrays?
+
+Yes, arrays are supported and compared using `Objects.deepEquals`.
+
+## How to test with mocks?
+
+Use a mock of the `Listener` interface. The key-based typing makes it easy to isolate in unit tests.
+
+---
+
+[[Home]] | [[API Reference]] | [[Programming Guide]]