An array is an object and it is an instance of a class which ought to document the length attribute. I don't think it's in the API docs at all, though. Arrays are one of those dirty little corners in the language where they can say "Everything works like this ... except array"
You can try to explore the class through reflection. This much works:
Look into the Class class for ways to explore the methods and attributes.
A good question is never answered. It is not a bolt to be tightened into place but a seed to be planted and to bear more seed toward the hope of greening the landscape of the idea. John Ciardi
The "length" property that arrays have in Java is not a real instance variable. It's just a special Java keyword. It's part of the Java language, not of the standard API, so it isn't documented in the API documentation.