Why are the data members of a class usually declared private?

Making the data private stops any code outside the class from changing it directly, so the class alone decides how its state may change and can validate every change in its methods. This keeps the object in a consistent state and lets the internal representation be changed later without breaking the code that uses the class.

No citable source has been recorded for this record. Treat it as practice material, not as fact.

Explanation

The methods that read and write private fields are usually called accessors and mutators.