Challenge 2: A Package-Private Field, Explained — Possible Solution
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Sensor.java:

package readings;

public class Sensor {
    double lastValue; // no modifier -- package-private

    // Accessible from:
    //   - any other class inside the "readings" package, e.g. a
    //     SensorLogger class also declared "package readings;"
    //   - NOT accessible from a class in a different package, even
    //     a subclass of Sensor, unless that subclass is itself also
    //     in the "readings" package
    //
    // Compared to the alternatives on this exact field:
    //   - private double lastValue;   -> only Sensor itself could
    //     read/write it, not even another class in "readings"
    //   - public double lastValue;    -> any class anywhere, in any
    //     package, importing Sensor, could read/write it directly
}

WHY THIS WORKS AS AN ANSWER
------------------------------
This states the real, exact boundary package-private draws (same
package only, regardless of subclassing) and contrasts it directly
against what private and public would each allow on the identical
field, matching the chapter's own compare-table row for the
no-modifier default.