Question

Strangely, instance variable brand is private scope, yet accessible the “public” way inside of method compareTo.

public class Car implements Comparable<Car> {
    private String brand;

    public String getBrand() {
        return brand;
    }

    public void setBrand(String brand) {
        this.brand = brand;
    }

    public int compareTo(Car o) {
        return this.brand.compareTo(o.brand);
    }
}

Answer 1

Class variable brand is private to other classes not the class Car itself.

for instance if you try

class Foo
{
     Foo()
     {
          Car car = new Car();
          string brand = car.brand; // <-- will not compile; 
                                    // should use car.getBrand()
     }
}