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Learning Exercise
The primary object-oriented construct in Java is the class, which is a combination of data (fields) and behavior (methods). The fields and methods of a class are known as its members.
Access to members can be controlled through access modifiers, the two most common ones being:
public: the member can be accessed by any code (no restrictions).private: the member can only be accessed by code in the same class.You can think of a class as a template for creating instances of that class.
To create an instance of a class (also known as an object), the new keyword is used:
class Car {
}
// Create two car instances
Car myCar = new Car();
Car yourCar = new Car();
Fields have a type and a name (defined in camelCase) and can be defined anywhere in a class (by convention cased in PascalCase):
class Car {
// Accessible by anyone
public int weight;
// Only accessible by code in this class
private String color;
}
One can optionally assign an initial value to a field. If a field does not specify an initial value, it will be set to its type's default value. An instance's field values can be accessed and updated using dot notation.
class Car {
// Will be set to specified value
public int weight = 2500;
// Will be set to default value (0)
public int year;
}
Car newCar = new Car();
newCar.weight; // => 2500
newCar.year; // => 0
// Update value of the field
newCar.year = 2018;
Private fields are usually updated as a side effect of calling a method.
Such methods usually don't return any value, in which case the return type should be void:
class CarImporter {
private int carsImported;
public void importCars(int numberOfCars)
{
// Update private field from public method
carsImported = carsImported + numberOfCars;
}
}
In this exercise you'll be playing around with a remote controlled car, which you've finally saved enough money for to buy.
Cars start with full (100%) batteries. Each time you drive the car using the remote control, it covers 20 meters and drains one percent of the battery.
The remote controlled car has a fancy LED display that shows two bits of information:
"Driven <METERS> meters"."Battery at <PERCENTAGE>%".If the battery is at 0%, you can't drive the car anymore and the battery display will show "Battery empty".
You have six tasks, each of which will work with remote controlled car instances.
Implement the (static) ElonsToyCar.buy() method to return a brand-new remote controlled car instance:
ElonsToyCar car = ElonsToyCar.buy();
Implement the ElonsToyCar.distanceDisplay() method to return the distance as displayed on the LED display:
ElonsToyCar car = ElonsToyCar.buy();
car.distanceDisplay();
// => "Driven 0 meters"
Implement the ElonsToyCar.batteryDisplay() method to return the battery percentage as displayed on the LED display:
ElonsToyCar car = ElonsToyCar.buy();
car.batteryDisplay();
// => "Battery at 100%"
Implement the ElonsToyCar.drive() method that updates the number of meters driven:
ElonsToyCar car = ElonsToyCar.buy();
car.drive();
car.drive();
car.distanceDisplay();
// => "Driven 40 meters"
Update the ElonsToyCar.drive() method to update the battery percentage:
ElonsToyCar car = ElonsToyCar.buy();
car.drive();
car.drive();
car.batteryDisplay();
// => "Battery at 98%"
Update the ElonsToyCar.drive() method to not increase the distance driven nor decrease the battery percentage when the battery is drained (at 0%):
ElonsToyCar car = ElonsToyCar.buy();
// Drain the battery
// ...
car.distanceDisplay();
// => "Driven 2000 meters"
car.batteryDisplay();
// => "Battery empty"
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