Question

so i was trying to understand the interfaces, but i almost only see articles that explains “how” to use the interface, my problem is to understand the “why” :
so it’s better to use Interface than creating and subclassing a class, which might be useless,
so we implement the interface methods in the class, but i don’t understand why this is a good thing,

Let’s say :
a class like Car.java defines all the code to make the car
we create the interface Working.java with several methods like start(), stop(), etc.
we implement the methods in Diesel_Car.java, Electric_Car.java, etc.
so what does it change for Car.java? This might not be the best example, as it seems that Car should be the parent of Diesel_Car.java etc,
but what was the meaning to implement the methods in those classes?
Is there a method in Car.java that somehow “calls” the Diesel_Car.java class and its interface methods?

I’ve read that the interface is like a “Contract”, but i only see the second part of this contract (where the method is implemented) and i’m having some trouble to imagine where the first part happen?

Thanks for your help

Answer 1

Lets take your example of a Base class of Car with Electric_Car and Diesel_Car Subclasses, and expand the model a bit.

Car may have the following Interfaces

1. Working : with start() & stop() methods
2. Moving : with move(), turn() & stop() methods

The Car might contain an instance of class AirConditioner which should also implement the interface Working.

The Driver object can interact with objects than implement Working , the driver can start() or stop() . (The driver can start or stop the car and the A/C seperatly).

also, since the Driver can walk around on his own (and does not always need a car) he should implement the interface Moving.
The object Ground can now interact with anything that implements Moving : either car or driver.