object-oriented-programming.md

Object Oriented Programming

Java is an object-oriented programming (OOP) language, which means that we design classes, objects and methods that hold certain data and can perform certain actions. This is important in the construction of larger and more complex Java programs.

Classes

The fundamental concept in object-oriented programming is the class. A class is a description of a set of data, and instructions for how that data should behave.

Java has a lot of pre-defined classes to use (String is one example of a class) but we can also define our own custom classes.

Defining a class

A class in Java is defined using the following syntax:

public class Car {

}

This creates a class named Car - although it doesn't contain any data or behaviour yet!

Instance variables

When creating a new class, there will be specific pieces of data that you want that class to hold. These are known as instance variables.

For instance, given a car, we might want to know how many doors it has and what colour it is. This can be done by defining instance variables like this:

public class Car {

    private int numberOfDoors;
    private String colour;

}

Constructors

Now that we've decided what data the Car class should contain, we need a way to actually create a Car and put some real data into it. In Java, this is done using a constructor.

A constructor is a special method on a class which contains a set of instructions for how to initialise a new instance of the class. Constructors take inputs which can be used as part of the initialisation.

Here's an example constructor for a Car which takes a number of doors and a colour as inputs, and sets the instance variables on Car to be equal to those given inputs:

public class Car {

    private int numberOfDoors;
    private String colour;

    public Car(int numberofDoors, String colour) {
        this.numberOfDoors = numberOfDoors;
        this.colour = colour;
    }
}

In Java, the name of the constructor must always be exactly equal to the name of the class - in this case, Car.

Instances

We now have a definition for what a Car should look like, and instructions for how to create a new one. We are now able to actually create an instance of the Car class as follows:

Car myCar = new Car(5, "silver");

This creates a variable (called myCar) which is of type Car. The new keyword is used to tell Java that we want a new Car and that we should use the constructor of the Car class to create it. Here, the inputs to our constructor were 5 and "silver". This would construct a car with 5 doors and which is silver-coloured.

Methods

So far we've decided what sort of data should be on the Car class, and worked out how to create instances of Car.

But classes do not only contain data - they also contain methods. A method is a pre-defined set of instructions that will be carried ot when you call the method.

We can add some methods to our Car class like this:

public class Car {

    private int numberOfDoors;
    private String colour;

    public Car(int numberofDoors, String colour) {
        this.numberOfDoors = numberOfDoors;
        this.colour = colour;
    }

    public void startEngine() {
        System.out.println("Vrooom!");
    }

    public int getNumberOfDoors() {
        return this.numberOfDoors;
    }

    public void paint(String colour) {
        this.colour = colour;
    }
}

A method definition consists of several parts:

1. An access modifier (usually public or private)
2. The type of object that the method returns
3. A name for the method
4. A list of inputs that the method needs, in parentheses
5. A block of code that will be executed when the method is called

The three methods we added to the Car class are broken down into their parts like this:

startEngine

1. Access modifier public
2. Returns nothing (this is known as void in Java)
3. Called startEngine
4. Takes no inputs
5. Prints "Vrooom!" to the console when called

getNumberOfDoors

1. Access modifier public
2. Returns an int
3. Called getNumberOfDoors
4. Takes no inputs
5. Returns the number of doors the car has when called

paint

1. Access modifier public
2. Returns nothing (void)
3. Called paint
4. Takes a single String input, called colour
5. Sets the colour of the car to be the input colour when called