Intro to Java

Classes & Object Oriented Programming

At a glance

• Understand the purpose of classes, objects and object-oriented programming
• Understand and utilize Java access modifiers
• Recognize the use cases of getters and setters
• Write your own Java classes

These are more advanced topics usually taught in Computer Science 30. If you don't retain some information that's okay!

You've learnt that repetitive sequences of code can be reused as methods.

But even with methods... our code files can still become very hard to navigate.

And even with methods... our code can still deal with LOTS of different unrelated logic

Robot-specific examples: a single code file could contain logic for:

• getting input from gamepads
• writing information to telemetry
• controlling many different motors, servos, etc.

That's where classes come in

Classes are a collection of data (variables) and actions that can modify that data (methods). We can create new classes in different files and have them be imported to be used in our main logic!

Triangle.java
public class Triangle {
    int base = 2; // <-- we call base and height, "attributes"
    int height = 3;

    public double getArea(){
        return base * height / 2;
    }
}

This allows us to group similar data into a common class

Classes are written using the "class" keyword followed by a name and a set of { } brackets

We can then instantiate these classes into objects

public class Main {
    public static void main(String[] args){
        Triangle t = new Triangle(); // <-- The "new" keyword instantiates a new object
        t.getArea() // Returns 3.0!
    }
}

By using the "new" keyword we can create new objects of a specified type. Think of this as a method returning a value that is your created object

But what is this "method"?

It is called a constructor.

public class Triangle {
    int base;
    int height;

    public Triangle(int b, int h) {
        base = b;
        height = h;
    }
}

Constructors are different

• They do not have an explicit return type
• Have the same name as the class it is creating

But also similar

• You can change their access modifier. (Though you will usually want to keep them public)
• You can pass parameters in for when you create an object

public class Triangle {
    int base;
    int height;

    public Triangle(int b, int h) {
        base = b;
        height = h;
    }
}

public class Main {
    public static void main(String[] args){
        // Creates a new triangle with base 3 and height 5
        // and assigns it to the variable, "t"
        Triangle t = new Triangle(3, 5);
    }
}

We use classes to group our data but we also want to protect it!

public class Triangle {
    public int base;
    public int height;

    ...
}

Currently anyone can change the values of base and height on our triangles.

Which is something we may not want!

We can use access modifiers to hide our data, then write methods called getters and setters to change values as we need.

In object-oriented programming we call this encapsulation.

public and private are the two main access modifiers (but there are more).

• public allows an attribute/method to be modified/used by anyone

• private only allows the object iself to modify/use/access the attribute/method

And if we want others access to these private attributes we use getters and setters.

public class Triangle {
    private int base; // <-- our private attributes
    private int height;

    public class Triangle(int b, int h){
        base = b;
        height = h;
    }

    public int getBase(){
        return base;
    }

    public int getHeight(){
        return height;
    }

    public void setHeight(int newHeight){
        if(newHeight > 0) { // <-- This allows allows you to validate the data
            height = newHeight
        }
    }
}

Altogether our new encapsulated triangle class looks like this:

public class Triangle {
    // Attributes
    private int base, height;
    
    // Constructor
    public Triangle(int b, int h){
        base = b;
        height = h;
    }

    // Getters
    public int getBase(){
        return base;
    }

    public int getHeight(){
        return height;
    }

    // Methods
    public int getArea(){
        return base * height / 2;
    }
}