Creating Basic Data Classes

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Table of contents

1. Creating Basic Data Classes
   1. Key Idea
   2. Drawing Program Classes
      1. Color class
      2. Point class
   3. Summary
2. Next Step

Key Idea

Data Classes allow us to combine data into a grouping and use that grouping as a data type in our programs.

Drawing Program Classes

Color class

If we examine the objects we have proposed for our drawing program (points, lines, rectangles, polygons, color) we can see that just about everything has a color. The definition for a type that represents color would be useful as then we could group the things that make up a color. For our example we want to store a color as three numbers between 0 and 255 representing the red, green, and blue intensities.

Our class should contain 3 numbers (Red, Green, and Blue). We can define our class as described previously, since this contains only the primitive type number.

class Color {
    public red: number = 0;
    public green: number = 0;
    public blue: number = 0;
}

Note the public keyword before each member variable (sometimes called a property) of the class. This denotes that the property is accessible by methods and code outside the class. We could also mark it as private or protected.

As you can see, our class definition is quite simple. We simply group the three components together and give it a name. We can then create objects of this type with the new keyword.

let myColor: Color = new Color();

And for a full example:

class Color {
    public red: number = 0;
    public green: number = 0;
    public blue: number = 0;
}
//We can use our new class to create a color object:
let myColor: Color = new Color();
//Then we can access its public members
myColor.red = 255;
myColor.blue = 128;
myColor.green = 10;
console.log(myColor);

Note: If red, green, and blue had been labeled private, then we could not have accessed them. More on this later in the text.

Point class

A point requires coordinates, x and y. These are both numbers. It also requires a color if we want points to be displayable (more on this later). We already have a definition for a color, so we can use that to define a point.

class Color {
    public red: number = 0;
    public green: number = 0;
    public blue: number = 0;
}
class Point {
    public x: number = 0;
    public y: number = 0;
    public color: Color = new Color();
}
let myPoint: Point = new Point();
myPoint.x = 100;
myPoint.y = 100;
myPoint.color.red = 255;
myPoint.color.blue = 128;
myPoint.color.green = 10;
console.log(myPoint);

Notice that we use the class Color inside of the class Point. This is referred to as composition and is a critical concept in understanding classes and Object Oriented Programming.

We can build up complex objects by including other objects inside of them. Now every point will have a position (x, y) and a color contained inside the point itself.

Summary

Complex objects can be built from simpler ones by creating a class to represent a new type.

Next Step

Next we’ll learn about constructors: Data Class Constructors »