Polymorphism

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Table of contents

1. Polymorphism
   1. Key Idea
   2. Motivation for Polymorphism
   3. Back to the drawing board
   4. Another Example
   5. Summary
2. Next Step

Key Idea

Polymorphism in Object Oriented Programming is the provision of a single interface to entities of different types.

Motivation for Polymorphism

From the fruit example in the previous section, it would be preferable if we could just store an array of fruits and call getDescription on each fruit. It would be great if the correct getDescription got called based on the type of fruit that was created, not the type of the array.

It turns out that this WORKS! For apples it will call the apple version of getDescription, and for oranges it will call the orange version.

class Fruit {
    constructor(private name: string) {}
    public getDescription(): string {
        return `This is a fruit called ${this.name}`;
    }
}

class Orange extends Fruit {
    constructor(protected subType: string) {
        super("orange");
    }
    public getDescription(): string {
        return super.getDescription() + " of type " + this.subType;
    }
}
class Apple extends Fruit {
    constructor(protected subType: string) {
        super("apple");
    }
    public getDescription(): string {
        return super.getDescription() + " of type " + this.subType;
    }
}
let fruits: Fruit[] = [
    new Apple("red"),
    new Apple("green"),
    new Orange("blood"),
    new Orange("navel"),
];
for (let fruit of fruits) {
    console.log(fruit.getDescription());
}

If either class did not implement getDescription(), then the superclass version would be called. This powerful behavior is a type of polymorphism and allows us to create very powerful class hierarchies, that are simple to access and use.

In other words, in our fruit example, we provided a public interface for all fruits that included the method getDescription(). Regardless of the type of fruit, the public interface does not change, and the language is able to dispatch the method call to the appropriate subclass for us automatically. This type of polymorphism is subclass or subtype polymorphism. There are other types of polymorphism including ad-hoc polymorphism and parametric polymorphism. We will examine parametric polymorphism later.

So with creative use of subclass polymorphism, we can provide a generic interface to all objects that share a base class, with a default behavior.

Back to the drawing board

Returning to the drawing example, if we added a draw method to the drawable class that does nothing, then implemented the draw method in each of our subclasses, then we could store a drawing as an array of Drawables, iterate through the array, and call the draw method. This is acceptable because Drawable has a draw method, but the correct draw method (depending on the type of object) will be called for us automatically. This is polymorphism:

class Drawable {
  public color: Color;
  constructor(color: Color) {
    this.color = color.clone();
  }
  clone(): Drawable {
    return new Drawable(this.color);
  }
  draw(page:Page): void {
    // Do nothing, I don't know how
  }
}

class Line extends Drawable {
   ...
  draw(page: Page): void {
    page.drawLine(
      this.start.getX(),
      this.start.getY(),
      this.end.getX(),
      this.end.getY(),
      this.color.toString(),
    );
  }
}
let obj:Drawable=new Line(new Point(0,0),new Point(1,1),
  new Color());
obj.draw(this.drawingSurface);

Note: You can install the drawing library used in this example with the page object using npm.
npm i --save @boots-edu/web-draw

It is safe to call draw on a Drawable object, it just doesn’t do anything. If we call it on a Line object, it draws the line. If we call it on a Line object stored in a Drawable variable (which is allowed since it is a Drawable), it calls the method in the Line class.

Another Example

Consider the following example. We would like the console.log expression to print the value of x, but the default behavior of Object.toString() is not what we really want.

  class myObj{
    x:number=42;
  }
  console.log(new myObj().toString());

To rectify this, we can override the toString() method to control how the conversion to a string takes place.

  class myObj extends Object{
    x:number=42;
    override toString():string{
      return this.x.toString();
    }
  }
  console.log(new myObj().toString());

Note: If you do not extend Object, you cannot use the override keyword, however this still works and provides the same behavior.

  class myObj{
    x:number=42;
    toString():string{
      return this.x.toString();
    }
  }
  console.log(new myObj().toString());

Summary

Polymorphism in general denotes the idea of several different types of objects having the same public interface. Specifically, in this section we examined subtype or subclass polymorphism which is when we override methods in a superclass allowing us to call the methods on a variable of the superclass type which contains an object of the subclass type. This causes the system to dispatch the call to the correct subclass.

Next Step

Next we’ll learn learn about abstract methods and classes: Abstract Classes »