Startup #

There should be tutoring now. I think they’re actually in this room.
Delay attendence until after the first example.

Record from Class: Car #

Simple Class

class Car {
    double tankSize; // gallons
    double mileage; // mpg
}

Problem: Those fields are mutable.

Why that’s a problem:

Unexpected mutation can create unexpected bugs.
Immutable structures are easier to reason about.
The bugs and reasoning considerations become much more significant in concurrent and distributed programs.

Add final to each field

Problem: Can’t say new Car(10.0, 20.0)

Add Constructor

    Car(tankSize, mileage) {
        this.tankSize = tankSize;
        this.mileage = mileage;
    }

Problem: No accessor methods.

Add accessors

Convention in Java says that a method should’t access fields on an instance of a different class. This allows interfaces to change, for example we might decide we want a maxRange to be a field and mileage to be calculated.

    double tankSize() { return this.tankSize; }
    double mileage() { return this.mileage; }

Problem: No equals, toString, or hashCode method

Add equals method

    boolean equals(Object obj) {
        if (obj instanceof Car) {
            Car that = (Car) obj;
            return this.tankSize == that.tankSize
                && this.mileage == that.mileage;
        }
        else {
           false; 
        }
    }
    
    // toString and hashCode are also free with a record

Take Attendence #

Nesting Dolls Example #

First, we’re going to have a doll wear a hat.

import image.*;
import world.*;

public class App {
    public static void main(String args[]) {
        var h0 = new Hat("blue");
        var d0 = new Doll("darkgreen", h0);
        var world0 = new DollWorld(d0);
        world0.bigBang();
    }
}

record Hat(String color) {
    Image draw() {
        return new Triangle(80, "solid", color);
    }
}

record Doll(String color, Hat hat) {
    Image draw() {
        // Template gives us color, hat
        // So we have access to all methods of hat
        return new Circle(80, "solid", color)
            .overlayxy(-40, 75, hat.draw());
    }
}

class DollWorld extends World {
    Doll doll;

    DollWorld(Doll d0) {
        this.doll = d0;
    }

    @Override
    public Scene onDraw() {
        var bg = new EmptyScene(800, 600);
        var fg = doll.draw();
        return bg.placeImage(fg, 400, 300);
    }
}

Let’s do nesting dolls.

record Doll(String color, Doll inner) {
    int depth() {
        if (inner == null) {
            return 1;
        }
        else {
            return 1 + inner.depth();
        }
    }

    Image draw() {
        // Template gives us color, inner
        var img = new Circle(20 * depth(), "solid", color);
        if (inner == null) {
            return img;
        }
        else {
            return img.overlay(inner.draw());
        }
    }
}

    public static void main(String[] args) {
        var d4 = new Doll("pink", null);
        var d3 = new Doll("blue", d4);
        var d2 = new Doll("yellow", d3);
        var d1 = new Doll("green", d2);
        var d0 = new Doll("orange", d1);
        var world0 = new DollWorld(d0);
        world0.bigBang();
    }

Overflow: #

Go back to Car and show toString for both a record and manually.