Kinds of data do we have so far

Primitive data types:

• Single values
• Not instances of classes / records (not created with “new”)

Objects:

• Composite values, with potentially multiple fields.
• A Car(tankSize, mileage) is (a double) AND (a double)
• A Doll(color, hat) is (a String) AND (a Hat)
• Fields can be primitive values or objects.
• Can be nested or even recursive.

New thing - Interfaces:

• What if we want to represent one of several different options?
• A Toy is (a Doll) or (a Puzzle)
• In Python, we have dynamic types, so we can just do this.
• In some languages there are “union types”.
• In Java, we’ve got two options: Inheritence and Interfaces
• Inheritence and Interfaces are very similar, but Interfaces are a bit more useful, so we’ll focus on those.

Starter Code

public class App {

    public static void main(String[] args) {
        var c1 = new Circle(10);
        var c2 = new Circle(12);

        printBigger(c1, c2);
    }

    static void printBigger(Circle xx, Circle yy) {
        if (xx.area() > yy.area()) {
            System.out.println("Bigger is: " + xx);
        }
        else {
            System.out.println("Bigger is: " + yy);
        }
    }
}

record Circle(double radius) {
    double area() {
        return Math.PI * Math.pow(radius, 2);
    }
}

Let’s make this more complicated:

record Square(double width) {
    double area() {
        return Math.power(width, 2);
    }
}

// Now let's compare a Square to a Circle...
    public static void main(String[] args) {
        var c1 = new Circle(10);
        var c2 = new Circle(12);
        var s1 = new Square(10);

        printBigger(c1, s1);
    }

That… doesn’t compile. Because the method doesn’t take Squares. Let’s fix that:

interface Shape {
    double area();
}

record Circle(double radius) implements Shape {
    public double area() {
        return Math.PI * Math.pow(radius, 2);
    }
}

record Square(double width) implements Shape {
    public double area() {
        return Math.pow(width, 2);
    }
}

Attendence #

Stuff from Lab01 #

Core Strategy For Assignments

• Expect to get stuck at least once
• Plan to get stuck soon enough to have time to get unstuck - optimally get stuck in lab on Tuesday.
• Make sure your program builds and runs. If the way you’re stuck is that the program won’t compile and you don’t know why, start over.
• Use help resources: Tutoring and my office hours.

Let’s Solve It

• Download Lab01
• Using 7zip, inspect the archive graphically.
• Unpack it.
• Run ‘mvn verify -q’
• Run the Perl script
• Run mvn verify
• Go look at the tests
• In the tests, match up literals to variables.
• Put the resulting formulas in Rocket.java
• Run the tests again.
• Pack up the archive.
• Using 7zip, inspect the new archive graphically.
  • Note that the new structure is the same as the old structure.
• Submit to Inkfish

Overflow: Drawable interface example #

• Doll, Hat, and Robot