CS 121Week 13 - Wednesday

Last time

What did we talk about last time? Color representationColor classPicture class

Questions?

Project 5

Mirroring

Mirroring an image horizontally Straightforward idea Flip an image around the y-axis Maybe you want to decipher some of

Leonardo’s writings No, the other one

How to perform a horizontal mirror

Given an image with width w and height h:

Moving from left to right in the original image, copy each column, storing each column from right to left in the new image

Horizontal mirror example

0 1 2

0 A

1 B

2 C

3 D

0 1 2

0 A

1 B

2 C

3 D

Original Mirrored

Horizontal mirror in code

What would the code for mirroring look like?

Picture picture = new Picture( file ); //the picture to be mirrored

Picture mirrored = new Picture( picture.width(),picture.height() );

for( int i = 0; i < picture.width(); i++ )for( int j = 0; j < picture.height(); j++ )

mirrored.set( picture.width() - i - 1, j, picture.get( i, j ) );

Mirroring vertically

Pretty much the same thing Instead of copying each column in

reverse order, copy each row in reverse order

Rotation

Rotating an image

Another straightforward idea Necessary if you are writing software for

a digital camera (the user might turn the camera for portrait instead of landscape)

How to perform a 90° right rotation

Given an image with width w and height h: Create a new image with width h and

height w Copy each column in the original image

into a row in the new image, remembering to move back along the row

Right rotation example

0 1 2

0 A

1 B

2 C

3 D

0 1 2 3

0 D C B A

1

2

OriginalRotated

Rotation in code

What would the code for a rotation look like?

Picture picture = new Picture( file ); //the picture to be rotated

Picture rotated = new Picture( picture.height(), picture.width() );

for( int i = 0; i < picture.width(); i++ )for( int j = 0; j < picture.height(); j++ )

rotated.set( picture.height() - j - 1, i, picture.get( i, j ) );

Rotating other amounts

Rotating 180°, 270°, -90°, -180°, or -270° can be done using similar techniques or simply performing right rotations multiple times

Rotations that are not multiples of 90° are much trickier, result in non-rectangular final images, and need some trigonometry

Don’t worry about rotations that are not multiples of 90°

Resizing

Resizing an image

You’ve seen this many times before Sometimes an image won’t fit nicely

in on a PowerPoint slide

How to perform a resize

Let's just focus on growing, because that's what you need to do in your project

If we are just doubling the image, we create a new image whose width and height are twice the original

Then, we go through the new image, copying the pixels from the original using pixels whose column and row are half as big as the ones we are putting into the new image

Resizing example

Doubling an image Each old pixels maps

to four new ones

0 1 2 3

0 A A B B

1 A A B B

2 C C D D

3 C C D D

0 1

0 A B

1 C D

Resizing issues

This kind of resize works, but is crude To shrinking an image, a clever resize might

average together pixels When growing an image, different

techniques can be done to fill in “guesses” for pixels that sit between pixels from the original image, instead of just duplicating them

People who program Photoshop have thought long and hard about how to do these tasks better

Concept of Inheritance

Inheritance

The idea of inheritance is to take one class and generate a child class

This child class has everything that the parent class has (members and methods)

But, you can also add more functionality to the child

The child can be considered to be a specialized version of the parent

Code reuse

The key idea behind inheritance is safe code reuse

You can use old code that was designed to, say, sort lists of Vehicles, and apply that code to lists of Cars

All that you have to do is make sure that Car is a subclass (or child class) of Vehicle

Subclass relationship

Java respects the subclass relationship

If you have a Vehicle reference, you can store a Car object in that reference

A subclass (in this case a Car) is a more specific version of the superclass (Vehicle)

For this reason, you can use a Car anywhere you can use a Vehicle

You cannot use a Vehicle anywhere you would use a Car

Subclass example

As long as Car is a subclass of Vehicle, we can store a Car in a Vehicle reference

Even in an array is fine

Storing a Vehicle into a Car doesn’t work

Vehicle v = new Car("Lancer Evolution");// perfectly okay

Vehicle[] vehicles = new Vehicle[100];for( int i = 0; i < vehicles.length; i++ )vehicles[i] = new RocketShip(); // cool

Car c = new Vehicle(); // gives error

Inheritance Mechanics

Creating a subclass

All this is well and good, but how do you actually create a subclass?

Let’s start by writing the Vehicle class

public class Vehicle{public void travel(String destination) {

System.out.println("Traveling to " + destination);

}}

Extending a superclass

We use the extends keyword to create a subclass from a superclass

A Car can do everything that a Vehicle can, plus more

public class Car extends Vehicle{

private String model;public Car(String s) { model = s; }

public String getModel() { return model; }

public void startEngine() {System.out.println("Vrooooom!");

}}

Power of inheritance

There is a part of the Car class that knows all the Vehicle members and methodsCar car = new Car("Camry");

System.out.println( car.getModel() ); //prints "Camry"car.startEngine();//prints "Vrooooom!"car.travel( "New York City" );//prints "Traveling to New York City"

A look at a Car

Each Car object actually has a Vehicle object buried inside of it

If code tries to call a method that isn’t found in the Car class, it will look deeper and see if it is in the Vehicle class

The outermost method will always be called

Car

model

getModel()startEngine()

Vehicle

travel()

Overriding Methods

Adding to existing classes is nice…

Sometimes you want to do more than add

You want to change a method to do something different

You can write a method in a child class that has the same name as a method in a parent class

The child version of the method will always get called

This is called overriding a method

Mammal example

We can define the Mammal class as follows:public class Mammal {

public void makeNoise(){

System.out.println("Grunt!");}

}

Mammal subclasses

From there, we can define the Dog, Cat, and Human subclasses, overriding the makeNoise() method appropriately

public class Dog extends Mammal {public void makeNoise() { System.out.println("Woof"); }

}

public class Cat extends Mammal {public void makeNoise() { System.out.println("Meow"); }

}

public class Human extends Mammal {public void makeNoise() { System.out.println("Hello"); }

}

Study on CS Education

Upcoming

Next time…

Inheritance examples Lab 13

Reminders

Start working on Project 5