Introduction to Objects

Introduction to Objects
> Click or hit Control-Enter to run the code above

These are the three main challenges that will occupy us for the remainder of the semester:

Algorithms: How do we use computers to solve problems?
Data structures: How do we structure information to enable efficient algorithms?
Software development: How do we write, debug, test, and publish good computer software?

Break our code into reusable, testable, and understandable pieces
Combine state and behavior
Document our code appropriately
Reuse preexisting solutions as much as possible
Share our code with others!

Programming is an intensely creative activity.

Java objects begin to expose you to the design aspects of computer science.

Ahead of us there are no right answers, just

creativity
imagination
holistic thinking
intuition
visualization

Objects combine state (like variables) and behavior (like functions).

What does it store? Objects allow us to structure data in more varied ways than we can achieve with primitive types alone.
What does it do? Objects allow us to place methods with the data that they naturally belong with.

The two core concerns of computer science.

Object: In the class-based object-oriented programming paradigm, "object" refers to a particular instance of a class where the object can be a combination of variables, functions, and data structures.

class Person {
  String name;
  int age;
  void printName() {
    System.out.println(this.name);
  }
}

A class determines how a particular type of object will behave.

You can think of the class as a blueprint that is used to create object instances of a particular type.

class Person {
  String name;
  int age;
  void printName() {
    System.out.println(this.name);
  }
}

Instances of object class Person:

Have a variable name of type String
Have a variable age of type int
Have a method printName that takes no arguments and returns nothing

class Person {
  String name;
  int age;
  void printName() {
    System.out.println(this.name);
  }
}

Unlike other languages, Java classes cannot be modified after the program is compiled (at runtime).

This is frustrating when writing small programs
This is sometimes essential when developing large programs

class Person {
  String name;
  int age;
  void printName() {
    System.out.println(this.name);
  }
}
Person geoffrey = new Person();

An instance of a class is an object of that type. We create an instance using the new keyword.

Person is a class—a type of object
geoffrey is an instance of type Person

class Person {
  String name;
  int age;
  void printName() {
    System.out.println(this.name);
  }
}
Person geoffrey = new Person();
geoffrey.age = 38;
System.out.println(geoffrey.age);

We access an object’s state and methods using dot notation.

> Click or hit Control-Enter to run the code above

class Dimensions {
  int width; // I'm a primitive type
  int height;
}
class Room {
  String name; // I'm another object
  Dimensions dimensions; // I'm defined above
}
Room diningRoom = new Room();
diningRoom.dimensions = new Dimensions();
diningRoom.dimensions.width = 10;

Instance variables can be both primitive types or other objects.

> Click or hit Control-Enter to run the code above

Java objects allow us to define our own types built from the primitive types we’ve already discussed.

This dramatically improves our ability to work with real-world data.

Remember MP0?

public static int farthestNorth(final double[] latitudes, final double[] longitudes,
                                final boolean[] validLocations) {
    return 0;
}

public static int farthestNorth(final double[] latitudes, final double[] longitudes,
                                final boolean[] validLocations) {
    return 0;
}

Synchronized indices is the wrong solution to this problem 1!

What do we really want?

One location type that stores
a latitude
a longitude
and whether it is valid or not

class Location {
  double latitude;
  double longitude;
  boolean isValid;
}
public static int farthestNorth(final Location[] locations) {
    return 0;
}

Sometimes known as records—but objects are much more than that…

class Dimensions {
  int width;
  int height;

  int area() {
    return this.width * this.height;
  }
}
Dimensions example = new Dimensions();
example.width = 10;
example.height = 20;
System.out.println(example.area());

Classes can also define methods that can be called on each instance.

> Click or hit Control-Enter to run the code above

class Dimensions {
  int width;
  int height;

  int area() {
    return this.width * this.height;
  }
}

Instance methods can refer to their instance variables using the this keyword.

this refers to the instance that is executing the method.

> Click or hit Control-Enter to run the code above

We frequently use Java objects to model real objects or entities.

Objects allow us to design software that deals with things in realistic and natural ways.

12 multiple-choice questions, mostly on code reading
3 programming questions:
- one using arrays,
- one using multi-dimensional arrays,
- and one using strings.
One programming question will be drawn directly from the HW125 practice problems. So use those to prepare.

The MP1 early deadline is today. Office hours until 5PM in Siebel 0403.
The first midterm exam on imperative programming starts tomorrow in the CBTF. Midterm exams cannot be dropped. Good luck!
My office hours will be 3–5PM today in Siebel 0403.

Onward

How Do We Structure Good Computer Programs?

Design

Design and Modeling

Objects

Objects: Uniting Algorithms and Data

Object Definition

Object Terminology: Class

Object Terminology: Class

Object Terminology: Class

Object Terminology: Instance

Dot Notation

Instance Variables

Objects as Custom Types

Custom Types: Location

Custom Types: Location

Custom Types: Location

Objects as Data Structures

Instance Methods

`this`

Object Modeling

Midterm Format

Midterm Questions?

Announcements