Introduction to Objects

Introduction to Objects
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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.

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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.

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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.

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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.

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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.

One question like this will appear on the midterm exam.

All lecture videos and EMP sessions have been posted to YouTube—and we’re getting better at this, so lectures should be up quickly now
Next Monday will be midterm review, with the midterm exam starting Tuesday
Office hours for MP2 continue—please get it done, since it will help you prepare for the midterm exam next week
MP3 will come out next Friday and be due two weeks from the following Monday

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 Data Structures

Instance Methods

`this`

Object Modeling

Midterm Preparation

Open-Ended Quiz Question

Announcements