Objects: Example Applications, toString, "this" and public vs. private

What we know so far about objects and classes

Classes allow us to create complex data types, using primitive types

Encapsulating related data in a single "type"
i.e. pieces of information that belong together

A representation of a Student

What we know so far about objects and classes

Objects are reference types

We are reserving `new` memory space for `s1`

`s1` is an instance of the `Student` class

What we know so far about objects and classes

To access or modify a property of an object, put a `.` after the variable name

We call these properties attributes of a class

What we know so far about objects and classes

We can use the `Student` class as any other type

Declaring an array of elements of type `Student`

Each element in `comp202_students` points an instance of the `Student` class, in the computer's memory

Each position in the array is null by default
We need to initialize each position in the array before using it

What we know so far about objects and classes

We can define methods inside a class. Inside class methods, we have acces to class attributes

What we know so far about objects and classes

We can define methods inside a class. Inside class methods, we have acces to class attributes

What we know so far about objects and classes

We can define methods inside a class. Inside class methods, we have acces to class attributes

What we know so far about objects and classes

We can define methods inside a class. Inside class methods, we have acces to class attributes

public class Student{
      //define class PROPERTIES here
      public int id;
      public String first_name;
      public String last_name;
      public String major_program;

//define class METHODS here
      // A constructor method, notice it does not declare a return type
      public Student(){
          // initialize properties and execute other code by default
      }

// Another constructor method, notice it does not declare a return type
      public Student(int new_id, String new_first_name, String new_last_name, String new_program){
          // initialize properties and execute other code by default
      }

public void printProperties(){
           // Each instance can acces its own properties from a class method
           System.out.println("My student id is: "+id);
           System.out.println("My student first_name is: "+first_name);
      }

public int compareTo(Student s2){
         // put some code to comapre this student with s2
         // s2 is a reference to another instance
      }
  }

The `toString` method

All objects in java share the `toString` method. By default it returns the Memory Address of the object.

An example - Creating a Course class

The Course class
- A Course has an course name, a course id, and an instructor name
- A Course has a list of registered students
- Write a method that assigns to each student a random major_program from { "B.A.", "B.Eng.", "B.Sc.", "B.Comm.", "M.Sc", "Ph.D" }
- Write a method that counts how many students are enrolled in each program

The `this` keyword

The `this` returns a memory reference to the current class

A simple exercise - Creating a 2D Point Class

The Point class
- A Point has two coordinates: call them x and y
- The coordinates should be real numbers ( double type )
- Write a class method that computes the distance to another Point

A simple exercise - Creating a 2D Point Class

The Point class
- A Point has two coordinates: call them x and y
- The coordinates should be real numbers ( double type )
- Write a class method that computes the distance to another Point
  - sqrt( ( x₁ - x₂ )² + ( y₁ - y₂ )² )

Public vs Private

Methods and variables declared as `public` are accessible from any other java file

Public vs Private

Methods and variables declared as `private` are accessible from within the class instance only

`private` methods and variables (members) can only be accessed through getter/setter methods

Public vs Private

Why do we care about declaring things as `private`

`private` methods and variables allow the designer of the code to control how the class is used ( to check for correct input, to allow for future modifications of the class, to give different values depending on the state of the program, etc)

Anoter exercise - Creating a 2D Point Class, and using in a Polygon class

The Polygon class
- A Polygon has a list of Point objects; its vertices.
- Write a class method that adds a new vertex to a polygon
- Write a class method that returns true if the Polygon is equilateral
- Write a class method that returns true if the Polygon is regular
- Write a class method that returns the area of a polygon using the Shoelace Algorithm

Using `Point` in `Polygon`

The Polygon class
- A Polygon has a list of Point objects; its vertices.

Adding a new `Point` to `vertices`

The Polygon class
- A Polygon has a list of Point objects; its vertices.
- Write a class method that adds a new vertex to a polygon

Adding a new `Point` to `vertices`

The Polygon class
- A Polygon has a list of Point objects; its vertices.
- Write a class method that adds a new vertex to a polygon

Completing the `Polygon` class

The Polygon class
- A Polygon has a list of Point objects; its vertices.
- Write a class method that adds a new vertex to a polygon
- Write a class method that returns true if the Polygon is equilateral
- Write a class method that returns true if the Polygon is regular
- Write a class method that returns the area of a polygon using the Shoelace Algorithm

Resources

Classes and Objects:
http://docs.oracle.com/javase/tutorial/java/javaOO/
The Shoelace Algorithm:
http://en.wikipedia.org/wiki/Shoelace_formula
Suggested reading:
How to think like a Computer Scientist, Chapter 11

Objects: Example Applications, toString, "this" and public vs. private

What we know so far about objects and classes

Classes allow us to create complex data types, using primitive types

A representation of a Student

What we know so far about objects and classes

Objects are reference types

We are reserving new memory space for s1

s1 is an instance of the Student class

What we know so far about objects and classes

To access or modify a property of an object, put a . after the variable name

We call these properties attributes of a class

What we know so far about objects and classes

We can use the Student class as any other type

Declaring an array of elements of type Student

Each element in comp202_students points an instance of the Student class, in the computer's memory

What we know so far about objects and classes

We can define methods inside a class. Inside class methods, we have acces to class attributes

What we know so far about objects and classes

We can define methods inside a class. Inside class methods, we have acces to class attributes

What we know so far about objects and classes

We can define methods inside a class. Inside class methods, we have acces to class attributes

What we know so far about objects and classes

We can define methods inside a class. Inside class methods, we have acces to class attributes

The toString method

All objects in java share the toString method. By default it returns the Memory Address of the object.

An example - Creating a Course class

The this keyword

The this returns a memory reference to the current class

A simple exercise - Creating a 2D Point Class

A simple exercise - Creating a 2D Point Class

Public vs Private

Methods and variables declared as public are accessible from any other java file

Public vs Private

Methods and variables declared as private are accessible from within the class instance only

private methods and variables (members) can only be accessed through getter/setter methods

Public vs Private

Why do we care about declaring things as private

private methods and variables allow the designer of the code to control how the class is used ( to check for correct input, to allow for future modifications of the class, to give different values depending on the state of the program, etc)

Anoter exercise - Creating a 2D Point Class, and using in a Polygon class

Using Point in Polygon

Adding a new Point to vertices

Adding a new Point to vertices

Completing the Polygon class

Resources

We are reserving `new` memory space for `s1`

`s1` is an instance of the `Student` class

To access or modify a property of an object, put a `.` after the variable name

We can use the `Student` class as any other type

Declaring an array of elements of type `Student`

Each element in `comp202_students` points an instance of the `Student` class, in the computer's memory

The `toString` method

All objects in java share the `toString` method. By default it returns the Memory Address of the object.

The `this` keyword

The `this` returns a memory reference to the current class

Methods and variables declared as `public` are accessible from any other java file

Methods and variables declared as `private` are accessible from within the class instance only

`private` methods and variables (members) can only be accessed through getter/setter methods

Why do we care about declaring things as `private`

`private` methods and variables allow the designer of the code to control how the class is used ( to check for correct input, to allow for future modifications of the class, to give different values depending on the state of the program, etc)

Using `Point` in `Polygon`

Adding a new `Point` to `vertices`

Adding a new `Point` to `vertices`

Completing the `Polygon` class