Objects: Creating your own data types

Announcements

  1. Assignment deadline extended until Sunday October 26th at midnight
  2. Midterm classroom locations posted in class website
  3. The midterm is 50 multiple choice questions.
    • Read and undertand a piece of code
    • Determine the value of variables during program execution
    • Count how many times a line of code is executed inside loops
  4. You can bring Letter size ( A4 ) crib sheet, to the midterm.
    • Challenge: Draw a priate ship (and nothing else) on your crib sheet and get bonus marks.

Types of Objects we have seen in Java

Scanner: An object that processes input from the keyboard

Random: An object that generates random numbers


These object types are also known as classes

What is a class

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

Example: Storing information about students enrolled in a course

Each student has

Student

Multiple Students

What is a class - 2

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

A representation of a Student

What is a class - 2

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

A representation of a Student in Java code

Syntax of a class

Use the class keyword

The public keyword, let's us access the properties of Student from outside the class definition

Declaring variable of type Student

s1 is an instance of the Student class

The constructor method

Defines some default code that is executed when we create a variable of our class

Declaring variable of type Student calls the constructor method of Student

s1 is an instance of the Student class

Using a class

We can now use Student as any other type


Declaring a variable of type Student


The computer will reserve some new space for s1 in the computer's memory

s1 will point to an instance of the Student class, in the computer's memory

Accessing properties of a class

Put a . after the variable name


To try for yourself

Write a method that
  • receives as input two objects s1 and s2 of the Student class
  • returns true if all of the properties of s1 are the same as the properties of s2
  • returns false otherwise


Write a method that receives a Student object as an input, and prints all of its properties

Objects are reference types


Memory Address Variable Type Variable name Value
@1001 Student s1 @1100
... ... ... ...
@1100 int id 0
@1101 String first_name ""
@1102 String last_name ""
@1103 String major_program ""

Objects are reference types


Memory Address Variable Type Variable name Value
@1001 Student s1 @1100
... ... ... ...
@1100 int id 1
@1101 String first_name "Banana"
@1102 String last_name "Phone"
@1103 String major_program "Math"

Objects are reference types


Memory Address Variable Type Variable name Value
@1001 Student s1 @1100
@1002 Student s2 @1200
... ... ... ...
@1100 int id 1
@1101 String first_name "Banana"
@1102 String last_name "Phone"
@1103 String major_program "Math"
... ... ... ...
@1200 int id 0
@1201 String first_name ""
@1202 String last_name ""
@1203 String major_program ""

References point to locations in the computer memory

References are what your computer uses to identify a variable


In the previous example, s1 points to memory location @1100, and s2 points to memory location @1200


Your computer uses @1100 and @1200 to identify/index/address s1 and s2

Null references

You can make a reference type point to nowhere in your memory


I.e. you can declare a reference type without the new statement


Memory Address Variable Type Variable name Value
@1001 Student s1 null
@1001 int[] array null

No new memory gets reserved for s1 or array

Using a class in Arrays

We can now use Student 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


Resources

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