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Classes &

Objects

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Week 6

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Helping Hand for Relief & Development

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Goal of this lesson

  • In this lesson, we will learn:
    • How to create classes
    • How to use classes
    • What access modifiers are
    • How to write a constructor

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Key Terms

  • Class:
  • Object:
  • Instance:
  • Constructor:
  • Field:
  • Method (review):
  • Access Modifier:

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We will learn about these terms in the next slides

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What are Classes?

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What are Classes?

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What are Classes?

  • “Blueprint” or “Template”
  • Group Common Things Together
  • Create your own objects with unique methods and fields
  • Do we know any examples of classes?

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Object

  • In real-life objects are around us
    • They have characteristics/attributes
    • Examples of attributes: Color, size, weight
  • In Java objects have attributes as well
    • Making a class lets us define them

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Examples

  • String is a commonly used class
  • Attributes/Characteristics
    • Capitalized first letter: String
    • Uses the dot operator
    • Has built in functions for you

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Why are Classes important?

  • Java is primarily used as an “Object Oriented Programming” (OOP) Language
  • Classes are more powerful than primitive data types
    • (int, float, boolean, ...) do not come with functions
    • (String, Integer, ...) do (String.charAt, String.length())

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How do we create one?

  • Let’s do example with coordinate points
  • What is a point?

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How do we create one?

  • Point has two main features: X Coordinate & Y Coordinate
  • Store with two integers
  • But what does the code look like?

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How do we create one?

  • File -> New -> Java Class
  • Name it with a capital letter (Point)
  • Add variables for what you want to store (Definition):

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How do we create one?

  • How do we use it (Creation)?
  • myPoint is an instance of the point class

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Instance Variables

  • Variables that are stored by the class are called fields or instance variables
  • Unique per instance of the class
    • You can create multiple instances of Point
    • They can all have unique X & Y Coordinate values

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Constructors

  • What if I had 50 instance variables?
  • Is there an easier way to make and set instances of our classes?

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Constructors

  • Constructors:
    • “Special function”
    • Used to make creating instances easier
    • “This” keyword refers to a field

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Constructors

  • Constructors use the same name as the class
  • Use input parameters to set the value of instance variables

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Constructors

  • Result: much cleaner code when creating them

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Your Turn! (Exercise 1)

  • Let us create a class together!
  • Create a book class:
    • What are the attributes of a book? Include at least 3
    • Make a constructor!
    • Make at least 2 instances of your class

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Solution

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Adding Methods

  • What do classes do right now?
    • Store information
  • Can classes do more than store things?
  • What about modifying/using the information stored?
  • How do we do this?

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Adding Methods

  • Our classes can have their own functions as well:

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Getters and Setters

  • Special functions
  • Purpose:
    • Used to get/retrieve variables
    • Used to set/modify variables
    • Exist when private is used

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Access Modifiers (Brief)

  • Package-private (default) - nothing outside of the package can access
  • Public - all classes present can access the data in a class
  • Private - nothing outside of the class where the variables are defined can access

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Getters and Setters

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Exercise 2

  • Go to your book class
    • Make all the variable private
    • Add an age parameter for your book
    • Add a method to increase the age of the book
    • Add Getters and Setters
    • Get the name for one of your instances of books

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Solution

  • ​

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Key Terms

  • Class:
  • Object:
  • Instance:
  • Constructor:
  • Field:
  • Method (review):
  • Access Modifier:

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Define these terms in your own words, we will go over them in a minute

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Key Terms

  • Class: A template used to create multiple objects/instances with similar characteristics
  • Object: Class with attributes in java
  • Instance: One specific version of an object of a class
  • Constructor: A special function used to make instances of classes simpler

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Key Terms

  • Field: A variable belonging to a class
  • Method (review): A function often belonging to a class
  • Access Modifier: Special keywords that modify what classes can access data from other classes

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What We Learned

  • Today, we learned:
    • How to create classes
    • How to use classes
    • What access modifiers are
    • How to write a constructor

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Homework

  • Here is the link to the homework.
  • Please email your java file by Thursday, March 4th to hhrd.compsci@gmail.com

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Extra Reading

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Questions?

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Feel free to ask!

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