1. Introduction to Computer Science and Object-Oriented Programming
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2. Examples What is a car? What is a truck?
Abstraction
Taking away inessential features until only the essence of some concept remains
Examples What is a car? What is a truck?
What is a mammal?
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3. We say a black box provides
Related to abstraction
A black box
Does something (often useful)
Provides an interface for controlling it
But its inner workings are hidden
Examples
Cell phone
Video game
Car
We say a black box provides
encapsulation
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4. Depends How You Look At It
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5. Who Cares?
Abstraction and black boxes help humans deal with complexity
Make things simpler and more efficient
You don’t care how things work inside
You care only the interface is well-defined
Leads to a separation of concerns
User of the black box
Designer and implementer of inner workings
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6. Let’s Try Abstraction
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7. What’s It Mean to Software?
Our black boxes are objects
OOP focuses on
Use
Interfaces
Design
of objects
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8. Encapsulation in OOP
The use of abstraction to design programs in terms of classes/objects
The definition of the class
Focused on the interface
On one outside - what users know and use
On the inside - what programmers design & code
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9. Encapsulation in OOP In COMP 111 we learn: To use
To define (the interface)
To design and implement (the inner workings)
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10. Example of Use We don’t know how the String class is implemented
But we do know the purpose and how to use public methods of String
e.g. length, toUpperCase, replace
But we know
What parameters (if any) we need to pass in
Their sequence and type of each
The type of the return value (if any)
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11. Example of Defining We need to define an abstraction Then translate
With attributes
With behavior
Then translate
Attributes to data
Behavior to methods
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12. Public Interface of a Class
Behavior (abstraction) for a Bank Account:
deposit money, withdraw money, get balance
Methods of a BankAccount class might be: deposit, withdraw, getBalance
harrysChecking.deposit(2000);
harrysChecking.withdraw(500);
System.out.println( harrysChecking.getBalance());
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13. Method Definitions access specifier (such as public or private)
return type (such as String or void)
method name (such as deposit)
list of parameters (such as double amount)
method body in { }
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14. Specifying Methods
accessSpecifier returnType methodName(parameters) { method body }
Where parameters is a comma-separated list of
parameterType parameterName
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15. Examples public void deposit(double amount) { . . . }
public void withdraw(double amount)
public double getBalance()
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16. Constructors
are similar to methods an access specifier, parameter list and body
but their job is not to define a behavior
but to properly initialize an object for later use
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17. Constructors . Named the same as the class, Never have a return value
All constructors of a class have the same name
Compiler can tell constructors apart because they take different parameters
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18. Constructors Constructor body is executed when a new object is created
Statements in constructor body will set the internal data of the object
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19. Commenting the Public Interface
Use documentation comments to describe the classes and public methods
Use the standard documentation notation
so program called javadoc can
automatically generate a set of HTML pages
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20. Commenting the Public Interface
Provide documentation comments for
every class
every method
every parameter
every return value
Javadoc comment are within /** and */
Best to write the method comments first, before writing code in the body
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21. Example /** Withdraws money from the bank account.
@param amount the amount to withdraw */
public void withdraw(double amount) {
// implementation filled in later }
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22. Example /** Gets the current balance of the bank account.
@return the current balance */
public double getBalance() {
// implementation filled in later }
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23. Generated by Javadoc
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24. Instance Fields A class is the pattern for its instances (objects)
Each object stores its data in instance fields (or instance variables)
A field is a term for a storage location in memory
An instance of a class is an object of that class
Thus, an instance field is a storage location present in each object of the class
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25. Defining Instance Fields
To define an instance field:
An access specifier (usually private)
The type of the instance field (such as double)
The name of the instance field (such as balance)
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26. Example public class BankAccount { . . . private double balance; }
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27. Instance Fields
Each object instance of a class has its own set of instance fields
Instance fields are generally declared with the access specifier private
The private specifier means
cannot be accessed directly
other than by methods defined in the class
must use methods
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28. More on Access The balance instance variable can be accessed
by the BankAccount deposit method
but not by the main method of another class
Since instance variables are private
all access to their values
must be through public methods
This is how we implement encapsulation
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29. Next Week Read Module 5 introduction and key points In Big Java
3-7 to 3.8
4.1 to 4.5
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30. Next Week Submit homework Assignment 4-2 Work on Assignment 5-1
Submit on or before May 29
Into Course Drop Box
Worth 15 points
Work on Assignment 5-1
Week 5 Guided Learning Activities
Nothing to submit
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