Chapter 1:
Object Oriented Paradigm
1.1 Data Abstraction and
Encapsulation
• OOP allows programmer to
– separate the details that are important to the user
• myCoin.flip();
myCoin.getValue();
– from the details that make the abstraction work
• if (Math.random() > 0.5) side = "heads"; else side="tails";
• Accomplished by organizing code into
– Interface: details important to user
– Implementation: “unimportant” details hidden from
user
• Who is "user?"
– person who writes the application…and/or…YOU !
Strings represented on Macs vs Linux
1.2 The Object Model
• A convenient design method
– program data managed by objects
– objects manage internal data, determines state
• point object manages its x,y coordinates
• coin object manages its side indicator
• student record object manages it's name, ID, GPA
– this helps manage chaos as complexity grows
• reduces unexpected linkages between parts of program
Primary focus of class
• how we implement and evaluate objects with
methods that are logically complex
• how we can use the objects we create
• objects mainly will be data structures,
– our primary interest!
• occasionally we will develop control structures
that manipulate other objects
1.3 Object-Oriented Terminology
• Applied to a Ratio class – see Demo1 in mod1
download
–
–
–
–
–
–
–
–
–
encapsulation
object
instance
class
fields
methods
constructor
utility methods (reduce, gcd)
static methods (gcd)
1.4 A Special-Purpose Class:
A Bank Account
• Manages data:
– balance and account number (or account name)
• Performs:
– getAccount
– getBalance
– deposit
– withdraw
BankAccount equals() method
public boolean equals(Object other)
// pre: other is a valid bank account
// post: returns true if this bank account is the same
as other
{
BankAccount that = (BankAccount)other;
// two accounts are the same if account numbers are
the same
return this.account.equals(that.account);
}
Class Object
• Every class is a descendant of Object
• Defines fundamental methods
– equals()
– toString()
– clone()
same data as another object
string representation for display
makes an identical copy
• If you don't define your own versions of these,
automatic (usually defective) ones will be
provided
Memory Model of BankAccount object
BankAccount jane;
jane
jane =
null 100
new BankAccount(“J. Doe”, 345.67);
account
J. Doe
balance
345.67
The number
100
indicates
the memory
location
where the
object
resides
Memory allocated by the declaration
BankAccount jane;
jane = new BankAccount(“J. Doe”, 345.67);
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Shallow vs Deep Copy
• There are two ways to “copy” an object
– Deep copy creates a clone of the object
• The objects data values are copied into the clone
– Shallow copy creates reference to the object
• The location of the object is copied into a reference variable
• After a
– Deep copy, two distinct objects exist
• Can change one object’s data without changing the other’s
– Shallow copy two references to one object exists
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Code of Shallow and Deep Copies
• Shallow copy is similar to copying primitives
jon = jane;
• Deep copy
– requires a class method (e.g., in the BankAccount class)
public BankAccount clone()
{ BankAccount copy = new BankAccount();
copy.account = account;
copy.balance = balance;
return copy;
}
– Invoked as
jon = jane.clone(); //copy jane into jon
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Memory Model of Two Types of Copies
jon
jane
100
200
location 100
account
balance
jon j. doe
j. doe
deep copy
187.95 345.67
345.67
location 200
account
balance
A Deep Copy of the Object jane into the Object jon
jon
100 200
shallow copy
jane
200
j. doe
location 200
345.67
A Shallow Copy of the Object jane into the Object jon
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Using Scanner and File
• Scanner – a class representing objects that
read input streams or files and extracts
primitive data types from them
• File – a class representing a path to a location
on disk or other media
• We will now apply these to our BankAccount
app
1.5 A General-Purpose Class:
An Association
• very general class represents a connection
between two kinds of objects
protected Object theKey; // the key of the key-value pair
protected Object theValue; // the value of the key-value pair
• Association can be used to link
– ID (key) with an employee record
– Two words from different languages (pig latin)
– Methods: setValue, setKey, getValue, getKey
Protected vs Private
• Usually, it is best to restrict methods as much
as possible.
– private: member is only used within class itself
– protected: member is available to other classes in
a package or to subclasses through inheritance
– public: member is available to any user
An app that uses Association
• atinLay – a Pig Latin translator
• This program uses the argument list set up in
main, as would be obtained from a “command
line” execution
• We (or you!) could convert it to read from the
System.in console with a Scanner object
1.6 Sketching an Example: A Word List
• Suppose we want to develop a Hangman app
– The program should :
1. Select a random word from a list of words (TODO!)
2. Receive user guesses and draw stick figure (demo)
• We propose to develop a WordList class to help
us with item number 1)
– BTW this is a “Data Structure”
• How do we design such a thing????
5 Step
Data
Structure
Design
Process
Sketch an example test application
helps familiarize operations needed
Sketch the “interface” for the class
names of methods, parameters, return types
Develop the implementation
• Choose an internal representation
– An array of words?
• Write the methods
• Test and debug using tester class
• Then write the complete implementation