Lecture Summary 3

Basic Concepts of OO

• An object has
• state: defined by the set of fields or attributes.
• behavior: defined by the set of methods or operation that can be applied to the object.
• identity: determined at the creation time.
• Class
• A template for creating objects.
• Objects of the same class exhibit the same behavior. They may have different states.
• In real world terms:
• An object represents an individual entity or thing.
• A class represents a group of objects that exhibit some common characteristics or behavior.
• Classes are resulted from classification.
• Examples of classes in real world:
• Students
• Graduate students
• Undergraduate students
• MS students
• Ph.D. students
• In programming terms:
• A class defines a self-contained software component that represents a real world class.
• The design of classes should follows the basic principles of object-orientation
• modularity
• encapsulation
• Each object is an instance of its class.
• A class defines the common characteristics of its instances:
• the behaviors and
• the templates of the fields.
• Each object has its own state, i.e., the values of the fields.
• Relationship among classes
• inheritance
• A mechanism to organize classes by commonalities.
• subclasses, specialization
• superclass, generalization
• Is-a relation
• Example:
• A graduate student is a student.
• A Master student is a graduate student.
• A Ph.D. student is a graduate student.
• An undergraduate student is a student.


Student
    GradStudent
        MSStudent
        PhDStudent
    UGStudent

• composition
• Has-a relation
• Example: a student has a
• address (type: Address)
• faculty advisor  (type: Faculty)
• etc.
• UML
• Class diagram

Classes and Methods

• Class declaration
• A class contains data (variable, field) declarations and method declarations
• Variables declared at the class level can be used by all methods in that class
• Variables declared within a method can only be used in that method
• A method declaration specifies the code that will be executed when the method is invoked (or called)
• Syntax
[ ClassModifiers ] class ClassName
  [ extends SuperClass ]
  [ implements Interface1, Interface2  ...] {
  ClassMemberDeclarations
}
• Class visibility modifiers:
• public

Accessible everywhere. One public class allowed per file. The file must be named ClassName.java
• default

Accessible within the current package.
• Other class modifiers
• abstract

A class that contains abstract methods
• final

No subclasses
• Field and method declaration
[ MethodModifiers ] ResultType  Name ( [ ParameterList ]  ) {
  Statements
}

[ FieldModifiers ] Type  FieldName1 [ = Initializer1 ] ,
   FieldName2 [ = Initializer2 ] ...;

• Encapsulation
• external view: for the users of the classs, client
• internal view: for the developers of the class, implementer
• visibility: public, protected, package (default), private
 

	public	protected	package	private
The class itself	Yes	Yes	Yes	Yes
Classes in the  same package	Yes	Yes	Yes	No
Subclasses in a  different package	Yes	Yes	No	No
Non-subclasses in  a different package	Yes	No	No	No

• Examples: Coin.java, CountFlips.java, FlipRace.java
• Instance field (data)
• Constructor
• has the same name as the class
• does not return a value
• has no return type, not even void
• sets the initial values of instance variables
• Parameters
• Method overloading
• Two or more methods/constructors with the same name but different numbers or different types of parameters:

void methodA(int i)
void methodA(int i, int j)

void methodB(int i)
void methodB(float f)

• Avoid overloading
• Special methods:
• public boolean equals(Object o)
• o1.equals(o2) versus o1 == o2
• The equals() method tests the equality of two objects.
• The == operator tests the identity of two objects.
• When compare two strings, use s1.equals(s2) instead of s1 == s2.
• public String toString()
• returns a string representation of the state of the object
• public void finalize()
• invoked by the Java runtime just before the object is garbage-collected
• Examples: Account.java, BankAccount.java
• Examples: PigLatinTranslator.java, PigLatin.java
• Translate an English phrase into Pig Latin. "This is a test" becomes "isthay isyay ayay estay."
• String manipulation.
• StringTokenizer class
• separates a string into words (tokens)
• white space as delimiters
• Constructors:

StringTokenizer(String str)
StringTokenizer(String str, String delimiter)
• Methods:

boolean hasMoreTokens()
String nextToken()