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
                              /               \
          UGStudent    GradStudent
                                          /           \
                PhDStudent     MSStudent

  • composition
    • Has-a relation
    • Example: a student has a
      • address (type: Address)
      • faculty advisor  (type: Faculty)
      • etc.
  
  
• UML (Unified Modeling Language)
  • 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
    
    Accessibility of class members:--- Yes: accessible,   No: Not accessible
     

    	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) In the class coin HEADS, TAILS and face are declared inside the class at class level. So they can be refered within any method that is a class member. This data is called instance data. Its memory space is created for each instance of the class.
  • Class methods: The method of Coin are: Coin, flip, getFace, and toString.
  
  
  
• Accessing members:
  objectReference.method (Parameters) ---> eg.   myCoin.HEADS
  objectReference.field ---> eg.   myCoin.getFace()



• 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
  
  
• Local Data and Scope
  The scope of a variable is the part of a program where the variable can be referenced. A variable declared inside a function is local to the function, so its scope is just the function definition body. It is not known outside this scope. Eg. The variable faceName declared in the toString method of Coin class is a local data.
  Formal parameters are local variables too, and have the function body as their scope.
  
  
  
• 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)

  • An example of overloaded function is the println method that can take as parameter a string, an int, a double, a char, a boolean, etc.
  
  
• Special methods:
  • public boolean equals(Object o)
    • o1.equals(o2) versus o1 == o2
    • The equals() method tests the equality of two objects, meaning the equality of their states. It defines the equivalence of objects on a per-class basis, meaning both o1 and o2 refer to the same object.
    • The == operator tests the identity of two objects. It refers to the equality of the two references, not the equality of the states of the objects refered to by them.
    • When compare two strings, use s1.equals(s2) instead of s1 == s2.
    
    
  • Examples: PalindromTester.java
    
    
    
  • public String toString()
    • returns a string representation of the state of the object

    Note that if the toString() method were not provided for the class Coin, a default version would be called, that returns a string containing the name of the class; in this case Coin. This because every class is derived implicitely from Object class that has a method called toString. But sisnce we want to print either "Heads" or "Tails" we did provide a version that overrides the one given in Object class.
    For Example: in FlipRace program, in the two statements:
                System.out.print ("Coin 1: " + coin1);
                System.out.println (" Coin 2: " + coin2);
    The toString method of the class Coin is called implicitely. In fact, when an object is used as an operand of the string concatenation operator (+), that object's toString method is automatically called. It is also called if an object is sent to a print or println by itself. If no toString method is defined, the default version of Object toString is called.
    

  
  
• Examples: Account.java,   BankAccount.java
  
  
• The StringTokenizer Class
  This class is provided by the Java standard class library in the java.util package.This class allows a sting to be broken into tokens, based on a set of delimiters such as space, tab, end of line, .
  
  
  • 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()
  • Examples:
    1. PalindromTester1.java     Modified version
    
    
    2. PigLatinTranslator.java,     PigLatin.java
    • Translate an English phrase into Pig Latin. "This is a test" becomes "isthay isyay ayay estay."
    • String manipulation.