Lecture Summary 4a

Classes and Methods (Cont.)

• Class declaration
  • A class contains 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
  
  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



• Previous Example: Coin.java,   CountFlips.java,   FlipRace.java
  
  
• 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 (review)
  • 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.
    
    
  • public String toString()
    • returns a string representation of the state of the object
  
  
• 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, .
  
  
  • 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()
    
    
    • More Examples: RationalNumbers.java,   Rational.java,