Transcript lecture 2 - classes and objects

CSC 205
Java Programming II
Java Review I
Classes and Objects
The StringBuffer Class
 StringBuffer objects are mutable

Used for holding strings to be changed, convert
into string objects after the changes
 Different set of operations (methods)
 append: to substitute for the concatenation +
 insert: to insert a character sequence into the middle
of a string
 replace: to replace a substring in the
StringBuffer object with a given character
sequence
Defining Your Own Classes
 A Java program is a collection of classes
 All program modules need to appear in one of these
classes that you define

There is no stand-alone method
 Every Java source code should include one of the
following statements



class Identifier …{…}
abstract class Identifier …{…}
interface Identifier …{…}
Types of Classes
 A class can be really simple, such as this Dummy
class:
public class Dummy { }



It compiles!
You may even instantiate an instance of it in another
class:
public class DummyDemo {
private Dummy d = new Dummy();
…
}
But it can’t do anything. You don’t want to define
something like it.
Types of Classes (cont’d)
 Only one class in your program needs to have a
main method, which serves as the entry point to your
application.


There might be hundreds of classes in your
application!
The signature of the main method is
public static void main(String[] args)
 Other classes might not have a main method, which
need to be accessed from other classes.
 Some classes might have static methods only, like
the Math class.
Defining A Class
 Class and object
 A class is a template for all objects of the same type.
 An object is an instance of the class that it belongs.
 A class usually has
 Data fields (or instance variables): what an object of
that class knows
 Methods: what an object of that class can do
 For information hiding, you should define private data
fields and public methods
Using Objects
 You should use the new keyword and one constructor
of the class to instantiate an object

The only exception is the String class
 An object may be used in
 (both sides of) an assignment statement; or as
 An argument
 A data field of another class