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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