OO Programming in Java

1. OO Programming in Java Objectives for today: Overriding the toString() method Polymorphism & Dynamic Binding Interfaces Packages and Class Path

2. The toString() Method: • The toString() method is declared in the Object Class. • It is commonly used by objects to return a String representation • of themselves to display their current state. • Because every Class in Java inherits from Object, every • Class has access to the toString() method. • The toString() method is commonly overridden in sub classes • so that developers can decide how objects will be textually • displayed at run time. • Objects are commonly displayed textually through the • System.out.println command or through List Boxes in GUI • environments.

3. OO Concepts • Polymorphism: • It is important to understand what happens when a method • call is made to objects of various types in an inheritance • hierarchy. • When you a call a method of a subclass using certain • parameters, here is what happens: • The subclass checks to see if it has that method with • exactly the parameters specified. If it does it uses it. • If the method is not found the super class is searched. If • the method is found in a super class the method is executed. • If the method is not found a compile time error occurs.

4. OO Concepts • Polymorphism: • An objects ability to decide what method to apply to itself in • the inheritance hierarchy is called polymorphism. • Methods with same signature (name and argument list) defined • in an inheritance hierarchy is called method overriding. • Objects in an inheritance hierarchy with overridden methods • can react differently to the same method call. • Polymorphism is also the ability of objects in an inheritance • hierarchy to react differently to the same method call.

5. OO Concepts • Dynamic Binding: • Dynamic binding is very closely related to polymorphism in Java. • Dynamic binding is the process of resolving the reference to a • method at run time. • When you call a method that has been overridden by at least • one subclass in an inheritance hierarchy, the JVM determines which • version of the method to call. • You do not have to do any special coding to take advantage of • dynamic binding because it is automatic.

6. OO Concepts • Interfaces • An interface is a special kind a Class that is used as a design • template to specify what kind of behavior a Class must implement. • An interface is a group of constants and method declarations that • define the form of a class. • However, interfaces provide no method implementation. • In essence, an interface allows you to specify what a class must do, • but not how it will get done. • Abstract Classes use abstract methods to ensure that any Class • inherited from the Abstract Class must implement that method.

7. OO Concepts • Interfaces • Interfaces are similar in that they define public abstract methods • that provide no implementation. • The Class implementing the interface is also • forced to implement these methods. • Interfaces can have fields, but they must be declared • as static and final. • If you omit the static and final qualifiers Java • provides them for you.

8. OO Concepts Syntax for Interfaces: [public] interface interface_name [extends interface_name] { /*Declare fields and methods*/ } How do Classes Implement Interfaces? public class clasName implements interfaceName[,interfaceName] { /*Class Body*/ }

9. Class Path & Packages • The CLASSPATH variable is how we tell applications written • in the Java programming language where to look for user Classes. • The Java interpreter (java) knows where to find the Core Java • Classes that come with Java. • The Java interpreter locates these classes needed to run your • code by using the CLASPATH environment variable. • We have used the Java Class String numerous times in our • programs so far. • The interpreter was able to find the String class because it • is in the system environment variable called CLASSPATH.