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Introduction To Object-Oriented Programming

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  1. Introduction To Object-Oriented Programming

  2. Object-Oriented Programming • Class: Code that defines the behavior of a Java programming element called an object • Object: An entity that has both state and behavior • State: Any data that the object might be keeping track of • Behavior: Actions that the object can perform

  3. Object-Oriented Programming The Difference Between Class and Object • Class – A plan for an object • Object – An instance of a class • Blueprint – A plan for a house • House – An implementation of a blueprint

  4. Object-Oriented Programming The Difference Between Class and Object • Class – A plan for an object • Object – An instance of a class • When an object is created, Java sets aside computer memory sufficient to hold the data stored by the object. • Each instance of a class has its own data independent of the data used by other instances of the same class

  5. Object-Oriented Programming Understanding Static Methods • You do not always have to create an instance of a class to use the methods of a class.

  6. Object-Oriented Programming Understanding Static Methods public class HelloApp { public static void main (String [] args) { System.out.println ("Hello, World!"); } } • Static can call the method without first creating an instance of the class. Static methods are called from classes, not from objects.

  7. Object-Oriented Programming Understanding Static Methods public class HelloApp { public static void main (String [] args) { System.out.println ("Hello, World!"); } } • Java does not create an instance of the application class. Instead, it calls the program’s static main method.

  8. Object-Oriented Programming Understanding Static Methods – Analogy Time! • A blueprint includes details about systems (electrical, water pipes etc.) but you can not turn on the hot water without building the house. • A blueprint can be used to give you dimensions of different rooms. • If a blueprint had a built-in calculator that displays the size of the living room when a the Living Room Button is pressed it would be like a static method in a class. • You can activate it from the blueprint alone. • Many java programs are entirely made up of static methods though more advanced programs rely on one or more objects.

  9. Creating An Object From A Class • Create a variable that provides a name you can use to refer to the object • Use the Keyword new to create an instance of the class • Assign the resulting object to the variable ClassNamevariableName = new ClassName ( );

  10. Creating An Object Class1 myClass1Object = new Class1 ( ) ; • Creates an object instance of a class named Class1 • Assigns Class1 to a variable named myClass1Object

  11. Creating An Object Class1 myClass1Object = new Class1 ( ) ; • Class name twice? • First time provides a type for the variable. You are saying that the variable you are creating here can be used to hold objects created from the Class1 class • Second time you are creating an object from the class. • new tells Java to create an object. The class name provides the name of the class to use to create the object • = is an assignment operator • Says to take the object created by the new keyword and assigns it to the variable.

  12. Creating An Object Class1 myClass1Object = new Class1 ( ) ; • This statement accomplishes 3 things: • Creates a variable named myClass1Object that can hold objects created from the Class1 class.

  13. Creating An Object Class1 myClass1Object = new Class1 ( ) ; • This statement accomplishes 3 things: 2. Creates a new object in memory from the Class1 class

  14. Creating An Object Class1 myClass1Object=newClass1 ( ) ; • This statement accomplishes 3 things: 3. Assigns this newly created object to the myClass1Object variable. You can now use the myClass1Object variable to refer to the object that was created.

  15. Viewing A Program That Uses An Object HelloApp2.java // This application displays a hello message on // the console by creating an instance of the // Greeter class and then calling the Greeter // object’s sayHello method. public class HelloApp2 { public static void main (String [ ] args) { Greeter myGreeterObject = new Greeter ( ) ; myGreeterObject.sayHello ( ) ; } } • Greeter.java • // This class creates a Greeter Object • // that displays a hello message on • // the console • public class Greeter • { • public void sayHello ( ) • { • System.out.println (“Hello, World!”) ; • } • } • Java requires that each public class be stored in a separate file with the same name as the class

  16. Viewing A Program That Uses An Object HelloApp2.java // This application displays a hello message on // the console by creating an instance of the // Greeter class and then calling the Greeter // object’s sayHello method. public class HelloApp2 { public static void main (String [ ] args) { Greeter myGreeterObject = new Greeter ( ) ; myGreeterObject.sayHello ( ) ; } } • End-of-line comments explain the functionality of the program and how it works.

  17. Viewing A Program That Uses An Object HelloApp2.java // This application displays a hello message on // the console by creating an instance of the // Greeter class and then calling the Greeter // object’s sayHello method. public class HelloApp2 { public static void main (String [ ] args) { Greeter myGreeterObject = new Greeter ( ) ; myGreeterObject.sayHello ( ) ; } } • Begins the HelloApp2 class with the public class-declaration • Since the public keyword is used, a file named HelloApp2.java must contain this class

  18. Viewing A Program That Uses An Object HelloApp2.java // This application displays a hello message on // the console by creating an instance of the // Greeter class and then calling the Greeter // object’s sayHello method. public class HelloApp2 { public static void main (String [ ] args) { Greeter myGreeterObject = new Greeter ( ) ; myGreeterObject.sayHello ( ) ; } } • A main method is declared as it is in ALL java programs

  19. Viewing A Program That Uses An Object HelloApp2.java // This application displays a hello message on // the console by creating an instance of the // Greeter class and then calling the Greeter // object’s sayHello method. public class HelloApp2 { public static void main (String [ ] args) { Greeter myGreeterObject = new Greeter ( ) ; myGreeterObject.sayHello ( ) ; } } • The body of the main method creates a variable named myGreeterObject that can hold objects created from the Greeter class. • Creates a new object using the Greeter class and assigns this object to the myGreeterObject variable

  20. Viewing A Program That Uses An Object HelloApp2.java // This application displays a hello message on // the console by creating an instance of the // Greeter class and then calling the Greeter // object’s sayHello method. public class HelloApp2 { public static void main (String [ ] args) { Greeter myGreeterObject = new Greeter ( ) ; myGreeterObject.sayHello ( ) ; } } • Calls the myGreeterObject object’s sayHello method (this method in Greeter.java displays “Hello, World!”)

  21. Viewing A Program That Uses An Object HelloApp2.java // This application displays a hello message on // the console by creating an instance of the // Greeter class and then calling the Greeter // object’s sayHello method. public class HelloApp2 { public static void main (String [ ] args) { Greeter myGreeterObject = new Greeter ( ) ; myGreeterObject.sayHello ( ) ; } } • Greeter.java • // This class creates a Greeter Object • // that displays a hello message on • // the console • public class Greeter • { • public void sayHello ( ) • { • System.out.println (“Hello, World!”) ; • } • }

  22. Viewing A Program That Uses An Object Greeter.java // This class creates a Greeter Object // that displays a hello message on // the console public class Greeter { public void sayHello ( ) { System.out.println (“Hello, World!”) ; } } • Starts with end-of-line comments that identify the function of the program

  23. Viewing A Program That Uses An Object Greeter.java // This class creates a Greeter Object // that displays a hello message on // the console public class Greeter { public void sayHello ( ) { System.out.println (“Hello, World!”) ; } } • Declares a public class declaration. • Allows HelloApp2 class to access Greeter class

  24. Viewing A Program That Uses An Object Greeter.java // This class creates a Greeter Object // that displays a hello message on // the console public class Greeter { public void sayHello ( ) { System.out.println (“Hello, World!”) ; } } • sayHello method is declared using the public keyword so that it is available to other classes that use the Greeter class. • void keyword indicates that this method does not provide any data back to the class that calls it. • sayHello provides the identifier for the method

  25. Viewing A Program That Uses An Object Greeter.java // This class creates a Greeter Object // that displays a hello message on // the console public class Greeter { public void sayHello ( ) { System.out.println (“Hello, World!”) ; } } • Body of method consists of one statement. • Displays “Hello, World!” on the console

  26. Viewing A Program That Uses An Object HelloApp2.java // This application displays a hello message on // the console by creating an instance of the // Greeter class and then calling the Greeter // object’s sayHello method. public class HelloApp2 { public static void main (String [ ] args) { Greeter myGreeterObject = new Greeter ( ) ; myGreeterObject.sayHello ( ) ; } } • Greeter.java • // This class creates a Greeter Object • // that displays a hello message on • // the console • public class Greeter • { • public void sayHello ( ) • { • System.out.println (“Hello, World!”) ; • } • } • Defines a class that knows how to say “Hello, World!” • Creates an object from that class and asks that object to say “Hello, World!” • The application does not know exactly how the Greeter object displays on the console, what it will display, or how it will be displayed

  27. Viewing A Program That Uses An Object HelloApp2.java // This application displays a hello message on // the console by creating an instance of the // Greeter class and then calling the Greeter // object’s sayHello method. public class HelloApp2 { public static void main (String [ ] args) { Greeter myGreeterObject = new Greeter ( ) ; myGreeterObject.sayHello ( ) ; } } • Greeter.java • // This class creates a Greeter Object • // that displays a hello message on • // the console • import javax.swing.JOptionPane; • public class Greeter • { • public void sayHello ( ) • { • JOptionPane.showMessageDialog (null, • <<Hello, World!>>, <<Greeter>>, • JOptionPane.INFORMATION_MESSAGE); • } • } • Accomplishes the same thing by making the Greeter class to use a Java library class • No changes are required for HelloApp2.java

  28. Importing Java API Classes • import trivia: • import statements must appear at the beginning of a class file, even before any class declarations • You can include as many import statements as are necessary to import all the classes used by your program • import javax.swing.*; • This statement can be used to import all classes from a particular package Greeter.java // This class creates a Greeter Object // that displays a hello message on // the console importjavax.swing.JOptionPane; public class Greeter { public void sayHello ( ) { JOptionPane.showMessageDialog (null, <<Hello, World!>>, <<Greeter>>, JOptionPane.INFORMATION_MESSAGE); } } • import keyword to lets the compiler know that the program is using a class that is defined by the Java API • imported class is called JOptionPane

  29. Importing Java API Classes Greeter.java // This class creates a Greeter Object // that displays a hello message on // the console import javax.swing.JOptionPane; public class Greeter { public void sayHello ( ) { JOptionPane.showMessageDialog (null, <<Hello, World!>>, <<Greeter>>, JOptionPane.INFORMATION_MESSAGE); } } • The Java API is full of thousands of classes • Java groups classes into manageable groups called packages • JOptionPane is contained within the javax.swing package