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Computer Science II 810:062 Section 01 Session 2 - Objects and Responsibilities

Learn the anatomy of an OO program using the MemoPad model, exploring objects, interactions, and responsibilities in Java. Focus on classes, access modifiers, and creating database objects.

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Computer Science II 810:062 Section 01 Session 2 - Objects and Responsibilities

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  1. Computer Science II810:062 Section 01Session 2 - Objects and Responsibilities

  2. The anatomy of an OO program The world consists of objects that interact to solve a problem.

  3. Last class we considered a MemoPad example Allows for the storage of a collection of memos that are associated with keys

  4. Object-Oriented Programs • MemoPad models the piece of the world of interest. • main() is the “big bang” that creates one or more objects for this MemoPad “world”. publicclass MemoPadApp { publicstaticvoid main( String [] args ) { MemoPad p = new MemoPad(); p.show(); } // end main } // end class MemoPadApp

  5. MemoDatabase key value MemoPad’s collaborating objects Field Panel Button Panel Message Panel MemoAssociation

  6. MemoPad UML Diagram

  7. MemoPad’s Constructor publicclass MemoPad extends CloseableFrame {private MemoDatabase database;private FieldPanel fieldPanel;private ButtonPanel buttonPanel;private MessagePanel messagePanel;public MemoPad() { database = new DefaultMemoDatabase(); ... fieldPanel = new FieldPanel(); ... buttonPanel = new ButtonPanel( this ); ... messagePanel = new MessagePanel( "..No message..”); ... } // end MemoPad constructor ...

  8. Objects interact by sending each other messages // in MemoPad class publicvoid find() { String value = database.find(fieldPanel.currentKey());if ( value == null ) messagePanel.setMessage( "Key not found." );else { fieldPanel.setValue( value ); messagePanel.setMessage( fieldPanel.currentKey() + " : " + value ); } // end if } // end find

  9. Focus on MemoDatabase interface • its interface is the set of messages to which it should respond • but to make an database object we need a class definition that adheres to the MemoDatabase interface, i.e., that implements the specified methods • here the DefaultMemoDatabase class is used

  10. Interface for the memo database public interface MemoDatabase { public String find ( String key ); public boolean remove ( String key ); public boolean insert (MemoAssociation m); public boolean containsKey( String key ); } // end interface MemoDatabase

  11. DefaultMemoDatabase class import java.util.Hashtable;publicclass DefaultMemoDatabase implements MemoDatabase{private Hashtable associations;public DefaultMemoDatabase() { associations = new Hashtable(); } // end DefaultMemoDatabase constructor publicboolean insert( MemoAssociation newEntry ) {if ( containsKey( newEntry.key() ) )returnfalse; associations.put( newEntry.key(), newEntry.value() );returntrue; } // end insert ...

  12. private vs. public? • What things are declared private? • What things are declared public? • Why are they declared private? • Why are they declared public?

  13. Access Modifiers • A Java programmer can control access to a class and its parts (both data and actions) through the use of Access Modifiers. • Any item that is declared public can be accessed by any piece of code that knows the name of the item. • Any item that is declared private can be accessed only be code within the same class.

  14. So when do we use what? Because of this, we normally followed the following conventions -- • Classes are usually declared public, because we want to create instances of the class that can help us solve a problem. • Methods are sometimes declared public, because they correspond to messages that other objects can send to the instance. • Instance variables are private, because we want the instance to have exclusive control over changes to their values.

  15. Access Modifiers But what if want to do things a little differently?  Could we have-- • a private class? • Sure.  You might want to declare a private class because you don’t want just anyone to create instances of it. • a private method? • You might want to declare a private method, because it is not a part of the object’s public interface. Perhaps it is simply a helper to one of the public methods. • a public instance variable? • You might want to declare a public instance variable simply for your convenience as a programmer. BUT..

  16. Homework #1 • Write a different class that implements the MemoDatabase using a different representation (e.g., array, ArrayList, vector, two arrays, ...) • Modify the MemoPad class to use your class. • Submission details to follow...

  17. Lunar Lander Example • Lander starts some initial distance above the surface of the moon with a limited amount of fuel • As time ticks off, the lander falls toward the moon, gaining speed according to the gravity of the moon. • The user can request to thrust the lander's engines which counteract the force of gravity and slow down the vehicle.  However, this also burns fuel. • The user can request multiple burns in any time interval. • If the vehicle hits the surface traveling slow enough, then the user wins!

  18. Lunar Lander Example • What objects can you identify? • What behaviors must these objects perform?

  19. For next class • Read chapter 3. • Read over all the code for the MemoPadApp. Much of the GUI (buttons, etc.) stuff will not make sense, but it might give you a feel for the code.

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