Computer Science II
810:062 Section 01
Session 2 - Objects and
Responsibilities
The anatomy of an OO program
The world
consists of objects
that interact
to solve a problem.
Last class we considered a MemoPad
example
Allows for the storage of a collection of
memos that are associated with keys
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”.
public class MemoPadApp {
public static void main( String [] args ) {
MemoPad p = new MemoPad();
p.show();
} // end main
} // end class MemoPadApp
MemoPad’s collaborating objects
Field Panel
Button Panel
Message
Panel
MemoDatabase
key
value
MemoAssociation
MemoPad UML Diagram
MemoPad’s Constructor
public class
private
private
private
private
MemoPad extends CloseableFrame {
MemoDatabase database;
FieldPanel
fieldPanel;
ButtonPanel buttonPanel;
MessagePanel messagePanel;
public MemoPad() {
database = new DefaultMemoDatabase();
...
fieldPanel = new FieldPanel();
...
buttonPanel = new ButtonPanel( this );
...
messagePanel = new MessagePanel( "..No
message..”);
...
} // end MemoPad constructor
...
Objects interact by sending each
other messages
// in MemoPad class
public void 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
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
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
DefaultMemoDatabase class
import java.util.Hashtable;
public class DefaultMemoDatabase implements MemoDatabase
{
private Hashtable associations;
public DefaultMemoDatabase() {
associations = new Hashtable();
} // end DefaultMemoDatabase constructor
public boolean insert( MemoAssociation newEntry ) {
if ( containsKey( newEntry.key() ) )
return false;
associations.put( newEntry.key(),
newEntry.value() );
return true;
} // end insert
...
Group #1
Group #2
“Shawn” Bancroft
Lucas Bergmann
Paul Chen
Zach Clark
Dustin Collins
Mathew Dahms
Terrence Glover
Alex Habeger
Michael Jaramillo
Group #3
Tressa McCabe
Angela McCauley
Paul Nadler
Tony Syndergaard
Bryan Zubrod
private vs. public?
• What things are declared private?
• What things are declared public?
• Why are they declared private?
• Why are they declared public?
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.
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.
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..
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...
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!
Lunar Lander Example
• What objects can you identify?
• What behaviors must these
objects perform?
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.