Transcript Creating and Using Objects

Why So Much for So Little?
• We want to use computers to solve complex
tasks.
• Complex problems require powerful
programming languages.
• Powerful programming languages have
more rules than simpler languages.
OOP with Java, David
J. Barnes
Creating and Using Objects
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What are Object-Oriented
Languages?
• Support an O-O view of problem solving.
• Objects categorized into classes.
• Objects interact with each other.
– Objects have behavior.
– Objects maintain state.
• Behavior and state are intimately related.
• Software reuse is facilitated.
OOP with Java, David
J. Barnes
Creating and Using Objects
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What are pure Object-Oriented
Languages?
• Support an O-O view of problem solving
exclusively. Examples include Smalltalk,
Emerald.
• Hybrid OO languages allow mixing values
and objects. Examples: Java, C++, Simula
68.
• Hybrid languages are considered messy by
the OO purists.
OOP with Java, Eric Jul
Creating and Using Objects
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Modeling the Operation of a Port
• Classes and objects in the scenario.
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The port.
Ships moving in and out.
Cargo and associated transportation.
Passengers, vehicles, car parks.
Customs officers, pilots.
Aspects of the natural environment.
OOP with Java, David
J. Barnes
Creating and Using Objects
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Interactions in the Port
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Transfer of cargo between ship and dock.
Passengers embarking and disembarking.
A pilot joining or leaving a ship.
A ship requesting permission to enter.
The weather affecting ship movements.
OOP with Java, David
J. Barnes
Creating and Using Objects
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Classes and Objects
• A class definition provides a description of
a typical object within that class.
– CargoShip, PassengerFerry.
• An individual object is an instance of a
class.
• Definition of behavior and attributes.
OOP with Java, David
J. Barnes
Creating and Using Objects
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Attributes
• Attributes store the state of an object.
• The state stored in an attribute is often a
reference to another object.
• Simple state is, in hybrid languages,
represented as values (non-object values).
OOP with Java, Eric Jul
Attributes
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Class and Instances
• Liner might capture the common
characteristics of ocean liners.
– RMS Titanic and RMS Olympic might be two
instances of that class.
• Class definition is like a blueprint or
template.
– Color, size, pattern might vary, but instances of
the same class come from the same mold.
OOP with Java, David
J. Barnes
Creating and Using Objects
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Creating an Object
// An illustration of object creation.
// (The ship class is assumed to preexist.)
class ShipMain1 {
public static void main(String[] args){
// Define a method variable to refer to
// a Ship object.
Ship argo;
// Construct a new Ship object.
argo = new Ship();
}
}
OOP with Java, David
J. Barnes
Creating and Using Objects
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Object Interactions
• Communication between people:
– Information: “I am going to a party.”
– Question: “What is the time?”
– Order/Request: “Please buy me some gum.”
• Objects communicate in similar ways.
– Passing messages to each other.
– Actor/Agent, Client/Server relationships.
OOP with Java, David
J. Barnes
Creating and Using Objects
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Sending a Message to a Ship
// An illustration of message sending.
class ShipMain2 {
public static void main(String[] args){
// Define a method variable.
// Make it refer to a new Ship object.
Ship argo = new Ship();
// Ask the ship to report its position,
// course and speed.
argo.report();
}
}
OOP with Java, David
J. Barnes
Creating and Using Objects
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Moving a Ship
// An illustration of ship movement.
class ShipMain3 {
public static void main(String[] args){
Ship argo = new Ship();
// Ask the ship to move.
argo.move();
}
}
OOP with Java, David
J. Barnes
Creating and Using Objects
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Passing Arguments with
Messages
• The report and move messages convey
implicit information.
– The Ship uses its own internal state to respond.
• Some messages need to be accompanied by
further explicit information.
– “Change course to ...”
– “Reduce speed to ...”
OOP with Java, David
J. Barnes
Creating and Using Objects
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Arguments
• Explicit information can be passed to an
object in a message by adding arguments to
the message.
• Arguments are objects containing
information.
• In hybrid languages, such as Java,
arguments can also be values.
OOP with Java, David
J. Barnes
Creating and Using Objects
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Passing Arguments
class ShipMain4 {
public static void main(String[] args){
Ship argo = new Ship();
argo.report();
argo.move();
// Ask it to change course, move and
// report its new settings.
argo.setCourse(90);
argo.move();
argo.report();
}
}
OOP with Java, David
J. Barnes
Creating and Using Objects
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Receiving Replies
• Human communication often requires a
reply.
– “What is the time?”
– “Are there any free berths in the port?”
– “What is the square-root of 2?”
• Similarly, we can request information from
a Ship.
• We often store the answer somewhere.
OOP with Java, David
J. Barnes
Creating and Using Objects
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Simple Example of Reply
• Ship’s speed is returned as a value – a
double which here merely is a number,
e.g., 17.3
• The value, we may assume, is in knots.
• The number is stored and used to request
the ship to go one knot faster.
OOP with Java, David
J. Barnes
Creating and Using Objects
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Requesting a Ship’s Speed
class ShipMain5 {
public static void main(String[] args){
Ship argo = new Ship();
argo.report();
// Define a variable to hold the speed.
double currentSpeed;
// Ask the ship what its current speed is.
currentSpeed = argo.getSpeed();
// Increase the ship's speed.
argo.setSpeed(currentSpeed+1);
argo.move();
argo.report();
}
}
OOP with Java, David
J. Barnes
Creating and Using Objects
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Variables and Objects
• Variables and objects are different.
• A variable may refer to a single object.
• Different variables may refer to the same
object - aliases.
• A variable may be switched from one object
to another.
• A variable may be uninitialized.
OOP with Java, David
J. Barnes
Creating and Using Objects
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Variables Contain References
• A variable contains either a value or a
reference to an object.
• A reference can be thought of as a pointer to
an object.
• Illustrated graphically.
OOP with Java, Eric Jul
Creating and Using Objects
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The Main Method's Class
• We (Barnes and Jul) choose never to create
an object of the main method’s class.
• This approach is not followed by many
other authorities.
• The main method is a static method.
– Static methods have special properties that we
shall explore in due course.
• A messy hack—just live with it for now.
OOP with Java, David
J. Barnes/Eric Jul
Creating and Using Objects
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Review
• O-O problem solving involves identifying
classes, objects and object interactions.
• Objects maintain state and exhibit classdefined behavior.
• Instances of the same class behave in similar
ways.
• Message passing illustrates object interaction.
OOP with Java, David
J. Barnes
Creating and Using Objects
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Review (cont.)
• Messages may contain additional
information: in the form of arguments.
• A result may be returned as the result of a
message.
• Variables and objects are distinct.
– Variables switch between different objects.
– An object may be simultaneously referred to by
different variables.
OOP with Java, David
J. Barnes
Creating and Using Objects
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Review (cont.)
• Variables contain references.
• In hybrid languages, variables may
alternatively contain values.
• References can be thought of as pointers.
OOP with Java, David
J. Barnes & Jul
Creating and Using Objects
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