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Class Relationships Part 1 : Composition and Association. CS 21a: Introduction to Computing I First Semester, 2013-2014. Class Relationships. More complex programs require multiple classes It is typical for objects to have fields that refer to other objects - PowerPoint PPT Presentation
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Class Relationships Part
1: Composition and Association
CS 21a: Introduction to Computing I
First Semester, 2013-2014
Class Relationships►More complex programs require multiple
classes► It is typical for objects to have fields that
refer to other objects► In class A, there may be a field whose
type is class B► There is a class relationship between A and
B►Examples of class relationships
► Composition or Aggregation► Association
Object Composition►Objects can be composed of other
objects►Have references to "parts" of the
class as fields of the class►Objects can create instances of
other objects ►Also called aggregation
Encapsulation►The idea of "hiding" implementation
details►What’s more important is the interface
►Users don’t need to know how a method works, just that it’s there and it works
►Objects know how to handle themselves …►users don’t need to know
Encapsulation►Data should be hidden with the object
that it belongs to►Changes to data should be done via
methods of object that contains the data►Again … objects should know how to
handle the data►Allows the object’s programmer to
change data representation►This is why we make fields private
Bank Example► A Bank encapsulates a set of BankAccount
objects► What’s important is the external interface► Users don’t need to know what goes on
inside the Bank, and Bank doesn’t need to know what goes on inside BankAccount
getBalance( "marsha")
withdraw( "john", 200 )
Bank and BankAccountBankAccou
nt balance1000
BankAccount balance
2000
BankBankAccount john
BankAccount marsha
Object Composition in Javapublic class Bank { private BankAccount john; private BankAccount marsha; public Bank() { john = new BankAccount( 1000 ); marsha = new BankAccount( 2000 ); } public void deposit(String name, double amt) { if ( name.equals( "john" ) ) john.deposit( amt ); ... } ...}
There are BankAccount fields in Bank
The fields are instantiated in Bank’s constructor
Bank has its own deposit method that calls BankAccount’s deposit method on the appropriate object
Using a Bank ObjectBank b = new Bank();b.deposit( "john", 200 );b.withdraw( "marsha", 100 );System.out.println( b.getBalance( "john" ) )
;System.out.println( b.getBalance( "marsha
" ) ); Prints:12001900
Object InteractionBankAccou
nt balance1000
BankAccount balance
2000
BankBankAccount john
BankAccount marsha
deposit( "john", 200 )
deposit( 200 )
Calling deposit on the Bank object causesdeposit to be called on a BankAccount object
The whole manages its parts
►In effect, Bank is a manager of BankAccounts
►Transactions are carried out through the Bank object but ultimately uses/affects a BankAccount object
►The one calling Bank’s methods does not even need to know about the BankAccount class this is exactly what encapsulation is about!
Composition with Array(List)s
►An object can be composed of a fixed number of other objects
►A fixed number of fields can implement that composition
►But in general, an object may be composed of an arbitrary number of instances of another object
Composition with Array(List)s
public class Bank { private ArrayList<BankAccount> accounts;
public Bank() { accounts = new ArrayList<BankAccount>(); }
public void openAccount( String name, double init ) { accounts.add( new BankAccount(name, init) ); } ...
There is an ArrayList of BankAccount objects in BankThe ArrayList is instantiated in Bank’s constructor
The actual BankAccounts are instantiated when they are needed.
Composition with Array(List)s
...
public void deposit( String name, double amt ) { BankAccount acct = find( name ); if( acct != null ) acct.deposit( amt ); }
...}
Deposit now requires a search for the correct BankAccount instance (done with a loop instead of hard-coded if-else)
Object Association►Association: a weaker kind of
relationship►Examples:
► Borrower and Book in a library system► Student, Class, Teacher in a university
system► WaterTank and Faucet
WaterTank-Faucet Example►A WaterTank object has methods that
cause it to be filled up with water or to dispense water
►A Faucet object is connected to a WaterTank and has methods to dispense or drain water
►Faucet needs a way to connect/associate to a WaterTank object► Note: we can connect several faucets to a
single water tank
WaterTank-Faucet Association
►Option 1: create WaterTank object, create Faucet object(s), and call a method on Faucet:
w = new WaterTank();f1 = new Faucet();f2 = new Faucet();f1.connect( w ); f2.connect( w );
►Option 2: Faucet’s constructor has a WaterTank parameter
w = new WaterTank();f1 = new Faucet( w ); f2 = new Faucet( w );
WaterTank and Faucetf1: FaucetWaterTank
tank
f2: FaucetWaterTank
tank
WaterTankdouble
waterLeft100.0
Object Association in Javapublic class Faucet
{
private WaterTank tank; public Faucet( WaterTank w )
{
tank = w; }
...
public void connect( WaterTank w )
{
tank = w; } ...
}
The association is represented by a WaterTank field
The field can be set in the constructor…
…or in a method
Object Interactionf1: FaucetWaterTank
tank
f2: FaucetWaterTank
tank
WaterTankdouble
waterLeft100.0
dispense( 20.0 )
flush()
dispense( 20.0 )
dispense( 80.0 )
Object Interactionpublic class Faucet{ private WaterTank tank; public Faucet( WaterTank w ) { tank = w; } … public void dispense( double
amt ) { tank.dispense( amt ); } public void flush() { tank.dispense(
tank.getWaterLeft() ); }}
public class WaterTank
{
private double waterLeft = 0;
...
public void fillTank() ...
public void dispense( double amt )
{
waterLeft = waterLeft - amt;
}
public double getWaterLeft()
{
return waterLeft;
}
}
Using Faucet andWaterTank Objects
WaterTank w = new WaterTank();WaterTank x = new WaterTank();w.fillTank(); // fills tank to capacity, say 100 gallonsx.fillTank(); // fills tank to capacity, say 100 gallonsFaucet f1 = new Faucet( w );Faucet f2 = new Faucet( w );f1.dispense( 20 );f2.flush();f1.connect( x );f1.dispense( 50 );System.out.println( w.getWaterLeft() );System.out.println( x.getWaterLeft() );
Prints:050
Composition versus Association
►In both cases, there appears to be a one-to-many relationship►One Bank, many BankAccounts►One WaterTank, many Faucets
►Why is it not correct to say that the WaterTank is composed of Faucets?
Composition versus Association
►Composition: The one manages the many► Implies whole-part relationship► The body is composed of cells, but the
cells do not need to be aware of the body►Association: The many use the one
► Does not imply a whole-part relationship► Many parasites can feed off a host, but
the host does not need to be aware of the parasites
Composition versus Association
►Composition: the whole is dependent on the parts
►Association: there is no such dependency
Composition► It doesn’t make sense to have Banks if
there is no concept of BankAccount (the class)► Although it’s perfectly okay to start a
Bank without any BankAccounts (the instances) yet
► In other words, if BankAccount is removed from the system, it no longer makes sense to retain Bank.
Association►It still makes sense to have a
WaterTank even if Faucets were never invented
►Faucets don’t need to be connected to WaterTanks. They can be connected to pipes, for example.
►In other words, if Faucet is removed from the system, it’s perfectly fine to retain WaterTank, and vice versa.
An Integrated Example►Grocery environment►Products are stocked and sold in the
grocery►Cashiers are front-end objects that carry
out a sale through a back-end price-and-stock Manager object► Multiple cashiers are associated to the
Manager object►The Manager object aggregates Product
objects (where prices and stock levels are stored)
Grocery Examplec1:
Cashier
c2: Cashier
Manager
product apples
product oranges
product pomelos
apples:Product
oranges:Product
pomelos:Product
Transactions are carried out through the Cashier objects
Product objects may be updated as a result
Prices are checked and purchase requests are made thru the Manager object
Exercise►Can you identify the class
relationships present in the Project 1 solution?