This document discusses classes and objects in Java. It explains how to write classes by defining attributes and methods, and how to create objects from classes using constructors. It covers encapsulation through visibility modifiers and accessor/mutator methods. Classes provide templates for objects, which have their own state defined by attribute values. Methods can manipulate attributes and call other methods.

1. Week 8
Classes & Objects:
Writing Classes,
Creating and Using
Objects

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3. Classes & Objects?
• Last time, we looked at the idea of classes to
group together data of different types about
something
• A Java class provides a template for the things in
the class, e.g. all bank accounts
• An object, built from the class template,
represents a single thing, i.e. one account
• Each object in the class will have the same set of
attributes, but each object can have different
values for the attributes
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4. Why Classes & Objects?
• Prime purpose: to group together data items
related to one thing
• Also: we can include methods in the class, so that
the data items can be manipulated in a consistent
way
• Also: we can protect the data items, so that the
only access to them is through the methods
provided
• This ensures that, despite bugs or malicious code,
our data remains intact and useful
 e.g. what if someone tried to withdraw money from a
bank account without knowing the PIN?
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5. Writing Classes
• The programs we’ve written in previous examples
have used classes defined in the Java standard
class library
• Now we will begin to design programs that rely on
classes that we write ourselves
• The class that contains the main() method is just
the starting point of a program
• True object-oriented programming is based on
defining classes that represent objects with well-
defined characteristics and functionality
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6. Structure of A Class
• A class can contain data declarations and method
declarations
int size, weight; Data declarations
char category;
Method declarations
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7. Classes: State & Behaviour
• The values of the attribute data define the state of
an object created from the class
• The functionality of the methods define the
behaviors of the object
• Given a class, we can create or construct an object
from the class, with its own state
• Then, the object uses its own methods to
manipulate its attributes
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8. Designing the BankAccount Class
• Let's go back to the BankAccount class from last
time. Here is what we designed so far:
public class BankAccount
{
int accountNumber;
double balance;
int PinNumber;
}
• These are the attributes, or the state, that each
object constructed from the class will have.
• The first method we need to write is a constructor
method.
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9. Constructors
• A constructor method is a special method that is
used to set up an object when it is initially created
from the class template
• A constructor has the same name as the class
• It normally takes as inputs the values that will be
put into the object's attributes
public BankAccount(int acct,
double bal,
int pin)
{
accountNumber= acct;
balance= bal;
PinNumber= pin;
}
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10. Constructors
• Note that we can use the names of the attribute
variables
• That's because the constructor method makes a
new object, and each object has its own attribute
variables
• Scope rule: if a variable isn't declared in a method,
look for it as an attribute variable in the class
• We can write several different constructors, as
long as the list of input parameters is different for
each one.
• Let's write a constructor where no balance is
given. What should the balance be set to?
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11. Multiple Constructors
public BankAccount(int acct,
int PinNumber)
{
accountNumber= acct;
balance= 0.0;
this.PinNumber= PinNumber;
}
• We set the balance to 0.0, thus keeping the
object's attributes safe and useful
• Note: conflict between the input parameter's name
and the name of the attribute
• To resolve this: we use this. to specify the
attribute in this object
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12. Making An Object in Bluej
• Compile, and right-click on the class file
• Choose one of the constructor methods
• Fill in the input parameters & click Ok
• The object appears down the bottom
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13. Inspecting An Object in Bluej
• Right-click on the object, and choose Inspect
• Bluej shows you the values of the attributes
• This creating/inspecting is only useful when
testing the classes that you write
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14. The toString() Method
• We need to add more methods to the class, to
make it useful
• All classes that represent objects should define a
toString() method: Java expects this
• The toString() method returns a String that
represents the object in some way
• It is called automatically when an object is
concatenated to a string or when it is passed to
the println() method
• It's up to you, the designer, to think of how to best
represent the object's attributes as a string
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15. toString() for BankAccount
public String toString()
{
return(“Account “ + accountNumber +
“, $” + balance);
}
• Once you have constructed an object with Bluej,
you can right-click on it, and run any of its
methods
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16. Gettor or Accessor Methods
• Accessor methods allow external access to the
object's attributes
• They also protect the object from unauthorised
use
• For the BankAccount class:
 int getAccountNumber()
 int getBalance(int pin)
 no method to get the PIN: user should know it
• Note that getBalance() requires knowledge of
the PIN: an example of the method protecting the
data kept inside the object
• Again, once an object is created, Bluej allows you
to run any of its methods on itself
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17. Settor or Mutator Methods
• Settor methods allow the object's attributes to be
set or updated
• Again, they also protect the object from being
corrupted
• For the BankAccount class:
 boolean deposit(double amount)
 boolean withdraw(int pin, double amount)
 boolean setPIN(int oldpin, int newpin)
• Anybody can deposit money, only people with the
PIN can withdraw or set a new PIN
• All methods in this example return true if the
operation succeeded.
• Note: all methods check their input values!
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Note the strict requirements for withdraw() 4-17

18. Instance Data
• The balance variable in the BankAccount class is
called instance data because each instance
(object) that is created has its own version of it
• A class declares the type of the data, but it does
not reserve any memory space for the variables
• Every time a BankAccount object is created, a new
balance variable is created as well
• The objects of a class share the methods in the
class, but each object has its own instance
variables
• That's the only way two objects can have different
states
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19. Scope and Local Variables
• The scope of a variable is the area in a program in
which that variable can be referenced (used)
• Variables declared at the class level can be
referenced by all methods in that class
• Variables declared within a method can be used
only in that method
• Variables declared within a method are called local
variables
• In the BankAccount class, the variable interest
is declared inside the calculateInterest()
method - it is local to that method and cannot be
referenced anywhere else
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20. Encapsulation
• We can take one of two views of an object:
 internal - the details of the variables and methods of the
class that defines it
 external - the services that an object provides and how
the object interacts with the rest of the system
• From the external view, an object is an
encapsulated entity, providing a set of specific
services
 accessor methods, settor methods etc.
• These services define the interface to the object
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21. Encapsulation
• One object (called the client) may use another
object for the services it provides
• The client of an object may request its services
(call its methods), but it should not have to be
aware of how those services are accomplished
• Any changes to the object's state (its variables)
should be made by that object's methods
• We should make it difficult, if not impossible, for a
client to access an object’s variables directly
• That is, an object should be self-governing
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22. Encapsulation
• An encapsulated object can be thought of as a
black box -- its inner workings are hidden from the
client
• The client invokes the interface methods of the
object, which manages the instance data
Client Methods
Data
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23. Visibility Modifiers
• In Java, we accomplish encapsulation through the
appropriate use of visibility modifiers
• A modifier is a Java reserved word that specifies
particular characteristics of a method or data
• Members of a class that are declared with public
visibility can be referenced anywhere
• Members of a class that are declared with private
visibility can be referenced only by the methods
within that class
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24. Visibility Modifiers
• Public variables violate encapsulation because
they allow the client to “reach in” and modify the
values directly; this is usually very bad
• Therefore instance variables should not be
declared with public visibility
• It is acceptable to give a constant public visibility,
which allows it to be used outside of the class
• Public constants do not violate encapsulation
because, although the client can access it, its
value cannot be changed
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25. Visibility Modifiers
• Methods that provide the object's services are
declared with public visibility so that they can be
invoked by clients
• Public methods are also called service methods
• A method created simply to assist a service
method is called a support method
• Since a support method is not intended to be
called by a client, it should not be declared with
public visibility
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26. Visibility Modifiers
public private
Violate Enforce
Variables
encapsulation encapsulation
Support other
Provide services
Methods methods in the
to clients
class
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27. Accessors and Mutators
• Because instance data is private, a class usually
provides services to access and modify data
values
• An accessor method returns the current value of a
variable
• A mutator method changes the value of a variable
• The names of accessor and mutator methods take
the form getX and setX, respectively, where X is
the name of the value
• They are sometimes called “gettors” and “settors”
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28. Mutator Restrictions
• The use of mutators gives the class designer the
ability to restrict a client’s options to modify an
object’s state
• A mutator is often designed so that the values of
variables can be set only within particular limits
• For example, the setFaceValue mutator of the
Die class should have restricted the value to the
valid range (1 to MAX)
• We’ll see in Chapter 5 how such restrictions can
be implemented
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29. Methods Calling Method
• Methods in your class are normal Java methods
like we have seen before
• They can have their own variables, and have
normal Java control structures
• Methods can call other methods
• If the other method is in the same class, we can
simply use its name
• If the other method is in a different class, or is a
method inside an object, we have to write the
name of the class/object, dot, then the method
name
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30. Method Control Flow
• If the called method is in the same class, only the
method name is needed
compute myMethod
myMethod();
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31. Method Control Flow
• The called method is often part of another class or
object
main doIt helpMe
obj.doIt(); helpMe();
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32. Using Objects in Java
• We have shown you how to create objects with
Bluej
• This is only for testing. Normally, you would write
a client program that uses a class to create objects
from that class
• For example, the MyBank program is an example
of a program that makes and uses BankAccount
objects
• WARNING!!! When you declare
BankAccount userAcct;
this DOES NOT create an object!
• It only declares the name of an object in the future
• You must still construct an object for it to be
created
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33. Constructing Objects in Java
• Use the new command to construct an object;
provide a set of parameters that will match one of
the constructor methods, e.g
computerAcct= new BankAccount(6475, 1123);
• Java runs the constructor, and a new object is
stored in the computer's memory.
• It doesn't appear in the Bluej window.
• To see it, use the debugger to inspect the
computerAcct variable
• You can pass any parameters to a constructor:
literal constants, variables holding values passed
in by the user etc.
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34. Using Objects in Java
• Once an object is constructed, we can use any of
the methods from the class, via the object.
object's name.method(parameters);
• For example:
userAccount.calculateInterest(0.08);
• In fact, you have been constructing and using
objects already:
Scanner scan = new Scanner(System.in);
question= scan.nextLine();
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35. The static Keyword
• A static method is a special method that does
not require an object to invoke
• It is used when a method is not specific to any
object
• A static method is invoked through the name of
the class rather than the name of an object of that
class
• For example, Math.max(int, int) returns the
value of the larger of two integer arguments
• Examples of use:
big = Math.max(a, b);
System.out.println( big );
• All of your main() programs have been static, so
you could run them without objects
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36. Classes: Not Just Real Things
• Classes are just to group related data together,
and to provide a set of methods to correctly
manipulate that data
• A Class, and its objects, don't have to represent
real things; just a group of related data
• Each pixel on a screen has a <red,green,blue>
colour, and possibly a transparency percentage
public class Pixel
{
byte red,green,blue;
byte transparency;
}
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37. Strings are Objects
• Strings in Java are objects, and there is a String
class that provides a useful set of methods to
manipulate Strings
• Each string literal (enclosed in double quotes)
represents a String object
• Once a String object has been created, neither its
value nor its length can be changed
• However, several methods of the String class
return new String objects that are modified
versions of the original
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38. Useful String Methods
• See the on-line String class documentation for
more details
 charAt() - Get a char at a particular position
 compareTo() - Compare a string to another one
 endsWith() - See if a string ends with a suffix
 indexOf() - Find first position of a char in string
 length() - Returns the string's length
 replace() - Replaces all of ch1 with ch2 in string
 trim() - Remove leading/trailing whitespace
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39. The Last Slide!
• Types that start with lowercase in Java are
primitive types, not classes/objects
 byte, short, int, long, char, float, double, boolean
• Any type that starts with an uppercase letter is a
class, and you must construct objects from the
class, and use class methods
• The String class is one exception, you can do
String message = “This is a message”;
• Just as we can have arrays or primitive types, we
can also have arrays of objects. To use the method
of one object in the array, we do
bankaccountList[8].withdraw(1234, 75.00);
array name index method
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