CMP-MX21: Lecture 6
Objects & Methods 1
Steve Hordley
Overview
So far we have seen the basics of JAVA: we know most of the words which make up the language and have seen some simple programs
In the remainder of the course we will look at how we design and construct more complex programs in JAVA
JAVA is an object oriented programming (OOP) language. To use it properly we need to understand what this means
What is an object?
In the real world we encounter objects all around us: ourselves, cats, dogs, cars, and houses etc. are all objects
All objects can be characterised by their state and their behaviour
An object’s state describes its attributes. For example a dog has a certain breed and colour and it can be hungry, thirsty or tired etc.
An object’s behaviour describes what it does. For example a dog can be barking, sitting, running or eating etc.
What is an object?
An object does not need to be something physical. It might be a bank account or a song or something even more abstract like a mathematical theory
In JAVA and OOP in general an object is simply a model of some real world object.
When designing an object oriented program we begin by determining which objects we need to model
An Example
Suppose we want to write a program to look after a bank’s customer accounts. We will need to keep track of the banks customers and their accounts. We will need to be able to keep up-to-date records of a customer’s personal information such as their address and to record and track any transactions which occur on their account.
An example
Given a rough program specification of this form the first step is to extract potential objects which we need to model.
Customer
Personal Information
Bank Account
AddressTransactions
An example
Once we have extracted all the objects we need to model we next need to think about the following questions:
What attributes of the objects do we need to model?
How will the different objects interact with one another?
What actions or behaviours do each of the objects have?
Note: this process is likely to be an iterative one: the actions we need to implement will affect what attributes of an object we will model and visa versa
Object Oriented Programming
Good modelling of objects and their interactions/behaviours is the key to making our code re-usable:
A good model makes it easy to extend our programs to perform new tasks
Once all the objects are modelled we must convert the model to JAVA code
Once this code is written we can use it to perform the required tasks.
Object Oriented JAVA
Key to modelling objects in JAVA is the concept of a class
We can think of a class as a blueprint for an object: it tells us how to make a particular object.
Two types of information are contained in a class: the attributes (or states) that an object has and the behaviours (or actions) it has
Attributes are represented using variables while behaviours are implemented with methods (we will discuss methods later).
An example
Class: bank account
Account number
Account holder
Account balance
Make Deposit
Make Withdrawal
Calculate Interest
These are the class attributes or states.
These are the behaviours or actions which are associated
with the bank account
Continuing with our example, let us begin by designing a simple class to represent a bank account. A simple bank account might be modelled thus:
Coding the class in JAVA
class bankAccount {
// Class variables (data)
private String accountName;
private String accountHolder;
private float accountBalance;
// Class actions (methods)
…
}
Coding the class in JAVA
class bankAccount {
...
}
A class definition begins with the class keyword and is contained between curly brackets {}
private String accountName;
private String accountHolder;
private float accountBalance;
Class attributes (data) are represented as variables which are declared in the same way as we have seen variable declarations previously:
The private keyword
private String accountName;
Note that the variable declaration here is modified with the private keyword:
Declaring a variable private means that it cannot be changed by code which is not in the class definition in which the variable is declared.
Keeping variables private to a class makes more work for the person who designs the class: we need to write code to change variables
The private keyword
Often changing a variable is not a simple matter: we might have to perform a series of checks to ensure that the variable is changed to a valid value
By forcing the change to be coded in the class definition we ensure that whenever the class is used, variables are changed in a valid way
This approach also has the advantage that the code to change (and check validity of) a variable only needs to be written once. It can then be called (used) many times with no further programming effort
The public keyword
String accountName;
In our programs so far we are used to seeing variable declarations of the form:
This declares a public variable rather than a private one. We can make this explicit by writing:
public String accountName;
If neither modifier public or private is included the variable defaults to public
The public keyword
A public variable can be modified outside of the class in which it is defined
When designing and implementing classes we should make all our variables private unless we can think of a good reason to make them public
There is a third modifier called protected which is somewhere between private and public. We will not discuss the function of this keyword in this course.
Classes versus objects
So far we have only a class: a blueprint of an object. We use the class definition to create actual instances (objects) of the given class:
Class: bank Account
Account number
Account holder
Account balance
Make Deposit
Make Withdrawal
Calculate Interest
Object 1
13456302
D. J. Smith
350.23
Object 2
63817845
R. G. Hunt
5034.62
Object 3
49714294
W. H. Freeman
534.60
Creating objects from classes
// Creating a bank account object
class myProgram {
public static void main( String [] args ) {
bankAccount myBankAcct;
myBankAcct = new bankAccount();
}
Class name
Call to the class constructor
method
We can define a particular instance of a class in a program with the following piece of code:
Object name
Creating objects from classes
bankAccount myBankAcct;The first line is just a variable
declaration. It declares a reference variable of type bankAccount which
is called myBankAcct
Remember a reference variable holds an address: it tells us where to find the object of type bankAccount.
After this line myBankAcct holds the address null: we haven’t yet actually created the object so it doesn’t have an address
Creating objects from classes
mybankAcct = new bankAccount();
It is the second line which actually creates the bankAccount object:
The keyword new is used to create a new object. The new keyword reserves the memory for the object which is being created.
The new keyword must be followed by a constructor method for the class of object which is being created.
But what is a constructor method and what does it do?
A word about methods
In JAVA a method is essentially a set of instructions which are given a particular name:
myMethod() {
statements;
}
The method name followed by a set of
parentheses denotes the start of a method.
The statements to be performed by the
method are placed between curly brackets
A word about methods
When we want to execute the statements in a method we call the method from our program:
// A simple program
class myProgram {
public static void main( String [] args ) {
myMethod();
}
This line “calls” the method with name myMethod
A word about methods
mybankAcct = new bankAccount();
Our code to create a bank account object contains a call to a method:
This method called bankAccount() has the same name as our class. It is a special method called a constructor method.
Whenever we create a class, JAVA automatically creates an associated method: the constructor method. It’s job is to initialise the values of the object’s variables
The constructor function
MEMORY
myBankAcctnull
MEMORY
null
0.0
Account numberAccount holderAccount balance
The default constructor method which JAVA creates assigns all an object’s variables their default values
(Created by the variable declaration)
(Created by new)
Creating multiple objects
// Creating a bank account object
class myProgram {
public static void main( String [] args ) {
bankAccount myBankAcct, anotherAcct;
myBankAcct = new bankAccount();
anotherAcct = new bankAccount();
}
We can declare as many instances of a class (i.e. objects) in our programs as we require:
Each time we create (construct) an object, memory is set aside for it by the constructor function:
Creating multiple objects
null
null
null
null
0.0
0.0
myBankAcct
anotherAcct
Acct. no.
Acct. holder
Acct. balance
Acct. no.
Acct. holderAcct. balance
Class variables
Every time we create an object we create new instances of all the relevant variables: e.g. creating a bank account object creates an account number, an account name and a balance
This makes sense because all these variables are particular to a given object: each bank account has a different number, and a potentially different holder and balance
Sometimes however, objects will contain variables which are common to all variables in the class
Class variablesFor example, suppose we extend our bank account class to include an interest rate. We could extend our class definition to add an extra variable:class bankAccount {
// Class variables (data)
private String accountName;
private String accountHolder;
private float accountBalance;
private float interestRate;
// Class actions (methods)
}
interestRate variable added here
Class variables
Now, each time we create (construct) a bank account object JAVA reserves space for four variables:
null
null
0.0
Acct. holder
0.0
Acct. no.
Acct. balanceInterest Rate
myBankAcct
But, the interest rate has a value which is common to all bank accounts. So it would be more efficient if we could somehow avoid keeping a copy of this variable with every object.
Class variables
We achieve this in JAVA by defining interestRate to be a class variable:
private static float interestRate;
A class variable is created by modifying the variable declaration with the keyword static.
Note: class variables are sometimes called static variables. However, this is a slightly confusing term since static variables are not fixed as the name suggests, rather they can change just as other variables.
Class variables
A class variable is created when the JAVA interpreter loads the class in which it is defined.
A class variable exists independently of any objects of the class. If our program has 10 bankAccount objects, there will still be only a single interestRate variable.
Using classes
Whenever we define a class in JAVA we should save it in its own file and this file should have the same name as the class with the .java extension.
So, our bankAccount class will be saved in a file called bankAccount.java
To use the class in our programs we must first compile the class:
C:\myjavacode>javac bankAccount.java
On successful compilation the JAVA compiler will produce a file called bankAccount.class
Using classes
Next, we need to write the code that actually uses the bankAccount class. (For example the program we saw earlier which creates a bankAccount object).
C:\myjavacode>javac myProgram.java
C:\myjavacode>java myProgram
We then compile and run this program in the usual way:
Note: To use the bankAccount class JAVA needs to be able to find the bankAccount.class file. Putting it in the same directory as our program code ensures this.
Classes and programs
Note: all the programs we have written so far have in fact been class definitions:
// Creating a bank account object
class myProgram {
public static void main( String [] args ) {
bankAccount myBankAcct, anotherAcct;
myBankAcct = new bankAccount();
}
They all begin with the class keyword. In fact any program we write in JAVA is also a class definition.
Classes and programs
The programs we have written up to now are special classes because they also contain the special method main.
Remember, if we want to be able to execute (run) a program it must contain a main method.
So, bankAccount.class is not executable because it doesn’t contain a main method.
To use the methods in bankAccount.class we must call them from other classes.
A summary so far
So far we have seen how to define a basic class and how to create an object of a particular class in a program
At the moment our class definition is not very useful because we can’t do anything with it apart from create bankAccount objects.
To make the class more useful we need to add methods to it which allow us to change the attributes of a bankAccount e.g. to change an account balance.
To do this we need to understand a little bit more about methods.
More about methods
We said earlier than a method is simply a named set of instructions which can be called in a program.
For example, consider the following method which provides the balance on an account:
// Get account balance
public float getBalance( ){
return accountBalance;
}
More about methods
public float getBalance( )
The first line begins the definition of a new method called getBalance:
Here we have two keywords in front of the method name: public and double.
These keywords modify (change) different aspects of the method’s behaviour
They are similar to the modifiers we place in front of a variable declaration
The method name: getBalance must be followed by parentheses ( )
The public keyword
Placing the keyword public in front of a method name means that the method is visible (can be called) from outside the class definition
So for example, we could write a program which creates bankAccount objects and then queries their balance using getBalance().
Sometimes we will want to write methods which are only visible (can only be called) within a certain class.
We call this hiding the method and we use the keyword private rather than public to define such a method.
The float keyword
Placing the keyword float in front of the method name tells JAVA that the method will return a value of type float.
A method can return a value of any type: e.g. int, boolean, double, or it can return a reference to an object of any defined class, e.g. String, bankAccount.
Some methods don’t return any variable this is indicated using the void keyword:
public void myMethod( )
The method statements
Method statements follow the method name and are placed between curly brackets:
public float getBalance( ){
method statements go here;
}
The method getBalance contains a single statement:
return accountBalance;
The return keyword
The return keyword tells JAVA to stop executing the method statements and return to the point where the method was called from
If the method returns a value the return keyword must be followed by a variable (or literal constant) of the relevant type
return accountBalance;
Here return is followed by the variable name accountBalance which has a type of float. This matches the return value of the method getBalance().
Calling a method
To use the getBalance method we would write the following piece of code:
class myProgram {
public static void main( String [] args ) {
float theBalance;
bankAccount myBankAcct;
myBankAcct = new bankAccount();
theBalance = myBankAcct.getBalance();
System.out.println(“The balance is ”+theBalance);
}
Calling a method
The method is called using the dot operator .: myBankAccount.getBalance()
In English this statement means call the method getBalance() for the object myBankAccount
When JAVA reaches this statement it executes the method getBalance(). This method simply returns the value of accountBalance for the relevant object.
The relevant object is the object for which the method is called: myBankAccount in this case.
Calling a method
Because getBalance() returns a value, we can assign this value to a variable of the appropriate type:
theBalance = myBankAcct.getBalance();
theBalance has a type of float so this assignment is valid
Note: we can call the method without an assignment too:
myBankAcct.getBalance();
In this case the returned value is lost and cannot be accessed.
Another method
In addition to getting the balance on an account we will likely want to perform transactions. For example we might want to deposit an amount in the account. The following method takes care of this:
// Make a deposit
public void makeDeposit( float amount ){
if (amount >= 0 )
accountBalance += amount;
else
System.out.println(“Cannot deposit a negative amount”);
}
The method definition
We want to be able to call this method from outside the class definition so we
define it public.
The method doesn’t need to
return a value so we use the
keyword void
public void makeDeposit( float amount ){
In this method definition we have included what looks like a variable declaration between the parentheses
This variable declaration is called a method argument. Method allow us to pass data (variables) to the method from where we call the method
Calling a method with arguments
class myProgram {
public static void main( String [] args ) {
float theBalance;
bankAccount myBankAcct;
myBankAcct = new bankAccount();
myBankAcct.makeDeposit(500.63f);
theBalance = myBankAcct.getBalance();
System.out.println(“The balance is ”+theBalance);
}
When calling a method with arguments we must pass all the arguments which the method requires:
Calling a method with arguments
myBankAcct.makeDeposit(500.63f);
We must ensure that the correct type is passed to the method. Here the f forces the value to be a floating point value
We can also pass a variable:
float theAmount = 500.63f;
myBankAcct.makeDeposit(theAmount);
Calling a method with arguments
Note: when we pass a variable to a method it is the value of the variable which is passed not the actual value:
float theAmount = 500.63f;
myBankAcct.makeDeposit(theAmount);
500.63
theAmount
public void makeDeposit(float amount) 500.63
amount
Main program calls the method
makeDeposit
When makeDeposit is called, JAVA creates a new variable: amount and
initialises its value to be the value of theAmount
Calling a method with arguments
Because arguments are passed in this way, the method cannot change the passed variable
That is, the method makeDeposit cannot change theAmount. So adding the following line to the method makeDeposit changes the variable amount not the variable theAmount
public void makeDeposit(float amount)
amount +=10;
...
Method arguments
A method can have multiple arguments of different types:
public void myMethod( int arg1, double arg2, boolean arg3 ){
Method arguments can be reference variables to objects. For example I might write a method which takes a bankAccount object as an argument:
public void myMethod( bankAccount aBankAccount ){
aBankAccount.makeDeposit(435.34f);
}
Method arguments
What happens when this method is called?:
class myProgram {
public static void main( String [] args ) {
bankAccount myBankAcct = new bankAccount();
myMethod( myBankAcct );
theBalance = myBankAcct.getBalance();
System.out.println(“The balance is ”+theBalance);
}
C:\myjavacode>java myProgram
The balance is 435.34
Method arguments
In this case the call to myMethod which takes myBankAccount as an argument actually changes the value of accountBalance for the object myBankAccount
null
null
0.0
Acct. holder
Acct. no.
Acct. balance
myBankAcctCreated by main program
aBankAccount
The method call copies the address to the
variable aBankAccount
The variable aBankAccount is created by the method myMethod
The address it contains refers to the location of the object created in the main program
Other methods
Retrieving the account number for an account can be done with the following method:
// Get account number
public String getAccountNumber( ){
return AccountNumber;
}
Similarly, retrieving the account holder:
// Get account holder
public String getAccountHolder( ){
return AccountHolder;
}
Other methods
We might also want to be able to make a withdrawal:
// Make a withdrawal
public void makeWithdrawal( float amount ){
if (amount >= 0 ){
if (accountBalance-amount>=0)
accountBalance -= amount;
else
System.out.println(“Can’t withdraw this much”);
}
else
System.out.println(“Cannot deposit a negative amount”);
}
Other methods
We might also want to calculate the daily interest on an account balance:
// Calculate daily interest
public float getDailyInterest( ){
return accountBalance*interestRate/100/365;
}
Other methods
We might also want to change the interest rate for the class:
// Change interest rate
public static void changeInterestRate( float newRate ){
interestRate = newRate;
}
This method is a little different to the methods we have seen so far. The variable it is changing is a class variable not an object variable
The method is particular to the class and independent of any particular class object
Class methods
We call methods which change class variables class methods. A method is declared as a class method using the keyword static:
public static void changeInterestRate( float newRate )
A class method cannot change none class variables. For example the following is an invalid method:
public static void changeBalance( float newBalance ){
balance = newBalance;
}
balance is not a class variable
Calling class methods
A class method should be called with the dot operator but using the class name rather than an object name:
class myProgram {
public static void main( String [] args ) {
bankAccount myBankAcct = new bankAccount();
bankAccount.changeInterestRate(4.5f);
myBankAcct.changeInterestRate(3.1f);
}
Using the name of any relevant class object will also work but should be avoided since the action is not applied to a particular object but the whole class
A summary
This lecture has introduced the concept of an object: a programming model of a real world object.
We have seen how to model objects in JAVA using the concept of a class
We have seen that classes model an object’s attributes using variables and its actions or behaviours using methods
We have looked at how to create an instance of a class (an object) from its class definition using a constructor method
A summary
We have introduced the concept of methods in JAVA and have seen how to write simple methods when developing a class
We have looked at how we can return values from methods and pass values to methods
We have looked at the distinction between class variables and instance variables and at the difference between class methods and instance methods
A summary
You should now be familiar with the following JAVA keywords and operators and their use in developing classes and methods:
class, private, public, protected, static, void
new, return
The dot operator “.”
Top Related