Download Classes and Objects

Objects, Classes, Methods
What is Object Oriented Programming?


Object oriented Programming languages (OOP for short) include all the features
of structured programming and add still more powerful ways to organize
algorithms and data structures. There are three key features of OOP languages:

encapsulation

inheritance

polymorphism
All of them are tied to the notion of a class
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Classes and Objects
 The primary distinguishing feature of OOP languages is the class. A class is a set of
variables (data members) that can be associated with the methods which act on an
object with the object itself.
 As the name object-oriented implies, objects are key to understanding objectoriented technology. You can look around you now and see many examples of realworld objects: your dog, your desk, your television set, your bicycle.
 These real-world objects share two characteristics: they all have state and they all
have behavior. For example, cars have state (current gear, number of seats, four
wheels, etc.) and behavior (braking, accelerating, slowing down, changing gears).
 Software objects are modeled after real-world objects in that they, too, have state
and behavior. A software object maintains its state in variables and implements its
behavior with methods.
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Encapsulation
 As you can see from the above diagram, an object's variables make up the center or
nucleus of the object. Methods surround and hide the object's nucleus from other
objects in the program. This is called encapsulation.
 Typically, encapsulation is used to hide unimportant implementation details from
other objects. Thus, the implementation details can change at any time without
affecting other parts of the program.
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The Benefits of Encapsulation
 Encapsulation provides two primary benefits to software developers:

Modularity -- the source code for an object can be written and maintained
independently of the source code for other objects. Also, an object can be
easily passed around in the system.

Information hiding -- an object has a public interface that other objects can
use to communicate with it. But the object can maintain private information
and methods that can be changed at any time without affecting the other
objects that depend on it
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What Are Classes?
 A class is a blueprint or prototype that defines the variables and methods common to
all objects of a certain kind.
Object3
Object2
Class
Object1
 In the real world, you often have many objects of the same kind. For example, your
car is just one of many cars in the world. Using object-oriented terminology, we say
that your car object is an instance of the class of objects known as cars. Cars have
some state (current gear, number of seats, four wheels, etc.) and behavior (change
gears, brake) in common. However, each car's state is independent of and can be
different from other cars.
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Defining a Class
 A class definition takes the following form:
class MyClass {
// the members of the class go here
}
 The class keyword is followed by the class name, which must be a valid Java
identifier.
 Example:
class Employee {
int empnum;
// data member
public int Num() {
return empnum;
}
// method
public void setNum(int newNum) {
empnum = newNum;
}
// method
}
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 In our example we will have many thousands of Employees. Each specific employee
is an object. The definition of a Employee though, which we gave above, is a class.
 This is a very important distinction. A class defines what an object is, but it is not
itself an object. An object is a specific instance of a class.
 Thus when we create a new object we say we are instantiating the object. Each class
exists only once in a program, but there can be many thousands of objects that are
instances of that class.
 To instantiate an object in Java we use the new operator. Here's how we'd create a
new employee:
Employee x = new Employee();
The members of an object are accessed using the . (dot) operator, as follows:
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// Employee.java
class Employee {
int empnum;
public int Num() {
return empnum;
}
public void setNum(int newNum) {
empnum = newNum;
}
}
// data member
// method
// method
class TestEmployee {
public static void main(String[] args) {
Employee emp1 = new Employee();
Employee emp2 = new Employee();
emp1.setNum(12651);
emp2.setNum(36595);
System.out.println("num of emp1 : " + emp1.Num());
System.out.println("num of emp2 : " + emp2.Num());
}
}
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Sample Run
$ java TestEmployee
num of emp1 : 12651
num of emp2 : 36595
 Note when you compile the file Employee.java, you get two class files:
Employee.class and TestEmployee.class.
 A member declared as public in a class is accessible to all other methods.
 A member declared as private in a class is accessible only to other members of the
same class.
 A member which is not marked is said to be friendly and can directly accessible to
other members of the same package. In particular, friendly accessible to all classes
within the same java source file.
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Overloading Methods
 Two methods can have the same name as long as they have different argument lists.
This is called method overloading.
 Example
// TestOverload.java
class Employee {
int empnum;
public int Num() {
return empnum;
}
public void Num(int newNum) {
empnum = newNum;
}
}
// data member
// get method
// set method
class TestOverload {
public static void main(String[] args) {
Employee emp = new Employee();
emp.Num(98165);
System.out.println("num of emp : " + emp.Num());
}
}
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Sample Run
$ java TestOverload
num of emp : 98165
Static Members (Class Members)
 Most properties, like the balance in bank account, are unique to the object. But some
properties are shared among all objects of a given class. For example, the interest
rate is a property shared by all saving accounts in the same bank.
 Such properties are called class member.
 Class members are defined using the keyword static. So class members are also
called static members (e.g. Math class).
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// TestStatic.java
class Employee {
static int MaxNum;
int empnum;
public int Num() {
return empnum;
}
public void setNum(int newNum) {
if (newNum<=MaxNum && newNum >1)
empnum = newNum;
}
}
// data member
// data member
// method
// method
class TestStatic {
public static void main(String[] args) {
Employee emp1 = new Employee();
Employee emp2 = new Employee();
emp1.MaxNum = 99999;
System.out.println("MaxNum of emp2 : " + emp2.MaxNum);
}
}
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Sample Run
$ java TestStatic
MaxNum of emp2 : 99999
Object3
Object2
Class (Static)
Members
Object1
(nonstatic
members)
MaxNum
empnum
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Static Methods
 Methods can also be declared static.
 A static method does not associate with any objects so that it cannot access
nonstatic class members.
 Java allows programmer to call a static method using the name of the class rather
than an object name.
 Example: The static method you are already very familiar with is
public static void main(String[] args)
 Advantage of static methods: you don't have to create an object before using a static
method.
 You can still call a static method using an object rather than a class name.
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// TestStaticMethod.java
class Employee {
static int MaxNum;
int empnum;
// data member
// data member
public static void setMaxNum(int newMaxNum) {
MaxNum = newMaxNum;
}
}
class TestStaticMethod {
public static void main(String[] args) {
Employee.setMaxNum(88888);
System.out.println("MaxNum of Employee : " + Employee.MaxNum);
Employee emp = new Employee();
emp.setMaxNum(99999);
System.out.println("MaxNum of emp : " + emp.MaxNum);
}
}
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Sample Run
$ java TestStaticMethod
MaxNum of Employee : 88888
MaxNum of emp : 99999
Final Members
 It is possible to define a data member as final, meaning that its value is not
changeable.
 A final variable must be initialised. (JDK1.1 introduced "blank final variable" which
is simply a final variable that doesn't have an initializer. A blank final variable must
be assigned an inital value, and that can be assigned only once.)
 Example:
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class Employee {
final static int MaxNum = 99999;
//.... so on like before
}
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// not modifiable
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Object References
 Java has no explicit pointers. Instead, Java offers references.
 You might have noticed that a class object is not created the same way an intrinsic
variable is created:
int i;
Employee emp = new Employee();
 The declaration Employee
an Employee object.
emp
// intrinsic variable
// class object
declares not an Employee object but a reference to
 Java will set any uninitialised reference to null. Any attempt to use a null reference
generates an exception.
 You can assign any reference to refer to a real object:
Employee emp;
emp = new Employee();
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// emp references the null object
// now emp references a real Employee object
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 It is legal for two references to refer to the same object:
Employee emp = new Employee();
Employee mgr = emp; // mgr refers to the same Employee as emp
So What is a Reference?
 You can think of a reference as a name for an object.
 I have a dog. The dog is an object. I can name it Lucky. Lucky is a reference to the
dog. I can give my dog several names: Lucky, Fido, Stupid. Each of these
references refers to the same dog (object).
 A reference can be redirected. For example, I got another new dog and name it
Lucky. When I say Lucky, I am referring to my new dog. This is similar to
assigning a reference to point to a different object.
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Passing References to Functions
 As all variables except intrinsic objects are references, a class object passed as an
argument to a function remains modified in the calling function (call by reference).
 However, an intrinsic object does not remain modified when passed as an argument
to a function (call by value).
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Call by Value
Call by Reference
boolean
byte
short
int
long
float
double
char
all other objects
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// PassRef.java
class Employee {
int empnum;
}
// data member
class PassRef {
public static void main(String[] args) {
Employee emp = new Employee();
emp.empnum = 81619;
System.out.println("num of emp : " + emp.empnum);
ModifyEmpNum(emp);
// call by reference
System.out.println("num of emp : " + emp.empnum);
}
private static void ModifyEmpNum(Employee Emp) {
Emp.empnum = 71621;
}
}
Sample Run
$ java PassRef
num of emp : 81619
num of emp : 71621
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Cleaning Up Lost Objects: Garbage Collection
 Object can get lost. Consider the following code segment:
public static void SomeFunc() {
//allocate an object
Employee emp = new Employee();
// ... does something and then exits
}
 Here the reference emp is local to the function SomeFunc. When the function exits,
emp goes out of scope and the object is no longer accessible. Java is free to reclaim
its memory and put it back into the heap.
 Java does the memory recovery in a process known as garage collection.
 Java performs garbage collection under the following circumstances:
 Whenever it needs to. When the amount of memory is not enough
 Whenever you ask. You can force garbage collection by calling System.gc.
 Whenever it gets around to it. Java continually runs a low priority background
task that looks for things to throw away.
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Constructor
 A constructor is a special method with the same name as the class that is invoked
automatically whenever a class object is created.
 A constructor initialises all the variables and does any work necessary to prepare the
class to be used.
 A constructor has no return type, not even void.
 If no constructor is defined by the programmer, a default "do-nothing" constructor is
created for you.
 Java does not provide a default constructor if the class define a constructor of its
own.
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// TestConstructor.java
class Employee {
int empnum;
public Employee() {
empnum = 99999;
}
public Employee(int newNum) {
empnum = newNum;
}
}
// data member
// constructor
// constructor
class TestConstructor {
public static void main(String[] args) {
Employee emp1 = new Employee();
System.out.println("num of emp1 : " + emp1.empnum);
Employee emp2 = new Employee(81263);
System.out.println("num of emp2 : " + emp2.empnum);
}
}
Sample Run
num of emp1 : 99999
num of emp2 : 81263
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What about Static Data?
 To initialise static data members, Java defines a special constructor for static
members, called static initialiser.
// TestStaticInit.java
class Employee {
static int MaxNum;
static {
MaxNum = 988;
}
}
// static data member
// static initialiser
class TestStaticInit {
public static void main(String[] args) {
System.out.println("MaxNum of Employee : " + Employee.MaxNum);
}
}
Sample Run
$ java TestStaticInit
MaxNum of Employee : 988
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Explicit Use of this pointer

Sometimes it is necessary to use the this pointer explicitly within the member
function. A typical example of this is to distinguish a local variable and data
member which have the same name.

You will find out more useof this pointer in the laboratory session.
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toString

All classes in Java are extending the class Object.

One method in the Object class is toString which which will return a string that
"textually represents" the object.
public class Car {
int seats;
public static void main (String args[]) {
Car c = new Car();
System.out.println("c = " + c.toString());
System.out.println("c = " + c);
}
}
Sample Output
c = Car@7672ed41
c = Car@7672ed41
 You can override the toString method to produce more meaningful message.
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class Car2 {
int seats = 4;
Sample Output
c = Honda 4
c = Honda 4
public String toString() {
return "Honda " + 4;
}
public static void main (String args[]) {
Car c = new Car();
System.out.println("c = " + c.toString());
System.out.println("c = " + c);
}
}
Additional Java features
 The break statement can also be labelled to allow control to pass out of multiple
loops at one time by using break label.
 The statement break outHere does not pass control to the label. Rather, it passes
control outside of the loop labelled outHere.
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Example - Labelled Break
// LabeledBreak.java
Sample Output
class LabeledBreak {
i = 0, j = 0
i = 0, j = 1
i = 0, j = 2
i = 1, j = 0
i = 1, j = 1
i = 1, j = 2
i = 2, j = 0
i = 2, j = 1
Program exits.
public static void main (String args[]) {
int i = 0;
outHere:
while (i<4) {
int j = 0;
while (j<3) {
System.out.println("i = "+i+", j = "+j);
j++;
if (i==2 && j==2)
break outHere;
}
i++;
}
System.out.println("Program exits.");
}
}
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Using the Standard I/O Objects
 Java provides three static standard objects for I/O:
Object
Type
System.in BufferedInputStream
System.out PrintStream
System.err PrintStream
 Java views the input data as a sequential stream of bytes and System.in has a lower
level method read() to read a number of bytes from the standard input.
Reading String
 We can use the function System.out.write to print the byte array read. Let us begin
by writing a very simple program that asks the user for their name and then prints a
personalized greeting.
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Remark : Don't run this example using FreeJava, use command line instead.
// PersonalHello.java
Sample Output
import java.io.*;
What is your name?
John
Hello John
class PersonalHello {
public static void main (String args[]) {
byte name[] = new byte[100];
int nr_read = 0;
System.out.println("What is your name?");
try {
nr_read = System.in.read(name);
System.out.print("Hello ");
System.out.write(name,0,nr_read);
}
catch (IOException e) {
System.out.print("I can't read your name.");
}
}
}
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 The block
try {
......
}
catch (......) {
......
}
is used to handle the exception generated by the run-time system. Some exceptions
can ignored but the IOException must be caught. So the main method must have a
try-catch block to handle any exception generated.
Reading Numbers
 The object InputStream doesn't offer anything more sophisticated than
read(bytes[]). So we have to convert the array of bytes to the type we need. This
involves some works to convert the byte array to a String object and then parse it to
the desired types.
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// IOTest.java
import java.io.*;
public class IOTest {
public static void main(String[] args) {
int i;
byte barray[] = new byte[80];
try {
System.out.println("Enter an integer : ");
System.in.read(barray);
Sample Run
$ java IOTest
Enter an integer :
15231
15231
// convert the byte array to String
String s = new String(barray);
// remove all spaces from s
// and convert the content to an integer
i=Integer.parseInt(s.trim());
System.out.println(i);
}
catch (IOException ioe)
{
System.out.println(ioe.toString());
}
}
}
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As parseInt() accepts a
string containing digits
only, trim() is used to
remove all spaces.
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 Another method is to use DataInputStream which is used to read formatted data.
// ReadNumber.java
Sample Run
import java.io.*;
$ java ReadNumber
Enter a number :
61347
i = 61347
class ReadNumber{
public static void main (String args[]) {
String line;
DataInputStream in = new DataInputStream(System.in);
try {
System.out.println("Enter a number : ");
line = in.readLine();
int i = Integer.valueOf(line).intValue();
System.out.println("i = " + i);
}
catch (Exception e) {
System.out.println(e);
}
}
}
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Reading Formatted Data
 If you enter any non-digit as the input to the above program, you will get a run-time
syntax error. It is because the method Integer.valueOf accept signed numbers only.
Sample Run
$ java ReadNumber
Enter a number :
7152__
java.lang.NumberFormatException: 7152
ended with two
spaces
$ java ReadNumber
Enter a number :
71846.
java.lang.NumberFormatException: 71846.
 To read text and numbers on the same line, you need to use the StreamTokenizer
class.
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Command Line Arguments
 You can pass command line arguments to your program.
command line arguments
java Hello May Tom Sue
program name
 Compile the following program usual and run it with some command line
arguments.
// Hello.java
class Hello{
Sample Output
public static void main (String args[]) {
System.out.print("Hello ");
if (args.length == 0)
System.out.println("whoever you are");
else {
for (int i=0; i<args.length; i++)
System.out.print(args[i]+" ");
System.out.println();
}
}
>java Hello May Tom Sue
Hello May Tom Sue
>java Hello
Hello whoever you are
}
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