Learning Objectives
After completing this session, you will be able to:
‰ Define a class
‰ Create objects
‰ Identify
packages, importstatement, and Objectclass
‰ Explain object
messaging
Defining a Class
A class is a basic building block of an object oriented
language.
A class is a template that describes the data and the
methods thatmanipulate the data.
Examples of class:
‰ Classroom
‰ Car
‰ Person
The following Java program defines a class Person with
data member name:
Class Person
private String name;
public void setName(String aName) {
name = aName;
}
public String getName() {
return name;
}
}
The data members in a class can be defined as follows:
‰ Instance
variable:
oThis differentiates one object from another, giving an
object its individuality. For
example, the particular name and address for a given
Person object is declared as
an instance variable.
oAn instance variable relates to an instance (object) of
its class.
‰ Class variable:
oThe data that is shared by all the objects are declared
as class variables.
oThere is only one copy of each of these variables no
matter how many objects are
created for the class.
oThese variables exist even if no object of the class has
been created.
oThese variables are also referred to as static fields
because they are declared with the keyword static.
oData members of the class are normally declared as
instance variables using the keyword private.
Creating Objects
An object is created from a class.
The following statement creates an object:
Person EdmundHillary = new Person();
The preceding statement has three parts:
‰ Declaration:This
notifies the compiler that you will use name to refer to data whose type is type;
Person EdmundHillary
‰
Instantiation:The new keyword is a Java operator that creates the
object.
EdmundHillary = new Person()
‰
Initialization:The new operator is followed by a call to a constructor.
The constructor will contain code that initializes the new object to the
desired values.
Packages
‰ Every class in
the Java library belong to a package.
‰ In the Java API,
classes are grouped into packages.
‰ A class has a
full name, which is a combination of the package name and the class name. For example, the class ArrayList is actually
java.util.ArrayList.
‰ To use a class
in a package other than java.lang, you must tell Java the full name of the class.
‰ Packages are
important for three main reasons:
oFirstly, they help in the overall organization of a
project or library.
oSecondly, packages give you a name-scoping,to help to
prevent collisions if many programmers in a company decide to make a class with the
same name.
oThirdly, packages provide a level of security, because
you can restrict the code, which you write so that only other classes in the same
package can access it.
‰ To put a class
in a package, put a package statement at the top of the source code file, before any import statement like package
com.mypack;.
‰ To be in a
package, a class must be in a directory structure that exactly matches the package structure. For a class, com.mypack.Book, the Book
class must be in a directory named mypack, which is in a directory named
com.
‰ Organizing your
classes into packages prevents naming collisions with other classes, if you preponed your reverse domain name on to the front
of a class name.
Import Statement
A typical set of import statements might look like the
following:
‰ import
java.io.*;
‰ import
java.util.ArrayList;
import
java.util.Date;
‰ import
java.util.Properties;
The import statements must come right after the package
statement, before the class statement.
Purpose of import statement:
‰ Because of the
rigid source code naming convention, the Java compiler can easily find the corresponding source or class files justfrom the
fully qualified name of a package and class. This can be illustrated with the following
example:
java.util.ArrayList myList = new java.util.ArrayList(50);
‰ The alternative
to this long-winded style of coding (as shown earlier), is to use import
statements like the following:
import java.util.ArrayList;
…
ArrayList myList = new ArrayList(50);
To derive a class from an external superclass, you must
first import the superclass using the import statement.
Object Class
Every class in Java extends class Object that is the
Object class is the mother of all classes. It is the superclass of everything.
Any class that does not explicitly extend anotherclass,
implicitly extends Object class.
Few of the important methods of the Object class are as
follows:
‰ equals(Object
obj): Indicates whether some other object is "equal to" this one and it returns a boolean value as true or false
‰
toString():Returns a string representation of the object
‰
hashCode():Returns a hash code value (of integer type) for the object
Object Messaging
‰ Objects
cooperate and communicate with other objects in a program by passing messages to one another (setting a value, returning a
value, or sending an email).
‰ When an object
invokes a method on itself or on the method of another object, it is said to pass a message to the object that contains the
target method.
‰ The message
contains the name of the method and any data that the method
requires.
‰ Example:
person.setName(“EdmundHillary”);
‰ Data that are
passed to a method are known as arguments. The required arguments for a method are defined by a parameter list of the
method.
‰ After performing
the operation, the body of the method uses the return statement to return the value to the calling object.
‰ thiskeyword:
oUsed to represent the object that invokes the method:
oUsed within the member methods of the class those are
not static
this keyword
‰ thiskeyword
refers to the object that is currently executing.
‰ thiskeyword also
allows one constructor to explicitly invoke another constructor in the same class.
‰ It is useful for
a method to refer to instance variables relative to thiskeyword when alocal variable hides the data member with the same name,
like the following code:
Class Person {
String name;
Person(String name) {
this.name = name;
}
…….
}
Try It Out
Problem Statement:
Illustrate the importance of the objects withrelevant to
entries in the address book.
Code:
class AddressBook {
String id;
int telephoneNumber;
public void getName() { }
public String setName(String aName) { }
public String changeName(String aName) { }
}
Refer File Name: AddressBook.javato obtain soft copy of
the program code
How It Works:
‰ One analogy for
objects is a packet of unused visiting cards in the address book.
‰ Each card has
the same blank fields (the instance variables).
‰ When you fill
out a card, you are creatingan instance (object), and the entries youmake on that card represent its state.
‰ The methods of
the class are the things you do to a particular card.
‰ getName() ,
changeName(String aName) , and setName(String aName) could all bethe methods for the class AddressBook.
‰ So, each card
can do the same things (getName() , setName(String aName) ,and so on), but each card knows things unique to that particular
card.
Tips and Tricks
What if you have a hundred classes or a thousand? Is not
that a big pain to deliver all those individual files? Can these classes be bundled into one
application thing?
Solution:Yes, it would be a big pain to deliver a huge
bunch of individual files to your end-users,but you will not have to. You can put all of your
application files into a Java Archive or a .jar file that is based on the pkzip format. In the jar file, you can
include a simple text file formatted as something called a manifest, that defines which class in
that jar holds the main() method that should run.
Summary
‰ Object-oriented
programming lets you to extend a program without having to touch the working code that is tested earlier.
‰ A class
describes how to make an object of that class type. A class is like a
blueprint.
‰ An object
knowsthings and doesthings.
‰ The things an
object knows about itself are called instance variables. They represent the state of an object.
‰ The things an
object does are called methods. They represent the behaviour of an object
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