Lecture 06 Java Classes and Packages

8/11/2019 Lecture 06 Java Classes and Packages

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Lecture 6


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To make types easier to find and use, to avoid naming conflicts, and to

control access, programmers bundle groups of related types intopac ages.

Java classes always exist in a class package There is a default package which doesnt have a name

Java core API is made up of several packages Class names in a package are qualified by the package name e.g Math class has the fully qualified name as java.lang.Math

Introduction to Java2


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The names of your classes and interfaces won't conflict with the

names in other packages because the package creates a newnamespace.

Names used for classes in one package will not interfere with the

names of classes in another package

Copackage classes enjoy special access to each other members ou on t ave to mport t ose c asses



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Creating packages Add a package statement as the first statement in your source file

containing the class definition n y commen s an an nes are a owe o prece e e pac age

statement

A package statement consist of the keyword package followed byt e pac age name terminate y a semico on.

You can specify a package name as a sequence of names separatedb eriods. If you do not use a package statement, your type ends up in an

unnamed package Use an unnamed acka e onl for small or tem orar a lications

Packages are intimately related to the directory structure in whichthey are stored


Class files must be in a directory named by the package


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as the first line of the code (except comments)acka e < acka eName>

package myownpackage;


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acka e SchoolClasses

public class StudentRecord {private String name;

private String address;private int age;.

}


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To use a public package member (classes and

interfaces) from outside its package, you mustdo one of the following Import the package member using import

statement Import the member's entire package using import

statement Refer to the member by its fully qualified name

(without using import statement)


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interpreter expects the directory structure containingthe executable classes to match the package hierarchy.

There should be same directory structure,./myowndir/myownsubdir/myownpackage directory forthe following package statement

Package myowndir.myownsubdir.myownpackage;


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.

package graphics;

}

name reflects the name of the package to

.....\graphics\Rectangle.java


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Directory Structure of Java Source

Files . , .

files should be in a series of directories that

Example : . pathname to source file: graphics/Rectangle.java .


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Directory Structure of Java Source

Files However, the path to the .class files does not have to

be the same as the path to the .java source files. Youcan arrange your source and class directoriesseparately, as:

\sources\com\example\graphics\Rectangle.java\classes\com\example\graphics\Rectangle.class

B doin this, ou can ive the classes director toother programmers without revealing your sources

You also need to manage source and class files in this

Machine (JVM) can find all the types your programuses


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su ose we lace the acka e schoolClasses under the C:

directory. We need to set the classpath to point to that directory so that

w en we ry o run , e w e a e o see w ere ourclasses are stored.

we t e this at the command rom t

C:\schoolClasses> set classpath=C:\ ,

anywhere by typing,C:\schoolClasses> java schoolClasses.StudentRecord


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anywhere. You can also set more than one,

;(for windows)=


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Defining a new class based on an existing class is called

The derived class is also called the direct subclass of thebase or su er class

You can also derive classes from the derived class and soon

Class A

Class B

Class C


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class B extends A{

definition of class B

}

subclass of class A

The class B can have additional members in addition to theinherited members of class A


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An inherited member of a base class is one that isaccessible within the derived class

Base class members that are not inherited still form part ofa derived class object

An inherited member of a derived class is a full member of

that class and is freely accessible to any method in the

Which members of the base class are inherited?


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The inheritance rules apply to class variables as well asinstance variables

SubClasspu ic BaseC ass

int a

protected int c;

private int d;


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You can define a data member in a derived class with the same nameas a a mem er n e ase c ass.

The data member of the base class is still inherited but is hidden bythe derived class member with the same name

The hiding will occur irrespective if the type or access modifiers are

the same or not.ny use o t e er ve mem er name w a ways re er to t e mem er

defined in derived class

To refer to the inherited base class member, you must qualify it withthe keyword super

Note that you cannot use super.super.something


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Methods in a base class excludin constructors are inherited in aderived class in the same way as the data members of the base class

Methods declared as private in a base class are not inherited

o e: ons ruc ors n e ase c ass are never n er e regar ess otheir attributes

Though the base class constructors are not inherited in your derived,

from your derived class constructor, the compiler will try to do it foryou


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If the first statement in a derived class constructor is not a call to aase c ass constructor, t e comp er w nsert a ca to t e e au t

class constructor i.e super ( ), for youDefault call for base class constructor is with no arguments. Thissometimes result in a compiler error. Why?

When you define your own constructor in a class, nodefault constructor is created by the compiler. Thus you

yourself so that in a derived class you dont get thecompile error due to call to default constructor of baseclass


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Exam lepublic class Person {

protected String name;protected String address;

/ * Default constructor */

public Person() {System.out.println(Inside Person:Constructor)

"" ""name = ; a ress = ;}. . . .

}public class Student extends Person {public Student(){ System.out.println(Inside Student:Constructor);. . . .

}

public static void main ( String[] args ){=

}


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A subclass can also explicitly call a constructor of its

.

This is done by using the super constructor call. A su er constructor call in the constructor of a subclass will

result in the execution of relevant constructor from the super

class, based on the arguments passed. ew t ngs to remem er w en us ng t e super constructor

call: The su er call must occur as the first statement in a constructor The super() call can only be used in a constructor (not in ordinary

methods)


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public Student(){super ome ame , ome ress ;

System.out.println("Inside Student:Constructor");}

Another use of super is to refer to members of the super class.

public Student() {super.name = somename;super.address = some address;}


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Object class is mother of all classes

In Java language, all classes are sub classed (extended)from the Object super class

class

Defines and im lements behavior common to all classes including the ones that you write

getClass() equals() toString()

Lecture 06 Java Classes and Packages

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