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PRESENTATIONPRESENTATION

Packages• Package is a container for classes

• A package is a grouping of related types (classes and interfaces) providing access protection and name space management.

• In simple words, packages is the way we

organize files into different directories

according to their functionality, usability as

well as category they should belong to.

Why do we need Packages?• One can easily determine that these types are related.• One knows where to find types that can provide task-

related functions.• The names of your types won't conflict with the type

names in other packages because the package creates a

new namespace.• One can allow types within the package to have

unrestricted access to one another yet still restrict access

for types outside the package.

How To Create a Package• Packages are mirrored through directory

structure.• To create a package, First we have to create

a directory /directory structure that matches the package hierarchy.

• Package structure should match the directory structure also.

• To make a class belongs to a particular package include the package statement as

the first statement of source file.

IntroductionIntroduction

Package access protection

• Classes within a package can access classes and members declared with default access and class members declared with the protected access modifier.

• Default access is enforced when neither the public, protected nor private access modifier is specified in the

declaration.

Exercise Creating Packagesmy package

mypackageBmypackageA

ABC DEG IJK XYZ

S1,S2,S3

S4,S5,S6

A,B,CA,B,C D,E,FD,E,F A,B,C,I,J,KA,B,C,I,J,K X,Y,ZX,Y,Z

Package ABC and IJK have classes with same name.Package ABC and IJK have classes with same name.

A class in ABC has name A class in ABC has name mypackage.mypackageA.ABC.A

A class in IJK has nameA class in IJK has name mypackage.mypackageB.IJK.A

How to make a class Belong to a Package

• Include a proper package statement as first line in source file

Make class S1 belongs to mypackageA

package mypackage.mypackageA;public class S1{public S1( ){System.out.println("This is Class S1");}} Name the source file as S1.java and

compile it and store the S1.class file in mypackageA directory

Make class S2 belongs to mypackageA

package mypackage.mypackageA;public class S2{public S2( ){System.out.println("This is Class S2");}}

Name the source file as S2.java and Name the source file as S2.java and compile it and store the S2.class file compile it and store the S2.class file in mypackageA directoryin mypackageA directory

Make class A belongs to IJK

package mypackage.mypackageB.IJK;public class A{public A( ){System.out.println("This is Class A in IJK");}}

Name the source file as A.java and compile it and store the A.class file in IJK directory

<< Same Procedure For all classes>>

Importing the PackageImporting the Package

• import statement allows the importing of package• Library packages are automatically imported irrespective of

the location of compiling and executing program• JRE looks at two places for user created packages (i) Under the current working directory (ii) At the location specified by CLASSPATH environment variable• Most ideal location for compiling/executing a program is

immediately above the package structure.

Example importing Example importing import mypackage.mypackageA.ABC;import mypackage.mypackageA.ABC;

import mypackage.mypackageA.ABC.*;import mypackage.mypackageA.ABC.*;class packagetestclass packagetest{{public static void main(String args[])public static void main(String args[]){{B b1 = new B();B b1 = new B();C c1 = new C();C c1 = new C();}}}}

<<packagetest.java>>

<< Store it in location above the << Store it in location above the package structure. Compile and package structure. Compile and Execute it from there>>Execute it from there>>

<< Store it in location above the << Store it in location above the package structure. Compile and package structure. Compile and Execute it from there>>Execute it from there>>

import mypackage.mypackageA.ABC.*;Import mypackage.mypackageB.IJK.*;class packagetest{public static void main(String args[]){A a1 = new A();}}

<< What’s Wrong Here>>

<< class A is present in both the imported packages ABC and IJK. So A has to be fully qualified in this case>>

mypackage.mypackageA.ABC.A a1 = new mypackage.mypackageA.ABC.A();

OR

mypackage.mypackageB.IJK.A a1 = new mypackage.mypackageB.IJK.A();

Using Packages

– Class in a named package can be referred to in two different ways

• Using the fully qualified name packagename.ClassName

• We can refer to the ElevatorPanel class in package elevator as

elevator.ElevatorPlanel

Common Mistakes

• Common mistakes while running the program– The directory structure for elevator program is C:\Project\

elevator.– Run the program from elevator directory, and we will get the

following error message

c:\project\elevator>java ElevatorSimulation Exception in thread "main"

java.lang.NoClassDefFoundError: ElevatorSimulation– The program runs successfully by running the program from

c:\project directory.

Solution

• Add c:\project to the CLASSPATH, and rerun the program.

• The program is launched successfully without any error messages.

Nested Classes

Nested classes are defined WITHIN another classThey are defined using the static keyword

The nested class is defined as part of the outer class. It can only be referenced through the outer class.

public class LinkedList{private static class LinkedListNode{

[...]}public static class LinkedListStats{[...]

}// Create an instance of the nested class LinkedListStats

LinkedList.LinkedListStats x = new LinkedList.LinkedListStats();

// Illegal access to private nested class -- compiler error

dLinkeList.LinkedListNode y = new LinkedList.LinkedListNode();

Local Classes

Local classes are defined within a methodThe scope is limited to the methodUsed for small helper classes whose applicability is within the method only

public class LinkedList{public void traverse(){

class MyIterator extends Iterator{

public void doTraversal(){

[...]}

}

MyIterator anIterator = new MyIterator();[...]

}}

Member Classes

Member classes are similar to nested classes, except they are not static.

They are used in the same manner as instance variables.

public class LinkedListpublic class LinkedList{{public class LinkedListStatspublic class LinkedListStats{{

[...][...]}} LinkedList aList = new Linked List();LinkedList aList = new Linked List();

LinkedList.LinkedListStats aStat = new LinkedList.LinkedListStats aStat = new aList.LinkedListStats();aList.LinkedListStats();

Access to members of the classes

Java packages can be stored in compressed files called JAR files,

allowing classes to download faster as a group rather than one at a time.

Demo:Access modifier private

package p1

class C1

private int x

class C2 extends C1

C1 c1; c1.x cannot be read or modified

package p2

class C4 extends C1

x cannot be read or modified in C2

class C5

C1 c1; c1.x cannot be read nor modified

Class C3

C1 c1; c1.x cannot be read or modified

Default access modifierprivate protected

package p1

class C1

private int x

class C2 extends C1

C1 c1; c1.x can be read or modified

package p2

class C4 extends C1

C1 c1; c1.x can be read or modified

class C5

C1 c1; c1.x cannot be read nor modified

Class C3

C1 c1; c1.x cannot be read or modified

Access modifier friendly

package p1

class C1

private int x

class C2 extends C1

C1 c1; c1.x can be read or modified

package p2

class C4 extends C1

C1 c1; c1.x cannot be read nor modified

class C5

C1 c1; c1.x cannot be read nor modified

Class C3

C1 c1; c1.x can be read or modified

Access modifier friendly

package p1

class C1

private int x

class C2 extends C1

C1 c1; c1.x can be read or modified

package p2

class C4 extends C1

C1 c1; c1.x can be read or modified

class C5

C1 c1; c1.x cannot be read nor modified

Class C3

C1 c1; c1.x can be read or modified

Access modifier publicpackage p1

class C1

public int x

class C3

C1 c1;c1.x can be read ormodified

package p2

class C2 extends C1

x can be read ormodified in C2

class C4

C1 c1;c1.x can be read normodified

Suppose you write a group of classes that representSuppose you write a group of classes that representgraphic objects, such as circles, rectangles, lines, andgraphic objects, such as circles, rectangles, lines, andpointspoints

● ● You also write an interface, Draggable, that classesYou also write an interface, Draggable, that classesimplement if they can be dragged with the mouseimplement if they can be dragged with the mouse//in the Draggable.java file//in the Draggable.java filepublic interface Draggable {}public interface Draggable {}//in the Graphic.java file//in the Graphic.java filepublic abstract class Graphic {}public abstract class Graphic {}//in the Circle.java file//in the Circle.java filepublic class Circle extends Graphic implements Draggable {}public class Circle extends Graphic implements Draggable {}

//in the Rectangle.java file//in the Rectangle.java filepublic class Rectangle extends Graphic implements Draggable { }public class Rectangle extends Graphic implements Draggable { }//in the Point.java file//in the Point.java filepublic class Point extends Graphic implements Draggable {}public class Point extends Graphic implements Draggable {}//in the Line.java file//in the Line.java filepublic class Line extends Graphic implements Draggable {}public class Line extends Graphic implements Draggable {}

Suppose you write a group of classes that representSuppose you write a group of classes that representgraphic objects, such as circles, rectangles, lines, andgraphic objects, such as circles, rectangles, lines, andpointspoints

● ● You also write an interface, Draggable, that classesYou also write an interface, Draggable, that classesimplement if they can be dragged with the mouseimplement if they can be dragged with the mouse//in the Draggable.java file//in the Draggable.java filepublic interface Draggable {}public interface Draggable {}//in the Graphic.java file//in the Graphic.java filepublic abstract class Graphic {}public abstract class Graphic {}//in the Circle.java file//in the Circle.java filepublic class Circle extends Graphic implements Draggable {}public class Circle extends Graphic implements Draggable {}

//in the Rectangle.java file//in the Rectangle.java filepublic class Rectangle extends Graphic implements Draggable { }public class Rectangle extends Graphic implements Draggable { }//in the Point.java file//in the Point.java filepublic class Point extends Graphic implements Draggable {}public class Point extends Graphic implements Draggable {}//in the Line.java file//in the Line.java filepublic class Line extends Graphic implements Draggable {}public class Line extends Graphic implements Draggable {}

Example Example PackagePackage

Example: Placing Example: Placing StudentRecordStudentRecordclass in SchoolClasses class in SchoolClasses pacakgepacakgepackage SchoolClasses;package SchoolClasses;public class StudentRecord {public class StudentRecord {private String name;private String name;private String address;private String address;private int age;private int age;::

Packages – Directory Paths

• The CLASSPATH– List of directories and/or jar files. The compiler will

look in these directories for any precompiled files it needs.

– The CLASSPATH can be set as an environmental variable or specified on the command line using –classpath option.

– The CLASSPATH is also used by the Java Virtual Machine to load classes.

Compile Package Classes

• Compile the program– To compile the program, we must change the working

directory to the source directory root, and issue the following command

c:\project> javac -d . elevator\*.java

– By compiling everything, we get the recent version of all the classes.

– Specify –d . option to tell the compiler to put the classes in a package structure starting at the root.

Benefits of Packaging● You and other programmers can easily determine that You and other programmers can easily determine that

these classes and interfaces are related.these classes and interfaces are related.● You and other programmers know where to find classes You and other programmers know where to find classes

and interfaces that can provide graphicsrelated functions.and interfaces that can provide graphicsrelated functions.● The names of your classes and interfaces won't conflict The names of your classes and interfaces won't conflict

with the names in other packages because the package with the names in other packages because the package creates a new namespace.creates a new namespace.

● You can allow classes within the package to have You can allow classes within the package to have unrestricted access to one another yet still restrict access unrestricted access to one another yet still restrict access for types outside the package.for types outside the package.

THANKS TO ALL THANKS TO ALL

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special thanks to-special thanks to-NOYONNOYON