4 Packages

8/14/2019 4 Packages

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1Java

Packages


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2Java

Java source file rules12

Static Imports11

Packages in JDK10

Imports9

jar command8

Jar7

Result of execution6

Compiling and executing5Default Access Specifier4

Solution !3

Executing classes in a package2

Definition1

CONTENTS


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3Java

What packages are in Java

How default access specifier works.

Java source file coding rules.

Some of the packages in java.

Know


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4Java

Be Able To

Use import and work with packages

Implement static import


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5Java

I want you to create a class

which will calculate gradesfor students based on

percentage of marks. A

student who gets above 90

has a grade O, a student

who gets between 80 and

90 gets a grade A, student

who gets between 70 and

80 gets B and between 60

and 70, C. Below 60 is

fail.

Project Manager


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6Java

Thats simple. I can do itwith my eyes closed .

public class Grade{/* get registrationnumber and % marksand assign thegrade */

}

You


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8Java

for reasons of performanceappraisal, we want teachers

grading also. Students of a class

will grade the faculty based on

his or her performance in the

class. The head of the

department will look at thegrades to do the performance

appraisal. We need to automate

this.

No problem!


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9Java

What should I name the teachers grade class. Icannot have Grade class again !

TeacherGrade class.

Yes ! But It doesnt look ok. For students

Grade class and for teachers TeacherGradeclass. Oh ! Should I then change the studentsgrade class to StudentGrade class. I will haveto do so much of rework. But I have alreadytold the PM that I am almost finished. He mayask for it tomorrow. I am stuck!

If only I had used packages!


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10Java

Definition

A grouping mechanism that holds classes andpackages

Classes can be grouped under a package for reasonsof similar / shared responsibilities, for reasons of

organization and so on Packages are implemented using file system

directories.

Syntax for creating Packages:

package package_name[.package_name];public class Class_name{}


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11Java

student teacher

Student

Grade

Teacher

Grade

student.StudentReferred to as

Student

Referred to as

Teacher

Referred to asteacher. Teacher

package student;

public classStudent{}

Changing the

student class


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12Java

package student;

public class Grade {

private Student student;private int mark[];private final static String O="Outstanding";private final static String A="V.Good";

private final static String B="Good";private final static String C="Average";private final static String F="Fail";

public static void main(String str[]) {Grade g=new Grade(new Student("Raja"),newint[]{ 80,85, 91,86, 82});System.out.println(g.getGrade()); }


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13Java

public Grade(Student s, int mark[]){this.student=s;this.mark=mark; }public String getGrade() {int percent=getPercent();if(percent>=90) return O;

else if(percent=80)return A;else if(percent=60)return B;

else if(percent=50)return C;elsereturn F;}


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14Java

int getPercent(){int tot=0;for(int i=0;i


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15Java

Let us first compile all the classes.Let us assumethat all the files are in D:\MyJava.

D:\MyJava> java Grade

Exception in thread "main"java.lang.NoClassDefFoundError: Grade (wrongname: student/Grade)

Run time error is generated.

Executing classes in a package


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16Java

Solution !

A student folder has to be created and all the.class files generated for the student packagehas to be shifted there. This could be donemanually or by using the shortcuts during the

compilation as given below. Shortcut during compilation:

javac -d *.java

D:\MyJava> javac d . *.java

D:\MyJava> java student.Grade

Executes the code successfully

Creates a folder in the

current directory and

puts Student.class and

Grade.class inside it.


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17Java

Default Access Specifier

If no access specifier is specified then by default it is

assumed to bepackage access specifierordefault

access specifier.

Classes or members having package access specifiercan be accessed only by the classes belonging to the

same package.


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18Java

package student;package teacher;

public class Student{

private int regNo;

public int getRegNo(){..}}

public class StudentTest{

void test1()

public void test2()

private void test3()

}

public class Teacher{

..

}


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19Java

Teacher.java

package teacher;public class Teacher{

private int factId;private String name;private static int id;

public int getFactId(){return factId;}public String getName(){return name;}

public void setName(String name){this.name=name;private int generateId(){id++;return id; }

public static int getId(){return id;} public

Teacher(String name){this.name=name;factId=generateId();

}}

G j


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20Java

Grade.javapackage teacher;public class Grade{

private Teacher faculty;

private student.Student student;private String subjectCode;private String grade;

public final static String A="EXCELLIENT";

public final static String B="GOOD";public final static String C="AVERAGE";public final static String D="POOR";public Grade(Teacher faculty,

student.Student student, String subjectCode,String grade){setFaculty(faculty);setStudent(student);setSubjectCode(subjectCode); setGrade(grade)

; }


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public Teacher getFaculty(){return faculty; }public void setFaculty(Faculty fact){

this.faculty=fact; }

public void setStudent(student.Studentstudent){this.student=student;}

public student.Student getStudent(){return student;}

public void setSubjectCode(StringsubjectCode){this.subjectCode=subjectCode; }public String getSubjectCode(){

return subjectCode;}


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public class Tester{public static void main(String str[]){

/*accessing student package */student.Student s1= newstudent.Student("Malini");System.out.println("Name of the student "

s1.getName()+"("+ s1.getRegNo()+")");student.Grade g=new student.Grade(news1,new int[]{ 80,85, 91,86, 82});System.out.println(g.getGrade());

/*accessing teacher package */teacher.Teacher f=new

teacher.Teacher("Tom");


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24Java

teacher.Grade gf= newteacher.Grade(f,s1,"001",teacher.Grade.B);

System.out.println("Teacher Name "+gf.getFaculty().getName()+"-"+gf.getFaculy().getId());

System.out.println("Student who graded "gf.getStudent().getName());

System.out.println("Grade "+gf.getGrade());}

}


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25Java

Compiling and executing Let us assume directory structure as given below:

D:\MyJava : Student.java and Grade.java

D:\MyJava1 : Teacher.java and Grade.java

D:\MyJava : Tester.java

Compile Student.java and Teacher.java first using doption.

Set the class path to

set classpath= %classpath%; D:\MyJava; D:\MyJava1

Then compile Grade classes with d option

Finally compile and execute Tester class.


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27Java

Jar

JAR(java archive) is a file which is a bundle of allthe class files required for an application.

It helps us distribute our application files easily andalso helps in setting the classpath.

Important thing about the jar file is that it maintainsthe directory structure. So if we want a classStudent.class to be inside student directory that can

be maintained in the jar file.

jar command can be used to create and update a jarfile.

It also be used to extract a jar file.


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28Java

jar command

Creation: jar cvf myjar.jar *.* Updation: jar uvf myjar.jar file.ext Extraction : jar xvf myjar.jar Example:

Copy student and teacher package folders intoD:\packages.

D:\packages >jar cvf college.jar *.class

This jar file can now be copied and placed anywhere. To compile and execute Tester class set the classpath

to the jar files location.

Set classpath =%classpath%;path\college.jar


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b) Importing all the classes in a package.

Example:import student.*;

public class Tester{

Student s= newStudent("John","M.C.A.");

}


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import student.*;import teacher.*;public class Tester1{

public static void main(String str[]){

/*accessing student package */Student s1= new Student("Malini");System.out.println("Name of the student "+s1.getName()+"("+ s1.getRegNo()+")");Grade g=new Grade(s1,new int[]{ 80,85,

91,86, 82});

System.out.println(g.getGrade());


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/*accessing teacher package */

Teacher f=new Teacher("Tom");Grade gf= new Grade(f,s1,"001",Grade.B);System.out.println("Teacher Name "+gf.getFaculty().getName()+"-"+gf.getFacu

lty().getId());System.out.println("Student who graded"+ gf.getStudent().getName());

System.out.println("Grade "+gf.getGrade());}}


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The program does not compile?

Can you figure out why ?

How will you solve it ?


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34Java

Some of the important packages :

java.lang : All the classes that are fundamental tothe Java programming language. String class is a

part of java.lang package.

java.util : All the utility classes like Date,LinkedList etc. which is frequently used by mostapplications.

java.io : All the classes related to IO.

java.sql : All the classes related to database.

java.awt : All the classes related to building GUI.

java.applet : Applet related classes.

Packages in JDK


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35Java

Static Imports New feature in java 1.5

Makes the static members of a class available to our codedirectly without us explicitly specifying the class name.

Syntax:

import static packageName.ClassName.*;

(imports all the static members)

Or

import static

packageName.ClassName.staticMember;(imports only the particularstaticMember available )


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Example

import static java.lang.System.out;import static teacher.Grade.*;import teacher.*;

class GradeTest{public static void main(String str[]){Teacher f=new teacher.Teacher("Tom");Grade gf= new Grade(f,new

student.Student("Malini"),"001",B);out.println("Grade "+ gf.getGrade());}} Instead of Grade.B

Instead of System.out.println

Static member of System

J fil l


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Java source file rules A java source file can contain:

b) a single package statement which will be the firststatement.

c) any no. of import statements. They should bebetween package and class statement.

d) any no. of classes with default access specifier. Insuch cases, the file name could be anything.java.

e) only one public class. If there is a public class thenthe file must be named after the public class name.

f) The package and the import statements apply to allthe classes in the source file.

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