Transcript Experience with Packages

Packages in Java: Extra Lecture
Overview
 Motivation
 Packages:
 Example
for Packages.
Some Definitions.
packages.
Packages: Extra Material
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Motivation for Packages

Until now, you have used classes from the Java API
library by importing the package containing the
class or classes you need.

A package is a group of related classes.

Packages are actually directory structures used to
organise classes and interfaces

Packages provide a mechanism for software reuse
and a convention for unique class names .

Packages make classes easier to find and use and
are used to control access to classes.

Packages potentially reduce compilation time
because if a large program is broken down into
several classes, only those classes that were
modified after the last compilation will be
recompiled.
Packages: Extra Material
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Steps for Creating Packages
1. Create as many directories as required.
2. Put related classes in the same directory.
3. Give the directory a meaningful name to reflect what the
classes in the directory do.
4. Tell the Java system that each of these classes belongs to this
package. You do this by adding a line like the following at
the top of each class file:
import myPackage;
Note that step 2 is not sufficient for the java system to
understand that the files in that directory belong to the same
package.
5. Remember to declare public all classes in the package that are
required to be used by other classes outside this package.
6. When your application is fully tested, debugged, and ready for
deployment, use the JavaTM Archive file format to deploy the
application.
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Experience with Packages: Example 1

Create a package, package1Lecture2, containing two
classes A.java and D.java given below.
package package1Lecture2;
public class A {
int
a = 2;
public
int b = 3;
protected int c = 5;
private
int d = 7;
public A(){
System.out.println("Constructor for
class A.");
System.out.println("Value of a: "+ a);
System.out.println("Value of b: "+ b);
System.out.println("Value of c: "+ c);
System.out.println("Value of d: "+ d);
}
}
------------------------------------------package package1Lecture2;
class D{
public static void main(String args []){
A aObject = new A();
}
}
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Experience with Packages: Example 1
(cont.)

Compile, run and observe the output of the program.

Make sure you understand every bit of the output, before
developing this example further.

Then, add the following class to package1Lecture2 and

Add the line B bObject = new B(); in the main()
method in class D shown in the preceding slide.
package package1Lecture2;
class B extends A {
public B(){
System.out.println("Constructor for
class B.");
System.out.println("Value of a:"+ a);
System.out.println("Value of b:"+ b);
System.out.println("Value of c:"+ c);
System.out.println("Value of d:"+ d);
}
}
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Experience with Packages: Example 1
(cont.)


The package now fails to compile after adding this
class. Use your knowledge of ICS102 to fix the error.
Add the following class in the same fashion as you did
for class B above. Remember to add the line
C cObject = new C();
in the main() method in class D.
package package1Lecture2;
class C {
public C(){
A obj = new B();
System.out.println("Constructor for
class C.");
System.out.println("Value of a:"+
obj.a);
System.out.println("Value of b:"+
obj.b);
System.out.println("Value of c:"+
obj.c);
System.out.println("Value of d:"+
obj.d);
}}
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Experience with Packages: Example 2

Create another package, package2Lecture2, using the
following 3 classes.

Notice that this new package uses classes in the package of
Example 1.

Use your knowledge of access modifiers to debug the
program by commenting out some lines classes P, and Q.
package package2Lecture2;
class P extends package1Lecture2.A {
public P(){
System.out.println("Constructor for
class P in package2.");
System.out.println("Value of a: "+ a);
System.out.println("Value of b: "+ b);
System.out.println("Value of c: "+ c);
System.out.println("Value of d: "+ d);
}
}
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Experience with Packages: Example 2
package package2Lecture2;
class Q {
public Q(){
package1Lecture2.A aObject = new
package1Lecture2.A();
System.out.println("Constructor for
class C.");
System.out.println("Value of a:"+
aObject.a);
System.out.println("Value of b:"+
aObject.b);
System.out.println("Value of c:"+
aObject.c);
System.out.println("Value of d:"+
aObject.d);
}
}
package package2Lecture2;
class R{
public static void main(String args []){
P aObject = new P();
Q bObject = new Q();
}
}
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Experience with Packages: Example 3
package package3Lecture2;
public class P5{
static int twice(int x){
return 2*x;
}
public static int twiceSquare(int
x){
return twice(x)*twice(x);
}
}
----------------------------------package package4Lecture2;
public class P6 extends
package3Lecture2.P5 {
public static int twice(int x){
//
super.twice(x);
return twiceSquare(x);
}
}
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Experience with Packages: Example 3
(cont.)

Test the given packages above using the following
import package3Lecture2.P5;
import package4Lecture2.P6;
class TestP5P6 {
public static void main(String []
args){
P6 p6Object = new P6();
System.out.println(p6Object.twice(
3));
}
}
--------------------------------
Can java packages be mutually recursive? That is,
can a package P refer to classes in another package
Q and vice-versa? Give an example to support your
answer.
Packages: Extra Material
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Package Structure for Our Programs
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For the rest of this course you are required to organise all
your programs into a package structure of the following form:
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