packages.ppt

PACKAGES
Explained By:
Sarbjit Kaur.
Lecturer, Department of Computer Application,
PGG.C.G., Sector: 42, Chandigarh

Packages
• Packages are Java’s way of grouping a number of related classes
and/or interfaces together into a single unit. That means, packages
act as “containers” for classes.
• The benefits of organising classes into packages are:
- The classes contained in the packages of other
programs/applications can be reused.
- In packages classes can be unique compared with classes in other
packages. That two classes in two different packages can have the
same name. If there is a naming clash, then classes can be accessed
with their fully qualified name.
- Classes in packages can be hidden if we don’t want other packages
to access them.
- Packages also provide a way for separating “design” from coding.

Java Foundation Packages
Java provides a large number of classes groped into different packages based on
their functionality
• The six foundation Java packages are:
- java.lang :
Contains classes for primitive types, strings, math functions, threads, and
exception.
- java.util :
Contains classes such as vectors, hash tables, date etc.
- java.io :
Stream classes for I/O .
- java.awt :
Classes for implementing GUI – windows, buttons, menus etc.
- java.net :
Classes for networking .
- java.applet :
Classes for creating and implementing app.

Using System Packages
• The packages are organised in a hierarchical structure .For example, a
package named “java” contains the package “awt”, which in turn contains
various classes required for implementing GUI (graphical user interface).
Java “java” Package containing
“lang”, “awt”,.. packages;
Can also contain classes.
awt package containing classes.
Classes containing methods
lang
graphics
font
image
awt

Accessing Classes from Packages
There are two ways of accessing the classes stored in packages:
• Using fully qualified class name
java.lang.Math.sqrt(x);
• Import package and use class name directly
import java.lang.Math
Math.sqrt(x);
• Selected or all classes in packages can be imported:
• Implicit in all programs: import java.lang.*;
• package statement(s) must appear first
import package.class;
import package.*;

Creating Packages
Java supports a keyword called “package” for creating user-defined
packages. The package statement must be the first statement in a Java
source file (except comments and white spaces) followed by one or more
classes.
package myPackage;
public class ClassA {
// class body
}
class ClassB {
}
Package name is “myPackage” and classes are considered as part of
this package; The code is saved in a file called “ClassA.java” and located in
a directory called “myPackage”.

Accessing a Package
• As indicated earlier, classes in packages can be
accessed using a fully qualified name or using a short-
cut as long as we import a corresponding package.
• The general form of importing package is:
import package1[.package2][…].classname
• Example:
import myPackage.ClassA;
import myPackage.secondPackage
• All classes/packages from higher-level package can be
imported as follows:
import myPackage.*;

Using a Package
• Let us store the code listing below in a file named“ClassA.java” within
subdirectory named “myPackage”within the current directory (say “abc”).
• package myPackage;
• public class ClassA {
• // class body
• public void display()
• {
• System.out.println("Hello, I am ClassA");
• }
• }
• class ClassB {
• // class body
• }

Using a Package
Within the current directory (“abc”) store the following
code in a file named“ClassX.java”
import myPackage.ClassA;
public class ClassX
{
public static void main(String args[])
{
ClassA objA = new ClassA();
objA.display();
}
}

Compiling and Running
• en ClassX.java is compiled, the compiler compiles it and places .class file in
current directly. If .class of ClassA in subdirectory “myPackage” is not
found, it comples ClassA also.
• Note: It does not include code of ClassA into ClassX ✞
• When the program ClassX is run, java loader looks for ClassA.class file in a
package called “myPackage” and loads it.

Using a Package
Let us store the code listing below in a file named“ClassA.java” within
subdirectory named“secondPackage” within the current directory
(say“abc”).
public class ClassC {
// class body
public void display()
{
System.out.println("Hello, I am ClassC");
}
}

Using a Package
• Within the current directory (“abc”) store the following code in a file
named“ClassX.java”
• import myPackage.ClassA;
• import secondPackage.ClassC;
• public class ClassY
• {
• public static void main(String args[])
• {
• ClassA objA = new ClassA();
• ClassC objC = new ClassC();
• objA.display();
• objC.display();
• }
• }

Output
• Hello, I am ClassA
• Hello, I am ClassC

packages.ppt

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