Date post: | 07-Aug-2015 |
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Accessing classes from
packageNaming conventio
ns
API packages
Creating packages
Packages and interfaces
PackagesPackages are java’s way of grouping a variety of classes and interfaces together.Packages acts as “containers ” for classes.
Benefits of using packages:
Reused Hiding separating “design” from “coding”. Two classes in two different packages can have same name.
packages
Packages and interfaces
Java API Packages
Java API provides a large number of classes grouped into different packages according to functionality.
Packages and interfacesPackage name Contents
java.lang Language support classes that java compiler itself uses and automatically imported . They include classes for primitive types , strings , math functions , threads and exceptions.
java.util Language utility classes such as vectors , hash tables , random numbers , date , etc.
java.io Input/output support classes that provide facilities for input and output of data.
java.awt Set of classes for implementing GUI like windows , buttons , lists , menus , colors , etc.
java.net Classes for networking for communication with local computer as well as with internet servers
java.applet Classes for creating and implementing applets
Packages and interfaces
Using systemPackages
java
PackageContaining awt package
PackageContaining classes
Classes Containing methods
Hierarchical representation of java.awt package
There are two ways of accessing the classes stored in the package.
Java.awt.Color;import packagename.classname; (or)import packagename.*;
First approach is perhaps easy and best way to access a class only once or when we need not have to access any other classes of the package.But, in many situations we want to use a class in number of places in program then we use second approach
Packages and interfaces
Accessing the classes stored in the package
There are known as import statements and must appear at the top of the file , before any class declarations , import is a keyword.
import Java.awt.Color;
This statement allows specified class in specified package to be imported
import java.awt.*;
This statement imports every class contained in the specified package.
Packages and interfaces
Accessing the classes stored in the package
Namingconventions
Packages begin with lowercase .
java.lang.Math.sqrt(x);
Packages and interfaces
Package name
Classname
Method name
Packages and interfaces
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Creating packages
We must first declare the name of the package using package keyword followed by package name.This must be the first statement in the source file except the comments and whitespaces followed by normal class definition.package firstPackage; //package declarationpublic class firstclass //class definition{………….…………..…………..}
Packages and interfaces
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Creating packagesRemember that the .class files must be located in a directory that has the same name as the package and this directory should be a subdirectory where classes will import the package are located.Creating ore own packages involve the following steps:Declare the package at the beginning of the file using the form package packagename;Define the class that is to be put in the package and declare it public.Create the subdirectory under the directory where the main source files are stored.Store the listing as the class name java file in the subdirectory created.Compile the file . This creates .class file in the subdirectory
Packages and interfacesAdding a class to a packageIt is simple to add a class to an existing package.
package p1; public class A { //body of A } if we want to add another class B to this package Define the class and make it public Place the package statements package p1; public class B{ //body of B } Store B.java file in p1 directory. Compile B.java file . This will create B.class file and place it in the directory p1.
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Implementing
interfaceNaming
conventions
Extending interface
Creating packages
Packages and interfaces
Why do we need Interfaces?
For the use of multiple inheritance which is not supported by JavaA java class cannot be a subclass of more than one super class , it can implement more than one interface , enabling us to create classes that build upon other classes without the problem created by multiple inheritance.
Interface
Packages and interfaces
Defining interfaces
An interface is basically a kind of class
What is the difference between an interface and a class?
Interface defines only abstract methods and fields. I .e. , the interface do not specify any code to implement these methods and data fields contain only constantsTherefore it is the responsibility of the class that implements an interface to define the code for implementation of these methods.
Packages and interfaces
Syntax for interface
interface interfacename{ variables declaration; methods declaration;}Here interface is the keyword .Example :interface area{final static float pi=3.14f;Float compute(float x , float y)void show();}
Like classes , interfaces can also be extended by the key word extendsSyntax :
interface name2 extends name1{//Body of name2}Example : interface ItemConstants{int code = 1001;String name=“fan”;}Interafce Item extends Item Constans{ void display();}
Packages and interfaces
Extending interfaces
Implementing interfaces
Interfaces are used as “ superclasses ” whose properties are inherited by classes.It is necessary to create a class that inherits the given interface.Syntax: class classname implements interfacename { //body of class } Here class implements the interface. implements is the keywordIf a class that implements an interface does not implement all the methods of the interface then the class becomes an abstract class and cannot be instantiated
Packages and interfaces
Implementing interface with an example
interface Area{final static float pi=3.14f;float compute (float x , float y );}Class Rectangle implements Area{ public float compute(float x, float y){Return(x*y);}}class Circle implements Area{ public float compute(float x, float y){return(pi*x*x);}}class InterfaceTest{ public static void main(String args[]) {{ Rectangle rect = new Rectangle(); Circle cir = new Circle(); Area a1; a1=rect; System.out.println(“Area of rectangle = ”+ a1.compute(10,20));
a1=cir; System.out.println(“Area of rectangle = ”+ a1.compute(10,0));}}
Packages and interfaces
Various Forms of interface implementationinterface
implementation
class
Extension
class
class
class
class
Extension
Extension
interface
interface
implementation
Extension
Various Forms of interface implementationinterface
implementation
classclass cla
ss
Extension
interface interface
implementation
Accessing interface variable
Packages and interfacesInterfaces can be used to declare a set of constants that can be used in different classesThis is similar to creating header files in C++ The constant class will be available to any class that implements the interface . The values can be used in any method.Ex : interface A{int m=10;Int n=50;}Class B implements A{ int x=m;void methodB ( int size{………..If (size<n)……….)