packages,interfaces and exceptions

JAVA concepts Packages and interfaces

Exception handling

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


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


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


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.


Accessing the classes stored in the package

Namingconventions

Packages begin with lowercase .

java.lang.Math.sqrt(x);


Package name

Classname

Method name


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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{………….…………..…………..}


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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 interfacesAccessing a package import package1 [.package2] [.package3].classname;

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


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


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.


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();}


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


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));}}


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)……….)

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packages,interfaces and exceptions

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