Session 3

Java Technology Interfaces and Exceptions

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Session Plan 

Interfaces in Java

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Packages in Java

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Need and Advantages of using package

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Creating a Package

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Using Existing Packages

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Java Class Libraries

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Session Plan 

Basic concepts of Exception Handling

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Errors and Exceptions

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Try – Catch – Finally block

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Throw & Throws



Some Built-in exceptions



User defined exceptions

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Interfaces in Java 

An interface is a set of predefined methods to be implemented by one or more classes in future.

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Java achieves multiple inheritance. By providing the interface keyword, Java allows to fully utilize the “one interface, multiple methods” aspect of polymorphism

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Interfaces do not have any instance variables. Any variable declared in an interface is automatically taken as public, static and final

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Creating an interface class public interface myinterface { public void method1(int x, int y) ; }

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Implementing Multiple Inheritance in Java Inter Paternal

Inter Maternal Inter GrandParent

INTERFACE Class Child Class Child implements Paternal, Maternal, GrandParent {............; } Nace Solutions (P) Ltd. Copyright © 2008,All Rights Reserved.

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Interfaces … 

Comparable to a pure abstract class

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Classes implement, by providing definitions to the methods of the interface

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Inheritance is possible in interfaces, even multiple inheritance is possible

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Interface vs Abstract classes 

Interfaces don’t worry about classes relationships. It just requires that the classes should implement them.

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Abstract classes force strict class relationships.

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Interface vs Multiple Inheritance 

Interfaces are not synonymous with multiple inheritance.

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However, a class can implement more than one interface.

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So in a way, we can tell interface is substitute for multiple inheritance.

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Packages in Java – Why ? What ? 

Reusability of code is one of the most important requirements in the software industry.

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Reusability saves time, effort and also ensures consistency.

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A class once developed can be reused by any number of programs wishing to incorporate the class in that particular program.

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Features of Packages 

Organize your classes into smaller units and make it easy to locate and use the appropriate class file.

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Avoid naming conflicts.

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Packages allow you to protect your classes, data and methods in a larger way than on a class-to-class basis.

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Package names can be used to identify your classes.

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Concept of Packages 

In Java, the code which can be reused by other programs is put into a “Package”.

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A Package is a collection of classes, interfaces and/or other packages.

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Packages are essentially a means of organizing classes together as groups.

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Usage of packages: 

Package creation 

In Java packages are created in the following manner. 

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Package packagename;

Package member can be accessed using import statements 

import package_name. class_name;

My_Package 

My_Sub_Package

import packagename,packagename.*;

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My_Class

Importing + Creating a Package 

While creating a package, care should be taken that the statement for creating a package must be written before any other import statements

LEGAL

ILLEGAL

package mypackage ;

import java.io;

import java.io;

package mypackage ;

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Standard Java Packages The Three Java Packages that are essential to any Java program are : 

java.lang 

Contains classes that form the basis of the design of the programming language of Java.

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You don’t need to explicitly import this package. It is always imported for you.

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The Wrapper classes, System class, Thread class etc, belong to this package.

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Standard Java Packages 

java.io 

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Input/Output operations in Java is handled by the java.io package.

java.util 

Contains classes and interfaces that provide additional utility.

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E.g. : creating lists, calendar, date etc.

java.net 

This package provides classes and interfaces for TCP/IP network programming.

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Standard Java Packages 

java.awt 



This package is useful to create GUI applications.

java.applet 

This package consists of classes that you need, to execute an applet in the browser or an appletviewer

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Math Class – java.lang package 

Math class – which provides mathematical functions like sine, cosine and square root.

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Some methods in the Math class (static methods) 

int abs(int i) - returns the absolute value of i

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double abs(double d) - returns the absolute value of d

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double ceil(double d) - returns as a double the smallest integer that is not less than d

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double floor(double d) - returns as a double the largest integer

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Exceptions in Java 

A Java exception is an object that describes an exceptional (abnormal) condition, that has occurred in a piece of code.

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Exceptions can be generated by the Java run-time system, or they can be manually generated by your code.

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Hierarchy Object

Throwable

Error Nace Solutions (P) Ltd. Copyright © 2008,All Rights Reserved.

Exception

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Exceptions and Errors 

Exceptions are situations within the control of an application, that it should try to handle

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When a dynamic linking failure or some other “hard” failure in the virtual machine occurs, the virtual machine throws an error.

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Java programs need not catch these errors, because these errors are beyond their scope.

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Some Java Errors 

ClassFormatError

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InternalError

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LinkageError

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OutOfMemoryError

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StackOverflowError

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UnknownError

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Types of Exceptions Exceptions

Checked Exceptions (Compile time)

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Unchecked Exceptions (Run time)

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Checked Exceptions 

These exceptions occur not because of the fault of programmer. The reasons being, not available I/O, network error, file not found for reading, etc.



These exceptions are checked during compile time. Hence appropriate code has to be written to either catch or throw the exceptions.

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Runtime Exceptions 

Examples NullpointerException ArrayOutOfBoundsException ArithmeticException

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It is the duty programmer to handle these exceptions.

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Exception Example 

int i=5/0;

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What happens when the above code is executed ?

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Exception is thrown (an object is thrown)

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How to recover from this ? (handle it !)

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Exception Handling Mechanism 

Try-catch block 

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Multiple catch block

Finally 

Try-catch-finally block

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Try- finally

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Throws

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Throw

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Try-catch 

A try block is always followed by a catch block, which handles or catches the exception. A catch block always monitors the preceding try block. The catch block is not executed if no exception is thrown.

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Try-catch block try { ……… ……… ………} catch (exceptionType name) { ……… ……… ………} Nace Solutions (P) Ltd. Copyright © 2008,All Rights Reserved.

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Example Exception class ExceptionExample { public static void main(String argx[]) { int n,i; try { i=0; n=12/i; System.out.println("This line will not be printed"); catch(ArithmeticException e) { System.out.println("Division by zero" + e); } System.out.println("After catch statement"); } } Nace Solutions (P) Ltd. Copyright © 2008,All Rights Reserved.

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}

Multiple Catch try

{ ……… ……… ……… } catch (exceptionType name) { ……… ……… ……… } catch (exceptiontype2 name2) { ……… ……… ……… }

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Finally Block 

The finally statement is associated with a try statement and identifies a block of statements that are executed regardless of whether or not an error occurs within the try block.

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Defines the code that is executed always

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If an exception arises with a matching catch block, then the finally block is executed after the catch block. If no exception arises, the finally block is executed after the try block.

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Try – catch - finally try

{ ……… ……… ……… } catch (exceptionType name) { ……… ……… ……… } finally { ……… ……… ……… } Nace Solutions (P) Ltd. Copyright © 2008,All Rights Reserved.

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Throws 

The throws keyword is used along with the declaration of a method that can throw an exception.

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The throws is used for handling method level exception rather than having more than one try catch block.

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This makes it mandatory for anyone calling the method to have it in try block else the compiler will give an error.

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Custom Exceptions 

Java provides numerous exception classes in its library.

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The user also can define his/her own exception and throw it when the undesired condition occurs.

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Use the throw clause to throw an exception

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Throw statement is used explicitly to throw an exception.

Syntax: throw ThrowableInstance; Nace Solutions (P) Ltd. Copyright © 2008,All Rights Reserved.

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Throwing Exceptions public void read() throws IOException { // Some code that cause IO Exception throw new IOException(); } Calling a method in main(): Try { Read(); } catch(Exception e){} Nace Solutions (P) Ltd. Copyright © 2008,All Rights Reserved.

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Advantages of Exception Handling 

Separates error handling code from regular code.



Groups error types and allows for error differentiation.

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Summary 

Interfaces in Java



Packages in Java



Need and Advantages of using package



Creating a Package



Using Existing Packages



Some standard Library classes 

Math, Util, string/stringbuffer

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Summary 

Exception Handling in Java



Errors and Exceptions



Try – Catch – Finally block



Throw & Throws



Some Built-in exceptions



User defined exceptions

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