Chapter 12: Handling Exceptions and Events

Chapter 12: Handling Chapter 12: Handling Exceptions and EventsExceptions and Events

JJavaava PProgramming:rogramming:

From Problem Analysis to Program From Problem Analysis to Program Design, Design,

Second EditionSecond Edition

Java Programming: From Problem Analysis to Program Design, Second Edition 2

Objectives Learn what an exception is. See how a try/catch block is used to handle

exceptions. Become aware of the hierarchy of exception

classes. Learn about checked and unchecked exceptions. Learn how to handle exceptions within a program. Discover how to throw and rethrow an exception. Learn how to handle events in a program.

Exception

An occurrence of an undesirable situation that can be detected during program execution.

Examples:

Division by zero.

Trying to open an input file that does not exist.

An array index that goes out of bounds.

import java.util.*;

public class ExceptionExample1 { static Scanner console = new Scanner(System.in);

public static void main (String[] args) { int dividend, divisor, quotient;

System.out.print("Line 2: Enter the “ + "dividend: "); dividend = console.nextInt();

System.out.println(); System.out.print("Line 5: Enter the “ + "divisor: "); divisor = console.nextInt();;

System.out.println(); quotient = dividend / divisor;

System.out.println("Line 9: Quotient = “ + quotient); }}

Sample Run2

Line 2: Enter the dividend: 24

Line 5: Enter the divisor: 0

Exception in thread "main“

java.lang.ArithmeticException: / by zero

at ExceptionExample1.main(ExceptionExample1.java:23)

Sample Run3

Line 2: Enter the dividend: 2e

Exception in thread "main" java.util.InputMismatchException

at java.util.Scanner.throwFor(Scanner.java:819)

at java.util.Scanner.next(Scanner.java:1431)

at java.util.Scanner.nextInt(Scanner.java:2040)

at java.util.Scanner.nextInt(Scanner.java:2000)

at ExceptionExample1.main(ExceptionExample1.java:15)

Handling Exceptions within a Program

Can use an if statement to handle an exception.

import java.util.*;public class ExceptionExample2{ static Scanner console = new Scanner(System.in); public static void main (String[] args) { int dividend, divisor, quotient; System.out.print("Line 2: Enter the " + "dividend: "); dividend = console.nextInt(); System.out.println(); System.out.print("Line 5: Enter the " + "divisor: "); divisor = console.nextInt();; System.out.println();

if (divisor != 0) { quotient = dividend / divisor; System.out.println("Line 10: " + "Quotient = " + quotient); }else System.out.println("Line 11: Cannot " + "divide by zero.");

Handling Exceptions within a Program

However, suppose that division by zero occurs in more than one place within the same block.

In this case, using if statements may not be the most effective way to handle the exception.

Java’s Mechanism of Exception Handling

With exception handling, a program can continue executing after dealing with a problem.

When an exception occurs, an object of a particular exception class is created.

Java provides a number of exception classes to effectively handle certain common exceptions, such as: Division by zero Invalid input File not found

Division by zero is:

An arithmetic error.

Handled by the class ArithmeticException.

When a division by zero exception occurs, the program creates an object of the class ArithmeticException.

When a Scanner object is used to input data into a program, any invalid input errors are handled using the class InputMismatchException.

The class Exception (directly or indirectly) is the superclass of all the exception classes in Java.

try/catch/finally Block Statements that might generate an exception are

placed in a try block.

The try block:

Might also contain statements that should not be executed if an exception occurs.

Is followed by zero or more catch blocks.

A catch block:

Specifies the type of exception it can catch.

Contains an exception handler.

try/catch/finally Block

The last catch block may or may not be followed by a finally block.

Any code contained in a finally block always executes regardless of whether an exception occurs, except when the program exits early from a try block by calling the method System.exit.

If a try block has no catch block, then it must have the finally block.

If no exception is thrown in a try block, all catch blocks associated with the try block are ignored and program execution resumes after the last catch block.

If an exception is thrown in a try block, the remaining statements in the try block are ignored.

The program searches the catch blocks in the order in which they appear after the try block and looks for an appropriate exception handler.

If the type of the thrown exception matches the parameter type in one of the catch blocks, the code of that catch block executes and the remaining catch blocks are ignored.

If there is a finally block after the last catch block, the finally block executes regardless of whether an exception occurs.

Order of catch Blocks

The heading of a catch block specifies the type of exception it handles.

A catch block can catch either all exceptions of a specific type or all types of exceptions.

A reference variable of a superclass type can point to an object of its subclass.

If you declare an exception using the class Exception in the heading of a catch block, then that catch block can catch all types of exceptions because the class Exception is the superclass of all exception classes.

In a sequence of catch blocks following a try block, a catch block that declares an exception of a subclass type should be placed before catch blocks that declare exceptions of a superclass type.

It is a syntax error if a catch that catches a superclass object is placed before a catch that catches an object of a subclass of that superclass

import java.util.*;public class ExceptionExample3{ static Scanner console = new Scanner(System.in);

public static void main (String[] args) { int dividend, divisor, quotient; try { System.out.print("Line 4: Enter the " + "dividend: "); dividend = console.nextInt(); System.out.println(); System.out.print("Line 7: Enter the " + "divisor: "); divisor = console.nextInt(); System.out.println(); quotient = dividend / divisor; //line 10 System.out.println("Line 11: Quotient = " + quotient); } catch (ArithmeticException aeRef) //line 12 { System.out.println("Line 13: Exception " + aeRef.toString()); } catch (InputMismatchException imeRef) { System.out.println("Line 15: Exception " + imeRef.toString()); } }}

Sample Run2

Line 13: Exception java.lang.ArithmeticException: / by zero

The statement in line 10 throws an ArithmeticException,which is caught by the catch block starting at Line 12.

Sample Run3

Line 7: Enter the divisor: 2f1

Line 15: Exception java.util.InputMismatchException

The statement in line 8 throws an

InputMismatchException. Tis exception is thrown by the

method nextInt of the class Scanner.

The catch block starting at Line 14 catches this exception .

2f1 stays as the next input token in the input stream

If the input token is invalid, the method nextInt does not remove that input token from the input stream.

catch (InputMismatchException imeRef) {

String str; str = console.next(); System.out.println("Line 17: Exception "

+ imeRef.toString() + " " + str );

Sample Run3

Line 7: Enter the divisor: 2f1

Line 17: Exception java.util.InputMismatchException 2f1

The exception classes provided by Java

Java’s Exception Class class Exception:

Subclass of class Throwable. Superclass of classes designed to handle exceptions.

Various types of exceptions: I/O exceptions. Number format exceptions. File not found exceptions. Array index out of bounds exceptions.

Various exceptions categorized into separate classes and contained in various packages.

Java’s Exception Class

After a method throws an exception, the runtime system attempts to find something to handle it.

The list of methods is known as the call stack

Searching the call stack for the exception handler.

Checked Exceptions

Any exception that can be recognized by the compiler.

Example:

FileNotFoundExceptions.

Unchecked Exceptions Exceptions that cannot be recognized when the

program compiles (must be checked for by programmer).

Examples: Division by zero Array index out of bounds

Syntax:throws ExceptionType1, ExceptionType2,...

ExceptionType1, ExceptionType2, and so on are names of exception classes

Checked and Unchecked Exceptions

If a checked exception is thrown by a method , or if that method calls methods that throw a checked exceptions, each of those exceptions must be declared in the throws clause of that method or caught in a try/catch in that method.

Exceptions Example Code

public static void exceptionMethod()

throws InputMismatchException,

FileNotFoundException

//statements

The method exceptionMethod throws exceptions of the type InputMismatchException and FileNotFoundException.

The class Exception and the Operator instanceof

A reference of a superclass type can point to objects of its subclass.

You can determine if a reference variable points to an object using the operator instanceof.

You can combine catch blocks using this facility.

try { System.out.print("Line 4: Enter the " + "dividend: "); dividend = console.nextInt(); System.out.println(); System.out.print("Line 7: Enter the " + "divisor: "); divisor = console.nextInt(); System.out.println(); quotient = dividend / divisor; System.out.println("Line 11: Quotient = " + quotient); }catch (Exception eRef){ if (eRef instanceof ArithmeticException) System.out.println("Line 14: Exception " + eRef.toString() + ”division by zero”); else if (eRef instanceof InputMismatchException) System.out.println("Line 16: Exception " + eRef.toString() + “invalid input”); }

Example 12-6

int dividend, divisor;double quotient; try {

System.out.print("Line 4: Enter the " + "dividend: ");

dividend = console.nextInt(); System.out.println();

System.out.print("Line 7: Enter the " + "divisor: "); divisor = console.nextInt(); System.out.println(); quotient = (double) dividend / divisor; System.out.println("Line 11: Quotient = " + quotient);

}catch (ArithmeticException aeRef) {

System.out.println("Line 13: Exception " + aeRef.toString());

}catch (InputMismatchException imeRef) {

System.out.println("Line 15: Exception " + imeRef.toString());

Sample run

Line 11: Quotient = Infinity

public static void main (String[] args) {

int dividend, divisor; double quotient; try { System.out.print("Line 4: Enter the " + "dividend: ");

dividend = console.nextInt(); System.out.println(); System.out.print("Line 7: Enter the " + "divisor: ");

divisor = console.nextInt(); System.out.println(); quotient = result(dividend,divisor); System.out.println("Line 11: Quotient = " + quotient); } catch (ArithmeticException aeRef){ System.out.println("Line 13: Exception " + aeRef.toString()); }

catch (InputMismatchException imeRef) { System.out.println("Line 15: Exception " + imeRef.toString()); }}

public static double result( int number1, int number2) throws ArithmeticException

{ if (number2 == 0) throw new ArithmeticException("Attempted to divid by zero"); return (double) number1 / number2; }

Sample run

Line 13: Exception java.lang.ArithmeticException: Attempted to divid by zero

Rethrowing and Throwing an Exception

When an exception occurs in a try block, control immediately passes to one of the catch blocks.

Typically, a catch block does one of the following: Completely handles the exception. Partially processes the exception.

In this case, the catch block either rethrows the same exception or throws another exception for the calling environment to handle the exception.

Rethrows the same exception for the calling environment to handle the exception.

Useful when:

Catch block catches exception but is unable to handle it.

Catch block decides exception should be handled by calling environment.

Allows programmer to provide exception handling code in one place.

Syntax:

throw exceptionReference;//checked or unchecked exception

import java.util.*;public class RethrowExceptionExmp1{ static Scanner console = new Scanner(System.in); public static void main(String[] args) { int number; try { number = getNumber(); System.out.println("Line 5: number = " + number); } catch (InputMismatchException imeRef) { System.out.println("Line 7: Exception " + imeRef.toString()); } }}

public static int getNumber() { int num; try { System.out.print("Line 11: Enter an “ + "integer: "); num = console.nextInt(); System.out.println(); return num; } catch (InputMismatchException imeRef) { throw imeRef; } }}

Sample Run

Line 11: Enter an integer: e

Example 12-8

public static int getNumber() throws InputMismatchException { int num; try { System.out.print("Line 11: Enter an “ + "integer: "); num = console.nextInt(); System.out.println(); return num; } catch (InputMismatchException imeRef) { System.out.println("Line16: Exception " + imeRef.toString()); throw new InputMismatchException ("getNumber"); } }}

Sample Run

Line 7: Exception java.util.InputMismatchException: getNumber

public static int getNumber() { int num; System.out.print("Line 11: Enter an “ + "integer: "); num = console.nextInt(); System.out.println(); return num; }}

Sample Run

public static int getNumber() { int num; try { System.out.print("Line 11: Enter an “ + "integer: "); num = console.nextInt(); System.out.println(); return num; } catch (InputMismatchException imeRef) { System.out.println("Line 7: Exception " + imeRef.toString());//this will cause a compile error even if you //define the exception using the throws clause of //the getNum method heading//compile error message: missing return statement } }}

The mechanism of rethrowing an exception allows a program to handle all the exceptions in one location rather than spreading exception-handling code throughout the program

The Method printStackTrace

Used to determine the order in which the methods were called and where the exception was thrown.

import java.io.*;public class PrintStackTraceExample1{ public static void main(String[] args) { try { methodA(); } catch (Exception e) { System.out.println(e.toString() + " caught in main"); e.printStackTrace(); } }

public static void methodA() throws Exception { methodB(); } public static void methodB() throws Exception { methodC(); } public static void methodC() throws Exception { throw new Exception("Exception generated " + "in method C"); }}

Example 12-9

Because the methods A and B do not handle the exception thrown by methodC, they contain the throws Exception clause in their heading

Sample Run:java.lang.Exception: Exception generated in method C

caught in mainjava.lang.Exception: Exception generated in method C at PrintStackTraceExample1.methodC - (PrintStackTraceExample1.java:31) at PrintStackTraceExample1.methodB - (PrintStackTraceExample1.java:26) at PrintStackTraceExample1.methodA - (PrintStackTraceExample1.java:22) at PrintStackTraceExample1.main - (PrintStackTraceExample1.java:11)

Example 12-10

import java.io.*;public class PrintStackTraceExample1{ public static void main(String[] args) { try { methodA(); } catch (Exception e) { System.out.println(e.toString() + " caught in main"); e.printStackTrace(); } }

public static void methodA() throws Exception{ methodB();}public static void methodB() throws Exception{ methodC();}public static void methodC(){ throw new Exception("Exception generated in method C"); }}//this will cause a compile error:Line 30 : unreported exception java.lang.Exception; must be caught or declared to be thrown//if the exception was unchecked exception it will not cause this compile error

Exception-Handling Techniques Terminate program.

Output appropriate error message upon termination. Fix error and continue.

Repeatedly get user input. Output appropriate error message until valid value is

entered. Log error and continue.

Write error messages to file and continue with program execution.

Example 12-11

FixErrorAndContinue

Chapter Summary

Exception definition Handling exceptions within a program:

try/catch/finally block. Order of catch blocks. Using try/catch blocks in a program. The class Exception and the Operator instanceof.

Rethrowing and throwing an exception.

Chapter Summary Exception

Hierarchy Classes

Checked and unchecked exceptions The method printStackTrace Exception handling techniques:

Terminate program. Fix error and continue. Log error and continue.

Chapter 12: Handling Exceptions and Events

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