09. exceptions

8/11/2019 09. Exceptions

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Le Hong HaiUET-VNUH

Exceptions


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What is an Exception?

Event that occurs when somethingunexpected happens null.someMethod();

(new int[1])[1] = 0;

int i = string;

File not exists

..

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Exception Handling

Older programming languages such as C/C++have some drawbacks in exception handing

The programmers are not made to aware of theexceptional conditions

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Example

if(file exists) {

open file;

while(there is more records to be processed) {

if (no IO errors) {

process the file record} else{

handle the errors }

} if(file is opened)

close the file;

} else{report the file does not exist;

}

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Handing Exception in Java

Java overcomes these drawbacks by building the

exception handling into the language rather than

leaving it to the discretion of the programmers Exception handling codes are separated from the

main logic [Via the try-catch-finally construct]

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Exception Handling Process

When an exception occurs inside a Java method, themethod creates an Exception object and passes(throw) the Exception object to the JRE

The JRE is responsible for finding an exception

handlerto process the Exception object. It searchesbackward through the call stack until it finds amatching exception handler for that particular classof Exception

If the JRE cannot find a matching exception handler inall the methods in the call stack, it terminates theprogram

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Exception Handling Process

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Exception message

To tell the code using your method thatsomething went wrongException in thread "main"

java.lang.ArrayIndexOutOfBoundsException: 5 at

RuntimeException.main (RuntimeException.java:8)

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Try-catch-finallytry{

// main logic, uses methods that may throw

Exceptions ...

} catch(Exception1 ex) {

// error handler for Exception1 ...

} catch(Exception2 ex) {

// error handler for Exception2 ... ...

} finally{

// finally is optional // clean up codes,

always executed regardless of exceptions ...

}

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The finally-block is meant for cleanup code such as closing the file,

database connection regardless of whether the try block succeeds.

The finally block is always executed (unless the catch-block pre-

maturely terminated the current method).


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Exception Classes

The base class for all Exception objects isjava.lang.Throwable, together with its twosubclassesjava.lang.Exceptionandjava.lang.Error

The Errorclass describes internal system errors. If such

an error occurs, there is little that you can do and theprogram will be terminated by the Java runtime

The Exceptionclass describes the error caused by yourprogram (e.g. FileNotFoundException IOException). Theseerrors could be caught and handled by your program

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Example using custom exceptionpublic class MyMagicExceptionTest {

public static void magic(int number) throws

MyMagicException {

if (number == 8) {

throw (new MyMagicException("you hit the magic

number"));}

System.out.println("hello");

}

public static void main(String[] args) {

try {magic(9); // does not trigger exception

magic(8); // trigger exception

} catch (MyMagicException ex) {

// exception handler

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Example

test assumptions

Value > 0

String is an Integer

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Enable Assertion

Assertion can be disabled to ensure that they arenot a performance liability in the productionenvironment

It is disabled by default. To enable use theruntime command-line option

enableassertions (or ea)

> java -eaAssertionSwitchTest // enableassertion

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Enable Assertion in Eclipse

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09. exceptions

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