MSc IT Programming Methodology (2). THROWS an EXCEPTION Errors?

MSc IT

Programming Methodology (2)

THROWS an EXCEPTION

Errors?

By the end of this lecture you should be able to:

• explain the term exception

• distinguish between checked and unchecked exception classes in Java

• claim an exception using a throws clause

• throw an exception using a throw command

• catch an exception in a try catch block;

• define and use your own exception classes.

Exceptions

Pre-defined exception classes in Java

Throwable

Throwable

Exception Error

Throwable

Exception Error

RuntimeExceptionIOException

Throwable

FileNotFoundException

Exception Error


Throwable


Exception Error


IllegalArgumentException IndexOutOfBoundsException

Throwable

NumberFormatException


Exception Error



Throwable

NumberFormatException ArrayIndexOutOfBoundsException


Exception Error



Throwable



Exception Error



Unchecked

Throwable



Exception Error



Unchecked

Throwable



Exception Error



Checked

Java Compiler

RuntimeException


IOException

FileNotFoundException WARNING!

Handling exceptions: an example

public class AptitudeTest{ public static void main (String[] args) { int score; System.out.print("Enter aptitude test score: "); score = TestException.getInteger( ); // test score here }} Let’s look at the

code for this method.

Outline TestException class

public class TestException{

public static int getInteger(){

// code for method goes here}

}

The read method of System.in

"hello"

System.in.read( [ ] )

array of bytes

104,101,108,108,111,13, 10

Coding the getInteger method

This is a first attempt, it will not compile!

byte [] buffer = new byte[512];

System.in.read(buffer);

String s = new String (buffer);

s = s.trim();

int num = Integer.parseInt(s);

return num;


The read method may throw a checked IOException

Dealing with exceptions..

main

getInteger

read

main

getInteger

read

IOException!

main

getInteger

read

catch

main

getInteger

read

IOException!

main

getInteger

read

throw

main

getInteger

IOException!

read

throw

main

getInteger

IOException!

read

main

getInteger

read

catch

main

getInteger

IOException!

read

main

getInteger

read

throw

main

IOException!

getInteger

read

throw

Claiming an exception

To claim an exception we add a throws clause to our method header

import java.io.*public class TestException{ private static int getInteger( ) throws IOException {

// as before }}

This method will pass on the IOException error.

Revisiting the AptitudeTest class

public class AptitudeTest

{

public static void main (String[] args)

{

int score;

System.out.print("Enter aptitude test score: ");

score = TestException.getInteger( );

// test score here

}

}

score = TestException.getInteger( );

main

IOException!

getInteger

read

throw

main

IOException!

getInteger

read

main

getInteger

read

catch

main

IOException!

getInteger

read

main

getInteger

read

throw

PROGRAM CRASH!!!!

import java.io.*; public class AptitudeTest{ public static void main (String[] args) throws IOException { int score; System.out.print("Enter aptitude test score: "); score = TestException.getInteger( ); if (score >= 50) { System.out.println("You have a place on the course!"); } else { System.out.println("Sorry, you failed your test"); } }}

A test run

Enter aptitude test score:

java.lang.NumberFormatException: 12w

at java.lang.Integer.parseInt(Integer.java:418)


at TestException.getInteger(TestException.java:10)

at AptitudeTest.main(AptitudeTest.java:11)

12w

A test run

Enter aptitude test score:

java.lang.NumberFormatException: 12w



at TestException.getInteger(TestException.java:10)

at AptitudeTest.main(AptitudeTest.java:11)

12w

A Stack Trace


byte [] buffer = new byte[512];


String s = new String (buffer);

s = s.trim();


return num;


Catching an exception.

In order to trap the exception object in a catch block you must surround the code that could generate the exception in a try block.

Syntax for using a try and catch block

try { // code that could generate an exception}catch (Exception e) { // action to be taken when an exception occurs}// other instructions could be placed here

Some methods of the Exception class

method description

printStackTrace prints (onto the console) a stack trace of the exception

toString returns a detailed error message

getMessage returns a summary error message

import java.io.*;public class AptitudeTest2{ public static void main (String[ ] args) { try { // as before score = TestException.getInteger( ); // as before } catch (NumberFormatException e) { System.out.println("You entered an invalid number!"); } catch (IOException e) { System.out.println(e); } System.out.println("Goodbye"); }}

Test Run of ApititudeTest2

Enter aptitude test score: 12w

You entered an invalid number!Goodbye



Generates a NumberFormatException




Exceptions in GUI applications

room should be a number

Using exceptions in your own classes

Look back at the Bank constructor:

public Bank(int sizeIn){ list = new BankAccount[sizeIn]; total = 0;}

A negative value would cause a NegativeArraySizeException.

Making use of exceptions: a first attempt

public Bank(int sizeIn) throws NegativeArraySizeException{ list = new BankAccount[sizeIn]; total = 0;}

Reveals that we are using an array.

Making use of exceptions: a second attempt

public Bank (int sizeIn) throws Exception{ if (sizeIn < 0) { throw new Exception ("can’t set a negative size"); } else { list = new BankAccount[sizeIn]; total = 0; }}

Testing for the exception

public class BankProgram{ public static void main(String[] args) { try { System.out.print(“Maximum number of accounts?“); size = EasyScanner.nextInt(); Bank myBank = new Bank(size);

// rest of code here } catch (Exception e) {

System.out.println(e.getMessage()); } } // other static methods here as before

Creating your own exception classes

Throwable



Exception Error



Throwable



Exception Error



NegativeSizeException

public class NegativeSizeException extends Exception{ public NegativeSizeException () { super("cannot set a negative size"); } public NegativeSizeException (String message) { super (message); }}

Amending the Bank constructor

public Bank (int sizeIn) throws Exception{ if (sizeIn < 0) { throw new Exception(); } else { list = new BankAccount[sizeIn]; total = 0; }}

public Bank (int sizeIn) throws NegativeSizeException{ if (sizeIn < 0) { throw new Exception(); } else { list = new BankAccount[sizeIn]; total = 0; }}

public Bank (int sizeIn) throws NegativeSizeException{ if (sizeIn < 0) { throw new NegativeSizeException(); } else { list = new BankAccount[sizeIn]; total = 0; }}

Testing for the NegativeSizeException

public class BankProgram{ public static void main(String[] args) {

try{

System.out.print(“Maximum number of accounts? “); size = EasyScanner.nextInt(); Bank myBank = new Bank(size);

// rest of code here}catch (NegativeSizeException e){ System.out.println(e.getMessage());

System.out.println(“due to error in Bank constructor”); }

catch (Exception e) { System.out.println(“Some unforseen error”); e.printStackTrace(); } // rest of code here }}

Re-throwing exceptions

public Bank (int sizeIn) throws NegativeSizeException{ if (sizeIn < 0) { throw new NegativeSizeException(); } else { list = new BankAccount[sizeIn]; total = 0; }}

public Bank (int sizeIn) throws NegativeSizeException{ if (sizeIn ≥ 0) { throw new NegativeSizeException(); } else { list = new BankAccount[sizeIn]; total = 0; }}

public Bank (int sizeIn) throws NegativeSizeException{ if (sizeIn ≥ 0) { list = new BankAccount[sizeIn]; total = 0; } else { throw new NegativeSizeException(); }}

public Bank (int sizeIn) throws NegativeSizeException{ try { list = new BankAccount[sizeIn]; total = 0; } catch ( ? ) { throw new NegativeSizeException(); }}

public Bank (int sizeIn) throws NegativeSizeException{ try { list = new BankAccount[sizeIn]; total = 0; } catch ( NegativeArraySizeException e ) { throw new NegativeSizeException(); }}

Practical Work

public class Exceptions{ public static void main(String[ ] args) {

int[ ] someArray = {12,9,3,11}; int position = getPosition(); display (someArray, position); System.out.println("End of program" ); } private static int getPosition() { System.out.println("Enter array position to display"); String positionEntered = EasyScanner.nextString(); return Integer.parseInt(positionEntered); } private static void display (int[ ] arrayIn, int posIn) { System.out.println("Item at this position is: " + arrayIn[posIn]); }}

return Integer.parseInt(positionEntered);

System.out.println("Item at this position is: " + arrayIn[posIn]);


ArrayIndexOutOfBoundsException

a) Re-write main so that it catches any exceptions it may now throw by displaying a message on the screen indicating the exception thrown.

public static void main(String[ ] args){

int[ ] someArray = {12,9,3,11}; int position = getPosition(); display (someArray, position); System.out.println("End of program" );}

public static void main(String[ ] args){ try { int[ ] someArray = {12,9,3,11}; int position = getPosition(); display (someArray, position); }

System.out.println("End of program" );}

// catches go here

b) Add an additional catch clause in main to catch any unaccounted for exceptions (within this catch clause print out the stack trace of the exception).



// old catches as before



// old catches as before

// add additional catch clause

Catches all exceptions not caught so far.

c) Create your own exception class InvalidPositionException (make this a checked exception).

public class InvalidPositionException{

}


}

Make this a checked exception


}

extends Exception


}

extends Exception

// add two constructors here

d) Re-write the display method so that it throws the InvalidPositionException from a catch block.

private static void display (int[ ] arrayIn, int posIn){ System.out.println("Item at this position is: " + arrayIn[posIn]);}

private static void display (int[ ] arrayIn, int posIn) throws InvalidPositionException { System.out.println("Item at this position is: " + arrayIn[posIn]);}

private static void display (int[ ] arrayIn, int posIn) throws InvalidPositionException { try { System.out.println("Item at this position is: " + arrayIn[posIn]); } catch ( ? ) { // code here } }

e) Re-write main to take account of this amended display method.



// old catches



// modify catches so this new exception is caught

MSc IT Programming Methodology (2). THROWS an EXCEPTION Errors?

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