Programming in Java Exceptions Handling Errors using...

Fachbereich Informatik und Elektrotechnik

Programming in Java, Helmut Dispert

Java

Programming in Java

Exceptions

Handling Errors using Exceptions



Exceptions

Exception = Exceptional Event

Exceptions are:

� objects,

� derived from java.lang.Throwable.

Throwable Objects:

� Errors (Java Structure Violation)

� Exceptions (Java Logic Violations)

� Runtime Exceptions



Exceptions

Throw Catch

Exception

Creating an Exception

Throw an Exception

Exception handler appropriate to handle the

thrown exception:

Catch the Exception



Exception Types

Object

Throwable

Exception

RuntimeException

Error

...

... ...



Exception Types

Errors:

Dynamic Linking Error, hard failure in the virtual machine

� VM throws an Error.

Exceptions:

Indication of a not very serious systematic problem

� Program throws an Exception.

Runtime Exceptions:

Exceptions that occur within the Java virtual machine

during runtime

(e.g. NullPointerException).

� VM throws a Runtime Exception.



Exception Types

Advantages of Java's Error Handling

1. Error Handling Code is separated from regular code.

2. Error Propagation up the Call Stack.

3. Grouping Error Types and Error Differentiation.



Call Stack

method1

{

x = method2(...)

}

method2

{

y = method3(...)

}

method3

{

z = method4(...)

}



Error Differentiation

Exception ⇒⇒⇒⇒ Instances of Throwable

or subclass of Throwable

Object

Throwable

ArrayException

InvalidIndexException NoSuchElementException

ElementTypeException



Error Differentiation

Leaf Class (class without subclass):

� represents a specific type of exception,

� specialized handler.

Node Class (class with one or more subclass):

� represents a group of related exceptions,

� general handler.

Leaf Class:

catch (InvalidIndexException e){

...}

Node Class:

catch (ArrayException e){

...}



Catch or Specify Requirement

� Catch

� Specify Checked Exception

Exception Types:

� Runtime Exceptions

� I/O-Exceptions

� Own Exceptions

A method either has to catch or specify all checked exceptions

that can be thrown within it's scope.

Runtime Exceptions:

Don't have to be caught or specified.

Checked Exceptions:

Not runtime exceptions, checked by the compiler;

checked exceptions must be caught or specified.



Catch or Specify Requirement

Checked Exceptions:

� Due to external circumstances that the programmer cannot

prevent

� Compiler checks that these exceptions are handled

� All IOExceptions are checked exceptions

Unchecked Exceptions:

� Programmers fault, can be prevented

� Included exceptions:

e.g. NullPointerException, ArithmeticException

� e.g. to avoid the NullPointerException the reference can be

checked for “null”



Try and Catch

Critical statements are capsuled in

try-catch statements

try

{

<some dangerous action>

}

catch (Exception e)

{

<some reasonable behaviour>

}

.

.

.

finally

{

<something absolutely necessary>

}



Try and Catch: Syntax Diagram

Syntax Diagram:

try block catch

finally block

(object) block

catch (object) block finally block



Exception Handler Parameter

e: exception handler parameter (variable)

� type of exception,

� exception message,

� stack trace.

Available methods:

� e.toString()

� e.getMessage()

� e.printStackTrace()



Try and Catch

// Hello program with exception handling

class ExceptionalHello2

{

public static void main (String args[])

{

try /* Now let's say hello */

{

System.out.println("Hello " + args[0]); // line 9

}

catch (Exception e)

{

System.out.println("Hello! Who are you?!");

// e.printStackTrace();

}

}

}

Example: HelloWorld with Exception Handling

continue

Java



Try and Catch

java ExceptionalHello2

Output:

Hello! Who are you?!

with (inside catch block):

e.printStackTrace();

Output:

Hello! Who are you?!

java.lang.ArrayIndexOutOfBoundsException:

at

ExceptionalHello2.main(ExceptionalHello2.java:9)

continued



Writing your own Exception / Example

class SampExcept extends Exception{

SampExcept (String Message) // Constructor{

super(Message); // pass message to parent}

}

public class ExTest{

static void range(int numb) throws SampExcept{

if(numb < 0 || numb > 100){

throw new SampExcept("0-100 expected"); // line 15}else{

System.out.println("Number is " + numb);}

}

continue





{

try

{

range(75);

range(250); // line 28

}

catch(SampExcept e)

{

System.out.println("Err: " + e.getMessage());

}

}

}

Number is 75

Err: 0-100 expected

Version 1

Output

Java





{

try

{

range(75);

range(250); // line 28

}

catch(SampExcept e)

{

System.out.println("\n\nError Messages:");

System.out.println("\nErr1:\n" + e.getMessage());

System.out.println("\nErr2:\n" + e.toString());

System.out.println("\nErr3: " );

e.printStackTrace();

}

}

}

Version 2




Number is 75

Error Messages:

Err1:

0-100 expected

Err2:

SampExcept: 0-100 expected

Err3:

SampExcept: 0-100 expected

at ExTest2.range(ExTest2.java:15)

at ExTest2.main(ExTest2.java:28)

Version 1Output

Class Class

Class Class

Class

Method

Object Object



I/O-Programmierung

import java.io.*;

public class IOTest2{

public static void main(String[] args){

try{

byte bArray[] = new byte[128];System.out.print("Texteingabe: ");System.in.read(bArray);String s = new String(bArray,0);System.out.print("Eingabe: ");System.out.println(s);

}catch(IOException ioe){

System.out.println(ioe.toString());ioe.printStackTrace();

}}

}

catchJava



I/O-Programmierung

import java.io.*;

public class IOTest22

{

public static void main(String[] args) throws IOException

{

byte bArray[] = new byte[128];

System.out.print("Texteingabe: ");

System.in.read(bArray);

String s = new String(bArray,0);

System.out.print("Eingabe: ");

System.out.println(s);

}

}

specifyJava

Programming in Java Exceptions Handling Errors using...

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