1unit 10

Exception HandlingException Handling

Run-time errors The exception concept Throwing exceptions Handling exceptions Declaring exceptions

basic programming

concepts

object oriented programming

in Java

topics in computer science

syllabus

2unit 10

Run-time errorsRun-time errors

Sometimes when the computer tries to execute a statement something goes wrong: • reading a file that does not exist or is inaccessible

• dividing a number by zero

• calling a method with improper arguments

In these cases we say that a run-time error has occurred

In Java, run-time errors are indicated by exceptions

3unit 10

ExceptionsExceptions

If a method wants to signal that something went wrong during its execution, it throws an exception; exceptions may be caught and handled by another part of the program

Throwing an exception involves:• creating an exception object that encloses information about

the problem that occurred• use of the statement throw to notify about the exception

A program can therefore be separated into a normal execution flow and an exception execution flow

4unit 10

Example: throwing an exceptionExample: throwing an exception

// Sets the time of the clock

// @param hour, minute, second - Tthe new time

public void setTime(int hour, int minute, int second) {

if (hour<0 || hour>59 || minute<0 || minute>59 ||

second<0 || second>59) {

throw new IllegalArgumentException(

“Invalid time”);

}

this.hour = hour;

this.minute = minute;

this.second = second;

}

5unit 10

The exception objectThe exception object

The information about the problem that occurred is enclosed in an exception object, including:• The type of the problem• The place in the code where the exception occurred• The state of the run-time stack• ... other information

An exception object comes with service methods such as getMessage or printStackTrace

The code that invoked the illegal operation will receive the exception object

6unit 10

The type of the exceptionThe type of the exception

The most important information is the type of the exception, indicated by the class of the exception object

The Java API defines classes for many types of exceptions (and you can define more of your own):• java.lang.ArithmeticException - thrown when an

exceptional arithmetic condition occurs, e.g., division by zero• java.io.FileNotFoundException - signals that a file

we tried to access could not be found• java.net.UnknownHostException - signals that a

computer we tried to communicate with cannot be located • NullPointerException - trying to refer to an object

through a reference variable whose value is null

7unit 10

ExampleExample

The setTime() method used the exception java.lang.IllegalArgumentException to signal that the values of the arguments were not valid

A method throws this type of exception if it wants to signal that an argument it got is not legal

8unit 10

Occurrence of an exceptionOccurrence of an exception

When a program performs an illegal operation the following happens:• An exception object is created and thrown

• The regular flow of the program stops

• The program may try to handle the exceptional situation

• If the program ignores the exception, execution stops; we sometimes say that the program crashed

9unit 10

Occurrence of an exceptionOccurrence of an exception

// ...class BucketsProblem {

public static void main(String[] args) { // ... int targetCapacity = 2*x-y; Bucket a = new Bucket(x); Bucket b = new Bucket(x); Bucket c = new Bucket(y); Bucket target = new Bucket(targetCapacity); // ... }} What happens if x<0 or y<0 or 2x-y<0 ?

10unit 10

The root of the exceptionThe root of the exception

in class Bucket (Bucket.java):

// Constructs a new Bucket. // @param capacity: the capacity of the bucket

// @exception: IllegalArgumentException if the // given capacity is negative

public Bucket(int capacity) { if (capacity<0) { throw new IllegalArgumentException( “Capacity must be positive!”); } this.capacity = capacity; }

11unit 10

Occurrence of an exceptionOccurrence of an exception

public static void main(String[] args) {

// ...

int z = 2*x-y; // capacity of target bucket

Bucket a = new Bucket(x);

Bucket b = new Bucket(x);

Bucket c = new Bucket(y);

Bucket target = new Bucket(z);

// ... the solution of the problem

}

A bucket must

have a positive

capacity

Hey, no one

cares to

listen!

I’ll crash the

method!

x = -3

12unit 10

Occurrence of an exceptionOccurrence of an exception

public static void main(String[] args) { // ... int z = 2*x-y; // capacity of target bucket Bucket a = new Bucket(x); Bucket b = new Bucket(x); Bucket c = new Bucket(y); Bucket target = new Bucket(z); // ... the solution of the problem }

A bucket must

have a positive

capacity.

No one cares, ...

I’ll crash the methodx = -3

public Bucket(int capacity) { if (capacity<0) { throw new IllegalArgumentException( “Capacity must be positive!”); } this.capacity = capacity; }}

capacity = -3

13unit 10

Exceptions happenExceptions happen

Sometimes we cannot avoid an exceptional state; for example when reading from a diskette we cannot tell if the diskette is readable or not without trying to read from it

Sometimes we want to handle the exceptional case outside the main block, so as not to complicate the readability of the code

If we ignore the exception, the program crashes

14unit 10

Exception HandlingException Handling

A program can deal with an exception in one of three ways:• ignore it (the program will ‘crash’)

• handle it where it occurs

• handle it an another place in the program

15unit 10

Handling ExceptionsHandling Exceptions

To process an exception when it occurs, the line that throws the exception is executed within a try block

A try block is followed by one or more catch clauses, which contain code to process an exception

Each catch clause has an associated exception type

When an exception occurs, processing continues at the first catch clause that matches the exception type

16unit 10

Handling exceptionsHandling exceptions

public static void main(String[] args) { // ... try { int z = 2*x-y; // capacity of target bucket Bucket a = new Bucket(x); Bucket b = new Bucket(x); Bucket c = new Bucket(y); Bucket target = new Bucket(z); // ... the solution of the problem } catch (IllegalArgumentException ia) { output.println(“Illegal input!”); }}

A bucket must

have a positive

capacity

17unit 10

Handling exceptionsHandling exceptions

public static void main(String[] args) { // ... try { int z = 2*x-y; // capacity of target bucket Bucket a = new Bucket(x); Bucket b = new Bucket(x); Bucket c = new Bucket(y); Bucket target = new Bucket(z); // ... the solution of the problem } catch (ArithmeticException ae) { // ... } catch (IllegalArgumentException ia) { output.println(“Illegal input!”); }}

A bucket must

have a positive

capacity

18unit 10

The The finallyfinally Clause Clause

A try statement can have an optional clause designated by the reserved word finally

If no exception is generated, the statements in the finally clause are executed after the statements in the try block finish their execution

In addition, if an exception is generated, the statements in the finally clause are executed after the statements in the appropriate catch clause complete execution

19unit 10

Exception PropagationException Propagation

If it is not appropriate to handle the exception where it occurs, it can be handled at a higher level

Exceptions propagate up through the method calling hierarchy until they are caught and handled or until they reach the outermost level

Any try block along the way, that contains a call to a method in which an exception is thrown, can be used to catch that exception

20unit 10

The The throwthrow Statement Statement

A programmer can define an exception by extending the appropriate class

Exceptions are thrown using the throw statement

Usually a throw statement is nested inside an if statement that evaluates the condition to see if the exception should be thrown

21unit 10

Generating Exception ObjectsGenerating Exception Objects

All the classes for indicating run-time errors are derived from the class java.lang.Throwable

The object you deliver to the throw statement must be an instance of class Throwable

The constructor of class Throwable initializes all the information about the location where the exception occurred, the state of the run-time stack etc (thus this information is set for every exception object)

22unit 10

Exceptions class hierarchyExceptions class hierarchy

Throwable

ExceptionError

RuntimeException

23unit 10

Exception & Error distinctionException & Error distinction

java.lang.Throwable has two direct subclasses: java.lang.Error and java.lang.Exception

Every run-time error is identified by a class that is either a subclass of Error or of Exception

The distinction is not very strict: • Error refers to run-time errors that are rooted in the

execution environment and are not in the hands of the application programmer

• Exception refers to all other run-time errors; the application programmer is responsible to see that all exceptions are handled

24unit 10

Errors examplesErrors examples

java.lang.InternalError - signals an error caused by an implementation bug in the JVM in which the application is running; the developer is not responsible for this error and should not try to handle it

java.lang.NoSuchMethodError - signals that a method that the application is trying to use does not exist; this may happen if the application is not installed correctly

java.lang.OutOfMemory - signals that the application ran out of memory

25unit 10

Exception examplesException examples

java.lang.ArithmeticException - occurs when you divide an integer by 0; the exception is rooted in the logic of the application, hence it is the programmer responsibility to handle it

java.io.FileNotFoundException - occurs when the application tries to access a file which does not exist; again, the programmer should treat this case

...

26unit 10

Errors and ExceptionsErrors and Exceptions

The application you write must handle all Exceptions but ignore Errors:• If an Error occurs you should let it crash the

application; the user will be notified for the error and act accordingly (buy more memory, reinstall the application, call tech support ...)

• On the other hand, it’s very uncool to let the application crash because you didn’t catch an exception

27unit 10

Exceptions class hierarchyExceptions class hierarchy

Throwable

ExceptionError

RuntimeException

28unit 10

Exceptions vs. RuntimeExcepitonExceptions vs. RuntimeExcepiton

Class Exception has a special subclass named RuntimeException, which makes another classification of exceptions: runtime-exceptions vs. regular exceptions

RuntimeException is characterized by:• The programmer could avoid the occurrence of the exception

• The exception can be thrown by common statements/methods

The name RuntimeException is confusing; every exception and Error occurs during the run-time of the application

29unit 10

RuntimeException examplesRuntimeException examples

java.lang.ArithmeticException

java.lang.NullPointerException

java.lang.IllegalArgumentException

java.lang.NegativeArraySizeException

java.lang.ArrayIndexOutOfBoundsException

30unit 10

Declaring for exceptionsDeclaring for exceptions

A method must declare all the non run-time exceptions it may throw

The declaration for exceptions a method can throw is done using the throws keyword

The user of the method is warned against possible exceptions this method can throw

The exceptions that might be thrown by a method should also be documented with the @exception tag

31unit 10

Example (using throws)Example (using throws)

// Creates an ElectronicGate of a given typepublic ElectronicGate createGate(String type) throws UnkownGateException { type = type.toUpperCase(); if (type.equals(“OR”)) { return new OrGate(); } if (type.equals(“AND”)) { return new AndGate(); } if (type.equals(“NOT”)) { return new NotGate(); } throw new UnknownGateException();}

32unit 10

Declaring for exceptionsDeclaring for exceptions

Because there is a possibility that an execution of createGate() will throw an UnknownGateException which is not a RuntimeException, createGate() must declare this exception

If you try to call the createGate() method from another method, the compiler will know that there is a possibility that this method will throw an UnknownGateException; it will then force you to:• try to catch this exceptionor• declare that the calling method also throws this type

of exception

33unit 10

Example: either catch ...Example: either catch ...

// Called when the user chooses to add a gateprivate void userAddsGate() { String type = ... look up the selected gate type try { Gate gate = createGate(type); GateFigure figure = createGateFigure(type); ... adds the gate to the model ... add the figure to the display ... } catch (UnknownGateException uge) { // ignore this, don’t add the gate }}

34unit 10

... or declare... or declare

// Called when the user chooses to add a gate

private void userAddsGate()

throws UnknownGateException {

String type = ... look up the selected gate type

Gate gate = createGate(type);

GateFigure figure = createGateFigure(type);

... adds the gate to the model

... add the figure to the display

...

}

35unit 10

Defining new exception typeDefining new exception type

// An exception thrown to indicate that a requested// type of electronic gate does not existpublic class UnknownGateException extends Exception { // Creates a new UnknownGateException public UnknownGateException() {

super(“This is an illegal gate type!”); }

// Creates a new UnknownGateException public UnknownGateException(String cause) { super(cause); }}

36unit 10

Several notesSeveral notes

The throws statement doesn’t affect the way the exception will be treated in run-time

You can also declare for RuntimeExceptions, but it is not necessary

When to declare and when to catch the exception within the method is a design decision

37unit 10

Exaple: class MyDateExaple: class MyDate

public class MyDate { public static final int JANUARY= 1; public static final int FEBRUARY= 2;

//data memberspublic int day;private int month;private int year;

//default constructor - recieves no paramaterspublic MyDate() {day= 1;month= 1;year= 2000;}

38unit 10

Exaple: class MyDateExaple: class MyDate

//insertion constructor public MyDate(int day,int month,int year) throws IllegalMyDateException { if (!legalDate(day,month,year))

throw new IllegalMyDateException();this.day= day;this.month= month;this.year= year;

}

private boolean legalDate(int day,int month,int year){if ( (day <1) || (day > 31) || (month<1) || (month>12) )

return(false);return(true);

}}

39unit 10

Exception class definitionException class definition

public class IllegalMyDateException extends Exception {

public IllegalMyDateException(String message){super(message);

}

public IllegalMyDateException(){super("this is not a legal date!");

}}