Java exception handling

Md.Tanvir Hossain
Department of CSE
Daffodil International University

 Exception is an abnormal condition that arises
when executing a program.
 In the languages that do not support exception
handling, errors must be checked and handled
manually, usually through the use of error codes.
 In contrast, Java:
1. provides syntactic mechanisms to signal, detect
and handle errors
2. ensures a clean separation between the code
executed in the absence of errors and the code
to handle various kinds of errors
3. brings run-time error management into object-
oriented programming
2

 An exception is an object that describes an
exceptional condition (error) that has
occurred when executing a program.
 Exception handling involves the following:
1. when an error occurs, an object
(exception) representing this error is
created and thrown in the method that
caused it
2. that method may choose to handle the
exception itself or pass it on
3. either way, at some point, the exception is
caught and processed
3

 Exceptions can be:
1. generated by the Java run-time system
 Fundamental errors that violate the rules of the
Java language or the constraints of the Java
execution environment.
2. manually generated by programmer’s code
 Such exceptions are typically used to report
some error conditions to the caller of a method.
4

 Five constructs are used in exception handling:
1. try – a block surrounding program statements
to monitor for exceptions
2. catch – together with try, catches specific
kinds of exceptions and handles them in some
way
3. finally – specifies any code that absolutely
must be executed whether or not an exception
occurs
4. throw – used to throw a specific exception
from the program
5. throws – specifies which exceptions a given
method can throw
5

 General form:
try { … }
catch(Exception1 ex1) { … }
catch(Exception2 ex2) { … }
…
finally { … }
 where:
 try { … } is the block of code to monitor for
exceptions
 catch(Exception ex) { … } is exception handler for the
exception Exception
 finally { … } is the block of code to execute after the
try block ends.
6

 All exceptions are sub-classes of the build-in
class Throwable.
 Throwable contains two immediate sub-classes:
1. Exception – exceptional conditions that
programs should catch. The class includes:
a) RuntimeException – defined automatically for user
programs to include: division by zero, invalid array
indexing, etc.
b) User-defined exception classes
2. Error – exceptions used by Java to indicate
errors with the runtime environment; user
programs are not supposed to catch them
7

 What happens when exceptions are not handled?
class Exc0 {
public static void main(String args[]) {
int d = 0;
int a = 42 / d;
}
}
 When the Java run-time system detects the
attempt to divide by zero, it constructs a new
exception object and throws this object.
 This will cause the execution of Exc0 to stop –
once an exception has been thrown it must be
caught by an exception handler and dealt with.
8

 As we have not provided any exception handler,
the exception is caught by the default handler
provided by the Java run-time system.
 This default handler:
1. displays a string describing the exception,
2. prints the stack trace from the point where the
exception occurred
3. terminates the program
java.lang.ArithmeticException: / by zero
at Exc0.main(Exc0.java:4)
 Any exception not caught by the user program is
ultimately processed by the default handler.
9

 The stack trace displayed by the default error
handler shows the sequence of method
invocations that led up to the error.
 Here the exception is raised in subroutine()
which is called by main():
class Exc1 {
static void subroutine() {
int d = 0;
int a = 10 / d;
}
Exc1.subroutine();
}
}
10

 Default exception handling is basically useful
for debugging.
 Normally, we want to handle exceptions
ourselves because:
1. if we detected the error, we can try to fix it
2. we prevent the program from automatically
terminating
 Exception handling is done through the try
and catch block.
11

 Try and catch:
 try surrounds any code we want to monitor for
exceptions
 catch specifies which exception we want to
handle and how.
 When an exception is thrown in the try
block:
try {
d = 0;
a = 42 / d;
System.out.println("This will not be
printed.");
}
12

 control moves immediately to the catch
block:
catch (ArithmeticException e) {
System.out.println("Division by zero.");
}
 The exception is handled and the execution
resumes.
 The scope of catch is restricted to the
immediately preceding try statement - it
cannot catch exceptions thrown by another
try statements.
13

 Resumption occurs with the next statement
after the try/catch block:
try { … }
catch (ArithmeticException e) { … }
System.out.println("After catch statement.");
 Not with the next statement after a =
42/d; which caused the exception!
a = 42 / d;
System.out.println("This will not be
printed.");
14

 The purpose of catch should be to resolve
the exception and then continue as if the
error had never happened.
 Try/catch block inside a loop:
import java.util.Random;
class HandleError {
int a=0, b=0, c=0;
Random r = new Random();
15

 After exception-handling, the program continues with the
next iteration:
for (int i=0; i<32000; i++) {
try {
b = r.nextInt();
c = r.nextInt();
a = 12345 / (b/c);
}
System.out.println("Division by zero.");
a = 0; // set a to zero and continue
}
System.out.println("a: " + a);
}
}
}
16

 All exception classes inherit from the
Throwable class.
 Throwable overrides toString() to
describe the exception textually:
try { … }
System.out.println(“Exception: “ + e);
}
 The following text will be displayed:
Exception: java.lang.ArithmeticException: / by zero
17

 When more than one exception can be raised
by a single piece of code, several catch
clauses can be used with one try block:
1. each catch catches a different kind of
exception
2. when an exception is thrown, the first one
whose type matches that of the exception is
executed
3. after one catch executes, the other are
bypassed and the execution continues after the
try/catch block
18

 Two different exception types are possible in
the following code: division by zero and
array index out of bound:
class MultiCatch {
try {
int a = args.length;
System.out.println("a = " + a);
int b = 42 / a;
int c[] = { 1 };
c[42] = 99;
}
19

 Both exceptions can be caught by the following
catch clauses:
catch(ArithmeticException e) {
System.out.println("Divide by 0: " + e);
}
catch(ArrayIndexOutOfBoundsException e) {
System.out.println("Array index oob: " +
e);
}
System.out.println("After try/catch blocks.");
}
}
20

 Order is important:
 catch clauses are inspected top-down
 a clause using a super-class will catch all sub-
class exceptions
 Therefore, specific exceptions should appear
before more general ones.
 In particular, exception sub-classes must
appear before super-classes.
21

 A try block with two catch clauses:
class SuperSubCatch {
try {
int a = 0;
int b = 42 / a;
//This exception is more general but occurs
first:
}
catch(Exception e) {
System.out.println("Generic
Exception catch.");
}
22

//This exception is more specific but occurs
last:
System.out.println("This is never
reached.");
}
}
}
 The second clause will never get executed. A
compile-time error (unreachable code) will be
raised.
23

 The try statements can be nested:
1. if an inner try does not catch a particular
exception
2. the exception is inspected by the outer try
block
3. this continues until:
 one of the catch statements succeeds or
 all the nested try statements are exhausted
4. in the latter case, the Java run-time system
will handle the exception
24

 An example with two nested try statements:
class NestTry {
//Outer try statement:
try {
/*Division by zero when no command-line
argument is present*/
int b = 42 / a;
25

//Inner try statement:
try {
/*Division by zero when one command-line
argument is present:*/
if (a==1)
a = a/(a-a);
/*Array index out of bound when two
command-line arguments are present:*/
if (a==2) {
int c[] = { 1 };
c[42] = 99;
}
26

/*Catch statement for the inner try statement,
catches the array index out of bound
exception:*/
}
catch(ArrayIndexOutOfBoundsException e)
{
System.out.println(e);
}
/*Catch statement for the outer try
statement, catches both division-by-zero
exceptions for the inner and outer try
statements:*/
}
System.out.println("Divide by 0: " + e);
}}}
27

 Nesting of try statements with method calls:
 method call is enclosed within one try statement
 the method includes another try statement
 Still, the try blocks are considered nested within
each other.
class MethNestTry {
static void nesttry(int a) {
try {
if (a==1) a = a/(a-a);
if (a==2) {
int c[] = { 1 }; c[42] = 99;}
} catch(ArrayIndexOutOfBoundsException e)
{ System.out.println(e);}}
28

 Method main contains another try block
which includes the call to nesttry:
try {
int b = 42 / a;
nesttry(a);
}}
System.out.println("Divide by 0: "
+ e);
}}}
29

 So far, we were only catching the exceptions
thrown by the Java system.
 In fact, a user program may throw an
exception explicitly:
throw ThrowableInstance;
 ThrowableInstance must be an object of
type Throwable or its subclass.
30

 Once an exception is thrown by:
throw ThrowableInstance;
1. the flow of control stops immediately
2. the nearest enclosing try statement is
inspected if it has a catch statement that
matches the type of exception:
1. if one exists, control is transferred to that
statement
2. otherwise, the next enclosing try statement is
examined
3. if no enclosing try statement has a
corresponding catch clause, the default
exception handler halts the program and prints
the stack
31

 Two ways to obtain a Throwable instance:
1. creating one with the new operator
 All Java built-in exceptions have at least two
constructors: one without parameters and another
with one String parameter:
 throw new NullPointerException("demo");
2. using a parameter of the catch clause
 try { … } catch(Throwable e) { … e … }
32

class ThrowDemo {
/*The method demoproc throws a
NullPointerException exception which is
immediately caught in the try block and re-
thrown:*/
static void demoproc() {
try {
throw new
NullPointerException("demo");
}
catch(NullPointerException e) {
System.out.println("Caught inside
demoproc.");
throw e;
}
}
33

 The main method calls demoproc within the
try block which catches and handles the
NullPointerException exception:
try {
demoproc();
}
System.out.println("Recaught: " + e);
}}}
34

 If a method is capable of causing an exception
that it does not handle, it must specify this
behavior by the throws clause in its
declaration:
type name(parameter-list) throws
exception-list {
…
}
 where exception-list is a comma-separated
list of all types of exceptions that a method
might throw.
 All exceptions must be listed except Error and
RuntimeException or any of their subclasses,
otherwise a compile-time error occurs.
35

 The throwOne method throws an exception
that it does not catch, nor declares it within
the throws clause.
class ThrowsDemo {
static void throwOne() {
System.out.println("Inside throwOne.");
throw new IllegalAccessException("demo");}
throwOne();}}
 Therefore this program does not compile.
36

 Corrected program: throwOne lists exception,
main catches it:
class ThrowsDemo {
static void throwOne() throws IllegalAccessException {
System.out.println("Inside throwOne.");
throw new IllegalAccessException("demo");
}
try {
throwOne();
} catch (IllegalAccessException e) {
System.out.println("Caught " + e);
}
}
}
37

 When an exception is thrown:
 the execution of a method is changed
 the method may even return prematurely.
 This may be a problem is many situations.
 For instance, if a method opens a file on
entry and closes on exit; exception
handling should not bypass the proper
closure of the file.
 The finally block is used to address this
problem.
38

 The try/catch statement requires at least one
catch or finally clause, although both are
optional:
try { … }
catch(Exception1 ex1) { … } …
finally { … }
 Executed after try/catch whether of not the
exception is thrown.
 Any time a method is to return to a caller from
inside the try/catch block via:
 uncaught exception or
 explicit return
 the finally clause is executed just before the
method returns.
39

 Three methods to exit in various ways.
class FinallyDemo {
 procA prematurely breaks out of the try by
throwing an exception, the finally clause is
executed on the way out:
static void procA() {
try {
System.out.println("inside procA");
throw new RuntimeException("demo");
} finally {
System.out.println("procA's finally");
}
}
40

 procB’s try statement is exited via a
return statement, the finally clause is
executed before procB returns:
static void procB() {
try {
System.out.println("inside procB");
return;
} finally {
System.out.println("procB's finally");
}
}
41

 In procC, the try statement executes
normally without error, however the
finally clause is still executed:
static void procC() {
try {
System.out.println("inside procC");
} finally {
System.out.println("procC's finally");
}
}
42

 Demonstration of the three methods:
try {
procA();
} catch (Exception e) {
System.out.println("Exception caught");
}
procB();
procC();
}
}
43

 The default java.lang package provides
several exception classes, all sub-classing the
RuntimeException class.
 Two sets of build-in exception classes:
unchecked exceptions – the compiler does not
check if a method handles or throws there
exceptions
 checked exceptions – must be included in the
method’s throws clause if the method generates
but does not handle them
44

 Methods that generate exceptions but do not
handle those exceptions need not declare
them in the throws clause:
45

 Methods that generate but do not handle
those exceptions must declare them in the
throws clause:
47

 Built-in exception classes handle some
generic errors.
 For application-specific errors define your
own exception classes.
 How? Define a subclass of Exception:
class MyException extends Exception { … }
 MyException need not implement
anything – its mere existence in the type
system allows to use its objects as
exceptions.
48

 Exception itself is a sub-class of Throwable.
 All user exceptions have the methods defined by the
Throwable class:
1. Throwable fillInStackTrace() – returns a
Throwable object that contains a completed stack
trace; the object can be rethrown.
2. Throwable getCause() – returns the exception that
underlies the current exception. If no underlying
exception exists, null is returned.
3. String getLocalizedMessage() – returns a
localized description of the exception.
4. String getMessage() – returns a description of the
exception
5. StackTraceElement[] getStackTrace() – returns
an array that contains the stack trace; the method at
the top is the last one called before exception.
49

 More methods defined by the Throwable class:
1. Throwable initCause(Throwable causeExc) –
associates causeExc with the invoking exception as its
cause, returns the exception reference
2. void printStackTrace() – displays the stack trace
3. void printStackTrace(PringStream stream) –
sends the stack trace to the specified stream
4. void setStackTrace(StackTraceElement
elements[]) – sets the stack trace to the elements
passed in elements; for specialized applications only
5. String toString() – returns a String object
containing a description of the exception; called by
print() when displaying a Throwable object.
50

 A new exception class is defined, with a
private detail variable, a one-parameter
constructor and an overridden toString
method:
class MyException extends Exception {
private int detail;
MyException(int a) {
detail = a;
}
public String toString() {
return "MyException[" + detail + "]";
}
}
51

 The static compute method throws the
MyException exception whenever its a
argument is greater than 10:
class ExceptionDemo {
static void compute(int a) throws MyException
{
System.out.println("Called compute(" + a
+ ")");
if (a > 10) throw new MyException(a);
System.out.println("Normal exit");
}
52

 The main method calls compute with two
arguments within a try block that catches
the MyException exception:
try {
compute(1);
compute(20);
} catch (MyException e) {
System.out.println("Caught " + e);
}}}
53

 The chained exception allows to associate
with a given exception another exception
that describes its cause.
 Throwable class includes two constructors
to handle chained exceptions:
 Throwable(Throwable causeExc)
 Throwable(String msg, Throwable causeExc)
 They both create an exception with
causeExc being its underlying reason and
optional msg providing the textual
description.
54

 Throwable class also includes two methods to
handle chained exceptions:
1. Throwable getCause() – returns an exception
that is the cause of the current exception, or null if
there is no underlying exception.
2. Throwable initCause(Throwable causeExc)
– associates causeExc with the invoking exception
and returns a reference to the exception:
a. initCause() allows to associate a cause with an
existing exception
b. the cause exception can be set only once
c. if the cause exception was set by a constructor, it is
not possible to set it again with initCause.
55

 The demoproc method creates a new
NullPointerException exception e,
associates ArithmethicException as its
cause, then throws e:
class ChainExcDemo {
static void demoproc() {
NullPointerException e = new
NullPointerException(“top layer”);
e.initCause(new ArithmeticException
(“cause”));
throw e;
}
56

 The main method calls demoproc within the try
block that catches NullPointerException, then
displays the exception and its cause:
try {
demoproc();
}
System.out.println(“Caught: “ + e);
System.out.println(“Cause: “ +
e.getCause());}}}
 A cause exception may itself have a cause. In fact,
the cause-chain of exceptions may be arbitrarily
long.
57

 New Java programmers should break the habit of
returning error codes to signal abnormal exit
from a method.
 Java provides a clean and powerful way to
handle errors and unusual boundary conditions
through its:
 try
 catch
 finally
 throw and
 throws statements.
 However, Java’s exception-handling should not
be considered a general mechanism for non-local
branching!
58

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