Although the default exception handler provided by the Java run-time system is useful for
debugging, you will usually want to handle an exception yourself. Doing so provides two
benefits. First, it allows you to fix the error. Second, it prevents the program from automatically
terminating. Most users would be confused (to say the least) if your program stopped running and printed a stack trace whenever an error occurred! Fortunately, it is quite easy
to prevent this.
To guard against and handle a run-time error, simply enclose the code that you want
to monitor inside a try block. Immediately following the try block, include a catch clause
that specifies the exception type that you wish to catch. To illustrate how easily this can be
done, the following program includes a try block and a catch clause that processes the
ArithmeticException generated by the division-by-zero error:
class Exc2 {
public static void main(String args[])
{
int d, a;
try { // monitor a block of code.
d = 0;
a = 42 / d;
System.out.println("This will not be printed.");
} catch (ArithmeticException e) { // catch divide-by-zero error
System.out.println("Division by zero.");
}
System.out.println("After catch statement.");
}
}
This program generates the following output:
Division by zero.
After catch statement.
Notice:- that the call to println( ) inside the try block is never executed. Once an exception
is thrown, program control transfers out of the try block into the catch block. Put differently,
catch is not “called,” so execution never “returns” to the try block from a catch. Thus, the
line “This will not be printed.” is not displayed. Once the catch statement has executed,
program control continues with the next line in the program following the entire try/catch
mechanism.
A try and its catch statement form a unit. The scope of the catch clause is restricted to
those statements specified by the immediately preceding try statement. A catch statement
cannot catch an exception thrown by another try statement (except in the case of nested try
statements, described shortly). The statements that are protected by try must be surrounded
by curly braces. (That is, they must be within a block.) You cannot use try on a single statement.
The goal of most well-constructed catch clauses should be to resolve the exceptional
condition and then continue on as if the error had never happened.
For example, in the nextprogram each iteration of the for loop obtains two random integers. Those two integers are divided by each other, and the result is used to divide the value 12345. The final result is put into a. If either division operation causes a divide-by-zero error, it is caught, the value of a is set to zero, and the program continues.
// Handle an exception and move on.
import java.util.Random;
class HandleError {
public static void main(String args[])
{
int a=0, b=0, c=0;
Random r = new Random();
for(int i=0; i<32000; i++) {
try {
b = r.nextInt();
c = r.nextInt();
a = 12345 / (b/c);
} catch (ArithmeticException e) {
System.out.println("Division by zero.");
a = 0; // set a to zero and continue
}
System.out.println("a: " + a);
}
}
}
Displaying a Description of an Exception
Throwable overrides the toString( ) method (defined by Object) so that it returns a string
containing a description of the exception. You can display this description in a println( )
statement by simply passing the exception as an argument. For example, the catch block
in the preceding program can be rewritten like this:
catch (ArithmeticException e) {
System.out.println("Exception: " + e);
a = 0; // set a to zero and continue
}
When this version is substituted in the program, and the program is run, each divide-byzero
error displays the following message:
Exception: java.lang.ArithmeticException: / by zero
While it is of no particular value in this context, the ability to display a description of
an exception is valuable in other circumstances—particularly when you are experimenting
with exceptions or when you are debugging.
Multiple catch Clauses
In some cases, more than one exception could be raised by a single piece of code. To handle
this type of situation, you can specify two or more catch clauses, each catching a different
type of exception. When an exception is thrown, each catch statement is inspected in order,
and the first one whose type matches that of the exception is executed. After one catch
statement executes, the others are bypassed, and execution continues after the try/catch
block.
--->The following example traps two different exception types:
// Demonstrate multiple catch statements.
class MultiCatch {
public static void main(String args[]) {
try {
int a = args.length;
System.out.println("a = " + a);
int b = 42 / a;
int c[] = { 1 };
c[42] = 99;
} 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.");
}
}
This program will cause a division-by-zero exception if it is started with no commandline
arguments, since a will equal zero. It will survive the division if you provide a
command-line argument, setting a to something larger than zero. But it will cause an
ArrayIndexOutOfBoundsException, since the int array c has a length of 1, yet the program
attempts to assign a value to c[42].
Here is the output generated by running it both ways:
C:\>java MultiCatch
a = 0
Divide by 0: java.lang.ArithmeticException: / by zero
After try/catch blocks.
C:\>java MultiCatch TestArg
a = 1
Array index oob: java.lang.ArrayIndexOutOfBoundsException:42
After try/catch blocks.
When you use multiple catch statements, it is important to remember that exception
subclasses must come before any of their superclasses. This is because a catch statement
that uses a superclass will catch exceptions of that type plus any of its subclasses. Thus, a
subclass would never be reached if it came after its superclass. Further, in Java, unreachable
code is an error. For example, consider the following program:
/* This program contains an error.
A subclass must come before its superclass in
a series of catch statements. If not,
unreachable code will be created and a
compile-time error will result.
*/
class SuperSubCatch {
public static void main(String args[]) {
try {
int a = 0;
int b = 42 / a;
} catch(Exception e)
{
System.out.println("Generic Exception catch.");
}
/* This catch is never reached because
ArithmeticException is a subclass of Exception. */
catch(ArithmeticException e) { // ERROR - unreachable
System.out.println("This is never reached.");
}
}
}
If you try to compile this program, you will receive an error message stating that the
second catch statement is unreachable because the exception has already been caught. Since
ArithmeticException is a subclass of Exception, the first catch statement will handle all
Exception-based errors, including ArithmeticException. This means that the second catch
statement will never execute. To fix the problem, reverse the order of the catch statements.
Nested try Statements
The try statement can be nested. That is, a try statement can be inside the block of another try.
Each time a try statement is entered, the context of that exception is pushed on the stack. If an
inner try statement does not have a catch handler for a particular exception, the stack is
unwound and the next try statement’s catch handlers are inspected for a match. This continues
until one of the catch statements succeeds, or until all of the nested try statements are exhausted.
If no catch statement matches, then the Java run-time system will handle the exception.
--->Here is an example that uses nested try statements:
// An example of nested try statements.
class NestTry {
public static void main(String args[]) {
try {
int a = args.length;
/* If no command-line args are present,
the following statement will generate
a divide-by-zero exception. */
int b = 42 / a;
System.out.println("a = " + a);
try { // nested try block
/* If one command-line arg is used,
then a divide-by-zero exception
will be generated by the following code. */
if(a==1) a = a/(a-a); // division by zero
/* If two command-line args are used,
then generate an out-of-bounds exception. */
if(a==2) {
int c[] = { 1 };
c[42] = 99; // generate an out-of-bounds exception
}
} catch(ArrayIndexOutOfBoundsException e) {
System.out.println("Array index out-of-bounds: " + e);
}
} catch(ArithmeticException e) {
System.out.println("Divide by 0: " + e);
}
}
}
As you can see, this program nests one try block within another. The program works as
follows. When you execute the program with no command-line arguments, a divide-by-zero
exception is generated by the outer try block. Execution of the program with one command-line
argument generates a divide-by-zero exception from within the nested try block. Since the
inner block does not catch this exception, it is passed on to the outer try block, where it is
handled. If you execute the program with two command-line arguments, an array boundary
exception is generated from within the inner try block. Here are sample runs that illustrate
each case:
C:\>java NestTry
Divide by 0: java.lang.ArithmeticException: / by zero
C:\>java NestTry One
a = 1
Divide by 0: java.lang.ArithmeticException: / by zero
C:\>java NestTry One Two
a = 2
Array index out-of-bounds:
java.lang.ArrayIndexOutOfBoundsException:42
Nesting of try statements can occur in less obvious ways when method calls are involved.
For example, you can enclose a call to a method within a try block. Inside that method is
another try statement. In this case, the try within the method is still nested inside the outer try
block, which calls the method. Here is the previous program recoded so that the nested
try block is moved inside the method nesttry( ):
/* Try statements can be implicitly nested via
calls to methods. */
class MethNestTry {
static void nesttry(int a) {
try { // nested try block
/* If one command-line arg is used,
then a divide-by-zero exception
will be generated by the following code. */
if(a==1) a = a/(a-a); // division by zero
/* If two command-line args are used,
then generate an out-of-bounds exception. */
if(a==2) {
int c[] = { 1 };
c[42] = 99; // generate an out-of-bounds exception
}
} catch(ArrayIndexOutOfBoundsException e) {
System.out.println("Array index out-of-bounds: " + e);
}
}
public static void main(String args[]) {
try {
int a = args.length;
/* If no command-line args are present,
the following statement will generate
a divide-by-zero exception. */
int b = 42 / a;
System.out.println("a = " + a);
nesttry(a);
} catch(ArithmeticException e) {
System.out.println("Divide by 0: " + e);
}
}
}
The output of this program is identical to that of the preceding example.
throw
So far, you have only been catching exceptions that are thrown by the Java run-time system.
However, it is possible for your program to throw an exception explicitly, using the throw
statement. The general form of throw is shown here:
throw ThrowableInstance;
Here, ThrowableInstance must be an object of type Throwable or a subclass of Throwable.
Primitive types, such as int or char, as well as non-Throwable classes, such as String and
Object, cannot be used as exceptions. There are two ways you can obtain a Throwable object:
using a parameter in a catch clause, or creating one with the new operator.
The flow of execution stops immediately after the throw statement; any subsequent
statements are not executed. The nearest enclosing try block is inspected to see if it has a
catch statement that matches the type of exception. If it does find a match, control is
transferred to that statement. If not, then the next enclosing try statement is inspected, and
so on. If no matching catch is found, then the default exception handler halts the program
and prints the stack trace.
the exception rethrows it to the outer handler.
// Demonstrate throw.
class ThrowDemo {
static void demoproc() {
try {
throw new NullPointerException("demo");
} catch(NullPointerException e) {
System.out.println("Caught inside demoproc.");
throw e; // rethrow the exception
}
}
public static void main(String args[]) {
try {
demoproc();
} catch(NullPointerException e) {
System.out.println("Recaught: " + e);
}
}
}
This program gets two chances to deal with the same error. First, main( ) sets up an exception
context and then calls demoproc( ). The demoproc( ) method then sets up another exceptionhandling
context and immediately throws a new instance of NullPointerException, which
is caught on the next line. The exception is then rethrown. Here is the resulting output:
Caught inside demoproc.
Recaught: java.lang.NullPointerException: demo
The program also illustrates how to create one of Java’s standard exception objects. Pay
close attention to this line:
throw new NullPointerException("demo");
Here, new is used to construct an instance of NullPointerException. Many of Java’s builtin
run-time exceptions have at least two constructors: one with no parameter and one that
takes a string parameter. When the second form is used, the argument specifies a string that
describes the exception. This string is displayed when the object is used as an argument to
print( ) or println( ). It can also be obtained by a call to getMessage( ), which is defined by
Throwable.
throws
If a method is capable of causing an exception that it does not handle, it must specify this
behavior so that callers of the method can guard themselves against that exception. You do
this by including a throws clause in the method’s declaration. Athrows clause lists the types
of exceptions that a method might throw. This is necessary for all exceptions, except those of
type Error or RuntimeException, or any of their subclasses. All other exceptions that a method
can throw must be declared in the throws clause. If they are not, a compile-time error will result.
This is the general form of a method declaration that includes a throws clause:
type method-name(parameter-list) throws exception-list
{
// body of method
}
Here, exception-list is a comma-separated list of the exceptions that a method can throw.
Following is an example of an incorrect program that tries to throw an exception that it
does not catch. Because the program does not specify a throws clause to declare this fact, the
program will not compile.
// This program contains an error and will not compile.
class ThrowsDemo {
static void throwOne() {
System.out.println("Inside throwOne.");
throw new IllegalAccessException("demo");
}
public static void main(String args[]) {
throwOne();
}
}
To make this example compile, you need to make two changes. First, you need to declare
that throwOne( ) throws IllegalAccessException. Second, main( ) must define a try/catch
statement that catches this exception.
The corrected example is shown here:
// This is now correct.
class ThrowsDemo {
static void throwOne() throws IllegalAccessException {
System.out.println("Inside throwOne.");
throw new IllegalAccessException("demo");
}
public static void main(String args[]) {
try {
throwOne();
} catch (IllegalAccessException e) {
System.out.println("Caught " + e);
}
}
}
Here is the output generated by running this example program:
inside throwOne
caught java.lang.IllegalAccessException: demo
debugging, you will usually want to handle an exception yourself. Doing so provides two
benefits. First, it allows you to fix the error. Second, it prevents the program from automatically
terminating. Most users would be confused (to say the least) if your program stopped running and printed a stack trace whenever an error occurred! Fortunately, it is quite easy
to prevent this.
To guard against and handle a run-time error, simply enclose the code that you want
to monitor inside a try block. Immediately following the try block, include a catch clause
that specifies the exception type that you wish to catch. To illustrate how easily this can be
done, the following program includes a try block and a catch clause that processes the
ArithmeticException generated by the division-by-zero error:
class Exc2 {
public static void main(String args[])
{
int d, a;
try { // monitor a block of code.
d = 0;
a = 42 / d;
System.out.println("This will not be printed.");
} catch (ArithmeticException e) { // catch divide-by-zero error
System.out.println("Division by zero.");
}
System.out.println("After catch statement.");
}
}
This program generates the following output:
Division by zero.
After catch statement.
Notice:- that the call to println( ) inside the try block is never executed. Once an exception
is thrown, program control transfers out of the try block into the catch block. Put differently,
catch is not “called,” so execution never “returns” to the try block from a catch. Thus, the
line “This will not be printed.” is not displayed. Once the catch statement has executed,
program control continues with the next line in the program following the entire try/catch
mechanism.
A try and its catch statement form a unit. The scope of the catch clause is restricted to
those statements specified by the immediately preceding try statement. A catch statement
cannot catch an exception thrown by another try statement (except in the case of nested try
statements, described shortly). The statements that are protected by try must be surrounded
by curly braces. (That is, they must be within a block.) You cannot use try on a single statement.
The goal of most well-constructed catch clauses should be to resolve the exceptional
condition and then continue on as if the error had never happened.
For example, in the nextprogram each iteration of the for loop obtains two random integers. Those two integers are divided by each other, and the result is used to divide the value 12345. The final result is put into a. If either division operation causes a divide-by-zero error, it is caught, the value of a is set to zero, and the program continues.
// Handle an exception and move on.
import java.util.Random;
class HandleError {
public static void main(String args[])
{
int a=0, b=0, c=0;
Random r = new Random();
for(int i=0; i<32000; i++) {
try {
b = r.nextInt();
c = r.nextInt();
a = 12345 / (b/c);
} catch (ArithmeticException e) {
System.out.println("Division by zero.");
a = 0; // set a to zero and continue
}
System.out.println("a: " + a);
}
}
}
Displaying a Description of an Exception
Throwable overrides the toString( ) method (defined by Object) so that it returns a string
containing a description of the exception. You can display this description in a println( )
statement by simply passing the exception as an argument. For example, the catch block
in the preceding program can be rewritten like this:
catch (ArithmeticException e) {
System.out.println("Exception: " + e);
a = 0; // set a to zero and continue
}
When this version is substituted in the program, and the program is run, each divide-byzero
error displays the following message:
Exception: java.lang.ArithmeticException: / by zero
While it is of no particular value in this context, the ability to display a description of
an exception is valuable in other circumstances—particularly when you are experimenting
with exceptions or when you are debugging.
Multiple catch Clauses
In some cases, more than one exception could be raised by a single piece of code. To handle
this type of situation, you can specify two or more catch clauses, each catching a different
type of exception. When an exception is thrown, each catch statement is inspected in order,
and the first one whose type matches that of the exception is executed. After one catch
statement executes, the others are bypassed, and execution continues after the try/catch
block.
--->The following example traps two different exception types:
// Demonstrate multiple catch statements.
class MultiCatch {
public static void main(String args[]) {
try {
int a = args.length;
System.out.println("a = " + a);
int b = 42 / a;
int c[] = { 1 };
c[42] = 99;
} 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.");
}
}
This program will cause a division-by-zero exception if it is started with no commandline
arguments, since a will equal zero. It will survive the division if you provide a
command-line argument, setting a to something larger than zero. But it will cause an
ArrayIndexOutOfBoundsException, since the int array c has a length of 1, yet the program
attempts to assign a value to c[42].
Here is the output generated by running it both ways:
C:\>java MultiCatch
a = 0
Divide by 0: java.lang.ArithmeticException: / by zero
After try/catch blocks.
C:\>java MultiCatch TestArg
a = 1
Array index oob: java.lang.ArrayIndexOutOfBoundsException:42
After try/catch blocks.
When you use multiple catch statements, it is important to remember that exception
subclasses must come before any of their superclasses. This is because a catch statement
that uses a superclass will catch exceptions of that type plus any of its subclasses. Thus, a
subclass would never be reached if it came after its superclass. Further, in Java, unreachable
code is an error. For example, consider the following program:
/* This program contains an error.
A subclass must come before its superclass in
a series of catch statements. If not,
unreachable code will be created and a
compile-time error will result.
*/
class SuperSubCatch {
public static void main(String args[]) {
try {
int a = 0;
int b = 42 / a;
} catch(Exception e)
{
System.out.println("Generic Exception catch.");
}
/* This catch is never reached because
ArithmeticException is a subclass of Exception. */
catch(ArithmeticException e) { // ERROR - unreachable
System.out.println("This is never reached.");
}
}
}
If you try to compile this program, you will receive an error message stating that the
second catch statement is unreachable because the exception has already been caught. Since
ArithmeticException is a subclass of Exception, the first catch statement will handle all
Exception-based errors, including ArithmeticException. This means that the second catch
statement will never execute. To fix the problem, reverse the order of the catch statements.
Nested try Statements
The try statement can be nested. That is, a try statement can be inside the block of another try.
Each time a try statement is entered, the context of that exception is pushed on the stack. If an
inner try statement does not have a catch handler for a particular exception, the stack is
unwound and the next try statement’s catch handlers are inspected for a match. This continues
until one of the catch statements succeeds, or until all of the nested try statements are exhausted.
If no catch statement matches, then the Java run-time system will handle the exception.
--->Here is an example that uses nested try statements:
// An example of nested try statements.
class NestTry {
public static void main(String args[]) {
try {
int a = args.length;
/* If no command-line args are present,
the following statement will generate
a divide-by-zero exception. */
int b = 42 / a;
System.out.println("a = " + a);
try { // nested try block
/* If one command-line arg is used,
then a divide-by-zero exception
will be generated by the following code. */
if(a==1) a = a/(a-a); // division by zero
/* If two command-line args are used,
then generate an out-of-bounds exception. */
if(a==2) {
int c[] = { 1 };
c[42] = 99; // generate an out-of-bounds exception
}
} catch(ArrayIndexOutOfBoundsException e) {
System.out.println("Array index out-of-bounds: " + e);
}
} catch(ArithmeticException e) {
System.out.println("Divide by 0: " + e);
}
}
}
As you can see, this program nests one try block within another. The program works as
follows. When you execute the program with no command-line arguments, a divide-by-zero
exception is generated by the outer try block. Execution of the program with one command-line
argument generates a divide-by-zero exception from within the nested try block. Since the
inner block does not catch this exception, it is passed on to the outer try block, where it is
handled. If you execute the program with two command-line arguments, an array boundary
exception is generated from within the inner try block. Here are sample runs that illustrate
each case:
C:\>java NestTry
Divide by 0: java.lang.ArithmeticException: / by zero
C:\>java NestTry One
a = 1
Divide by 0: java.lang.ArithmeticException: / by zero
C:\>java NestTry One Two
a = 2
Array index out-of-bounds:
java.lang.ArrayIndexOutOfBoundsException:42
Nesting of try statements can occur in less obvious ways when method calls are involved.
For example, you can enclose a call to a method within a try block. Inside that method is
another try statement. In this case, the try within the method is still nested inside the outer try
block, which calls the method. Here is the previous program recoded so that the nested
try block is moved inside the method nesttry( ):
/* Try statements can be implicitly nested via
calls to methods. */
class MethNestTry {
static void nesttry(int a) {
try { // nested try block
/* If one command-line arg is used,
then a divide-by-zero exception
will be generated by the following code. */
if(a==1) a = a/(a-a); // division by zero
/* If two command-line args are used,
then generate an out-of-bounds exception. */
if(a==2) {
int c[] = { 1 };
c[42] = 99; // generate an out-of-bounds exception
}
} catch(ArrayIndexOutOfBoundsException e) {
System.out.println("Array index out-of-bounds: " + e);
}
}
public static void main(String args[]) {
try {
int a = args.length;
/* If no command-line args are present,
the following statement will generate
a divide-by-zero exception. */
int b = 42 / a;
System.out.println("a = " + a);
nesttry(a);
} catch(ArithmeticException e) {
System.out.println("Divide by 0: " + e);
}
}
}
The output of this program is identical to that of the preceding example.
throw
So far, you have only been catching exceptions that are thrown by the Java run-time system.
However, it is possible for your program to throw an exception explicitly, using the throw
statement. The general form of throw is shown here:
throw ThrowableInstance;
Here, ThrowableInstance must be an object of type Throwable or a subclass of Throwable.
Primitive types, such as int or char, as well as non-Throwable classes, such as String and
Object, cannot be used as exceptions. There are two ways you can obtain a Throwable object:
using a parameter in a catch clause, or creating one with the new operator.
The flow of execution stops immediately after the throw statement; any subsequent
statements are not executed. The nearest enclosing try block is inspected to see if it has a
catch statement that matches the type of exception. If it does find a match, control is
transferred to that statement. If not, then the next enclosing try statement is inspected, and
so on. If no matching catch is found, then the default exception handler halts the program
and prints the stack trace.
the exception rethrows it to the outer handler.
// Demonstrate throw.
class ThrowDemo {
static void demoproc() {
try {
throw new NullPointerException("demo");
} catch(NullPointerException e) {
System.out.println("Caught inside demoproc.");
throw e; // rethrow the exception
}
}
public static void main(String args[]) {
try {
demoproc();
} catch(NullPointerException e) {
System.out.println("Recaught: " + e);
}
}
}
This program gets two chances to deal with the same error. First, main( ) sets up an exception
context and then calls demoproc( ). The demoproc( ) method then sets up another exceptionhandling
context and immediately throws a new instance of NullPointerException, which
is caught on the next line. The exception is then rethrown. Here is the resulting output:
Caught inside demoproc.
Recaught: java.lang.NullPointerException: demo
The program also illustrates how to create one of Java’s standard exception objects. Pay
close attention to this line:
throw new NullPointerException("demo");
Here, new is used to construct an instance of NullPointerException. Many of Java’s builtin
run-time exceptions have at least two constructors: one with no parameter and one that
takes a string parameter. When the second form is used, the argument specifies a string that
describes the exception. This string is displayed when the object is used as an argument to
print( ) or println( ). It can also be obtained by a call to getMessage( ), which is defined by
Throwable.
throws
If a method is capable of causing an exception that it does not handle, it must specify this
behavior so that callers of the method can guard themselves against that exception. You do
this by including a throws clause in the method’s declaration. Athrows clause lists the types
of exceptions that a method might throw. This is necessary for all exceptions, except those of
type Error or RuntimeException, or any of their subclasses. All other exceptions that a method
can throw must be declared in the throws clause. If they are not, a compile-time error will result.
This is the general form of a method declaration that includes a throws clause:
type method-name(parameter-list) throws exception-list
{
// body of method
}
Here, exception-list is a comma-separated list of the exceptions that a method can throw.
Following is an example of an incorrect program that tries to throw an exception that it
does not catch. Because the program does not specify a throws clause to declare this fact, the
program will not compile.
// This program contains an error and will not compile.
class ThrowsDemo {
static void throwOne() {
System.out.println("Inside throwOne.");
throw new IllegalAccessException("demo");
}
public static void main(String args[]) {
throwOne();
}
}
To make this example compile, you need to make two changes. First, you need to declare
that throwOne( ) throws IllegalAccessException. Second, main( ) must define a try/catch
statement that catches this exception.
The corrected example is shown here:
// This is now correct.
class ThrowsDemo {
static void throwOne() throws IllegalAccessException {
System.out.println("Inside throwOne.");
throw new IllegalAccessException("demo");
}
public static void main(String args[]) {
try {
throwOne();
} catch (IllegalAccessException e) {
System.out.println("Caught " + e);
}
}
}
Here is the output generated by running this example program:
inside throwOne
caught java.lang.IllegalAccessException: demo
Tags:
Java