Java exception handling

Exception Handling
• An exception is an abnormal condition that
arises in a code sequence at run time.
• In other words, an exception is a run-time
error.
• Exceptions can be generated by the Java run-
time system, or they can be manually
generated by your code.

• Exceptions thrown by Java relate to fundamental
errors that violate the rules of the Java language
or the constraints of the Java execution
environment.
• Manually generated exceptions are typically used
to report some error condition to the caller of a
method.
• Java exception handling is managed via five
keywords: try, catch, throw, throws, and finally

• The general form :
try {
// block of code to monitor for errors
}
catch (ExceptionType1 exOb) {
// exception handler for ExceptionType1
}
catch (ExceptionType2 exOb) {
// exception handler for ExceptionType2
}
// ...
finally {
// block of code to be executed before try block ends
}

• All exception types are subclasses of the built-
in class Throwable.
• Exception, Error are the subclasses of
Throwable class.

Exception
• This is also the class that you will subclass to
create your own custom exception types.
• Eg:
division by zero
invalid array indexing.

Error
• which defines exceptions that are not
expected to be caught under normal
circumstances by your program.
• Eg:
Stack overflow

Using try and catch
• exception handler allows you to fix the error.
Second, it prevents the program from
automatically terminating.
• To guard against and handle a run-time error,
simply enclose the code that you want to
monitor inside a try block.

• If an exception occurs within the try block, it is
thrown.
• Your code can catch this exception (using
catch) and handle it in some rational manner.

Sample
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.");
}
}

• Output:
Division by zero.
After catch statement.
• Once an exception is thrown, program control
transfers out of the try block into the catch
block.
• Once the catch statement has executed,
program control continues with the next line
in the program following the entire try/catch
mechanism.

• The statements that are protected by try must
be surrounded by curly braces.

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.

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.
• After one catch statement executes, the
others are bypassed, and execution continues
after the try/catch block

Sample
class MultiCatch {
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.");
} }

• When you use multiple catch statements, it is
important to remember that exception
subclasses must come before any of their
superclasses.

• Checked exceptions :
– Checked Exceptions forces programmers to deal
with the exception that may be thrown.
– Checked exceptions must be caught at compile
time.
• Unchecked exceptions:
– Unchecked exceptions , however, the compiler
doesn’t force the programmers to either catch the
exception or declare it in a throws clause.

Nested try Statements
• The try statement can be nested.
• Sample
class NestTry {
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);
}
}
}

• Output:
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

throw
• It is possible for your program to throw an
exception explicitly, using the throw
statement.
• The general form :
throw ThrowableInstance;
• ThrowableInstance must be an object of type
Throwable or a subclass of Throwable.

• 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 the exception.

Sample
class ThrowDemo {
static void demoproc() {
try {
throw new NullPointerException("demo");
} catch(NullPointerException e) {
System.out.println("Caught inside demoproc.");
}
}
try {
demoproc();
} catch(NullPointerException e) {
System.out.println("Recaught: " + e);
}
} }

throws
• If a method is capable of causing an exception
that it does not handle, it must specify this
behaviour 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.
• The general form:
– type method-name(parameter-list) throws
exception-list
– {
– // body of method
– }

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);
}
} }

finally
• Any code that absolutely must be executed
before a method returns is put in a finally
block.
• The finally block will execute whether or not
an exception is thrown.
• This can be useful for closing file handles and
freeing up any other Resources.
• The finally clause is optional.

Sample
class FinallyDemo {
// Through an exception out of the method.
static void procA() {
try {
System.out.println("inside procA");
throw new RuntimeException("demo");
} finally {
System.out.println("procA's finally");
}
}

Java’s Built-in Exceptions
• unchecked exceptions - because the compiler
does not check to see if a method handles or
throws these exceptions.
• checked exceptions- Java defines several other
types of exceptions that relate to its various
class libraries

Creating Your Own Exception
Subclasses
• To create your own exception types to handle
situations specific to your applications, to do
so just define a subclass of Exception.

Sample
class MyException extends Exception {
private int detail;
MyException(int a) {
detail = a;
}
public String toString() {
return "MyException[" + detail + "]";
}
}
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");
}
try {
compute(1);
compute(20);
} catch (MyException e) {
System.out.println("Caught " + e);
}
} }

Chained Exceptions
• The chained exception feature allows you to
associate another exception with an
exception.

Java exception handling

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