Java Exception Try-Catch-Finally & Handling Exception

Simmi S Dept Of CS
Exception Handling In Java
Ms Simmi S
Assistant Professor
Department of Computer Science UG
Kristu Jayanti(Deemed To Be University

Simmi S Dept Of CS
Exception
• In Java, Exception is an unwanted or unexpected event, which
occurs during the execution of a program, i.e. at run time, that
disrupts the normal flow of the program’s instructions.
• Exceptions can be caught and handled by the program. When an
exception occurs within a method, it creates an object.
• This object is called the exception object.
• It contains information about the exception, such as the name and
description of the exception and the state of the program when the
exception occurred.

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Major reasons why an exception Occurs
• Invalid user input
• Device failure
• Loss of network connection
• Physical limitations (out-of-disk memory)
• Code errors
• Opening an unavailable file

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Errors
• Errors represent irrecoverable conditions such as Java virtual
machine (JVM) running out of memory, memory leaks, stack
overflow errors, library incompatibility, infinite recursion, etc.
• Errors are usually beyond the control of the programmer, and
we should not try to handle errors.

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Exception Hierarchy
• All exception and error types are subclasses of the class
Throwable, which is the base class of the hierarchy.
• Exception This class is used for exceptional conditions that
user programs should catch. NullPointerException is an
example of such an exception.
• Error is used by the Java run-time system(JVM) to indicate
errors having to do with the run-time environment itself(JRE).
StackOverflowError is an example of such an error.

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Exception Hierarchy

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Types of Exceptions

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Exceptions can be categorized in two ways:
• Built-in Exceptions
• Checked Exception
• Unchecked Exception
• User-Defined Exceptions

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Built-in Exceptions
Built-in exceptions are the exceptions that are available in Java
libraries. These exceptions are suitable to explain certain error
situations.
• Checked Exceptions: Checked exceptions are called compile-
time exceptions because these exceptions are checked at
compile-time by the compiler.
• Unchecked Exceptions: The unchecked exceptions are just
opposite to the checked exceptions. The compiler will not check
these exceptions at compile time. In simple words, if a program
throws an unchecked exception, and even if we didn’t handle or
declare it, the program would not give a compilation error.

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User-Defined Exceptions:
• Sometimes, the built-in exceptions in Java are not able to
describe a certain situation. In such cases, users can also
create exceptions, which are called ‘user-defined Exceptions’.

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How Does JVM Handle an Exception?
• Default Exception Handling: Whenever inside a method, if an
exception has occurred, the method creates an Object known
as an Exception Object and hands it off to the run-time
system(JVM).
• The exception object contains the name and description of
the exception and the current state of the program where the
exception has occurred.
• Creating the Exception Object and handling it in the run-time
system is called throwing an Exception.

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How Programmer Handle an Exception?
• Customized Exception Handling: Java exception handling is
managed via five keywords: try, catch, throw, throws,
and finally.

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Checked Exception

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UnChecked Exception

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Syntax of try-catch Block
try {
// Block of code to try
}
catch(ExceptionName obj) {
// Block of code to handle errors
}

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Internal Working of Java try-catch block

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Unchecked Exception

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Methods To Print Exceptions In Java
• 1. java.lang.Throwable.printStackTrace() method:
By using this method, we will get the name(e.g.,
java.lang.ArithmeticException) and description(e.g., / by zero) of
an exception separated by a colon, and the stack trace (wherein
the code, that exception has occurred) in the next line.
• 2. toString() method :
Using this method will only get the name and description of
an exception. Note that this method is overridden in the
Throwable class.

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3.java.lang.Throwable.getMessage() method:
Using this method, we will only get a description of an
exception.

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Finally Blocks
• A finally keyword is used to create a block of code that follows a
try block. A finally block of code is always executed whether an
exception has occurred or not.
• Using a finally block, it lets you run any cleanup type
statements that you want to execute, no matter what happens in
the protected code. A finally block appears at the end of catch
block.

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Finally Blocks
Try
{
// Block of code to try
}
catch(ExceptionName obj) {
// Block of code to handle
errors
}
Finally
{
// cleanup code
}

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Finally Blocks

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Java Multi-catch block
• A try block can be followed by one or more catch blocks.
• Each catch block must contain a different exception handler.
So, if you have to perform different tasks at the occurrence of
different exceptions, use java multi-catch block.
• At a time only one exception occurs and at a time only one
catch block is executed.
• All catch blocks must be ordered from most specific to most
general, i.e. catch for ArithmeticException must come before
catch for Exception.

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Java Multi-catch block

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Nested Try Block
• A try block can be nested within another try block.
• This structure is termed as Nested try block. Whenever an
exception is raised within a nested try block, its exception is
pushed to Stack.
• The exception propagates from child to parent try block and so
on.

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Nested Try Block

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Throw Keyword
• The Java throw keyword is used to throw an exception explicitly.
• Java throw
• The throw keyword in Java is used to explicitly throw an
exception from a method or any block of code.
• We can throw either checked or unchecked exception. The throw
keyword is mainly used to throw custom exceptions.
• We specify the exception object which is to be thrown. The
Exception has some message with it that provides the error
description. These exceptions may be related to user inputs,
server, etc.

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Syntax:
• Throw new throwable_subclass;

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Throwing Exception(But not handling it)

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Throwing Exception(Example 1)

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Throwing Exception(Example 2)

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User defined Exception

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Java throws keyword is
• The Java throws keyword is used to declare an exception. It
gives an information to the programmer that there may occur an
exception.
• So, it is better for the programmer to provide the exception
handling code so that the normal flow of the program can be
maintained.
• Exception Handling is mainly used to handle the checked
exceptions

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return_type method_name() throws exception_class_name
{
//method code
}

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Throwing The Exception But Not Handled
Exception is not handled Execution will stop

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Throwing The Exception : Handled

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Throwing The Exception From Main Method
Exception is not handled Execution will stop

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Difference Between Throw & Throws

Java Exception Try-Catch-Finally & Handling Exception

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