12. Java Exceptions and error handling

Handling Errors During the Program Execution
Exception Handling
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Table of Contents
1. What Are Exceptions?
 The Exception Class
 Types of Exceptions and Their Hierarchy
2. Handling Exceptions
3. Raising (Throwing) Exceptions
4. Best Practices
5. Creating Custom Exceptions
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What are Exceptions?
The Paradigm of Exceptions in OOP

 Simplify code construction and maintenance
 Allow the problematic situations to be processed
at multiple levels
 Exception objects have detailed information about
the error
What Are Exceptions?
5
There are two ways to write error-free programs; only the third
one works. (Alan J. Perlis)

 Exceptions in Java are objects
 The Throwable class is a base for
all exceptions in JVM
 Contains information for the cause of the error
 Message – a text description of the exception
 StackTrace – the snapshot of the stack at the
moment of exception throwing
The Throwable Class
6

 Java exceptions inherit from Throwable
 Below Throwable are:
 Error - not expected to be caught under normal
circumstances from the program
 Example - StackOverflowError
 Exception
 Used for exceptional conditions that user programs should catch
 User-defined exceptions
Types of Exceptions
7

 Exceptions are two types:
 Checked - an exception that is checked (notified) by the
compiler at compilation-time
 Also called as Compile Time exceptions
 Unchecked - an exception that occurs at the time of execution
 Also called as Runtime Exceptions
Exceptions
8
public static void main(String args[]) {
File file = new File("E://file.txt");
FileReader fr = new FileReader(file);
} FileNotFoundException

Handling Exceptions
Catching and Processing Errors

Handling Exceptions
 In Java exceptions can be handled by the
try-catch construction
 catch blocks can be used multiple times to process
different exception types
11
try {
// Do some work that can raise an exception
} catch (SomeException) {
// Handle the caught exception
}

Multiple Catch Blocks – Example
12
String s = sc.nextLine();
try {
Integer.parseInt(s);
System.out.printf(
"You entered a valid integer number %s.", s);
} catch (NumberFormatException ex) {
System.out.println("Invalid integer number!");
}

 When catching an exception of a particular class, all its
inheritors (child exceptions) are caught too, e.g.
 Handles IndexOutOfBoundsException and its descendants
ArrayIndexOutOfBoundsException and
StringIndexOutOfBoundsException
Handling Exceptions
13
try {
// Do some work that can cause an exception
} catch (IndexOutOfBoundsException ae) {
// Handle the caught arithmetic exception
}

Find the Mistake!
14
String str = sc.nextLine();
try {
Integer.parseInt(str);
} catch (Exception ex) {
System.out.println("Cannot parse the number!");
System.out.println("Invalid integer number!");
}
Should be last
Unreachable code

 Unmanaged code can throw other exceptions
 For handling all exceptions (even unmanaged) use the
construction:
Handling All Exceptions
15
try {
// Do some work that can raise any exception
} catch (Exception ex) {
// Handle the caught exception
}

 The statement:
 Ensures execution of a given block in all cases
 When exception is raised or not in the try block
 Used for execution of cleaning-up code, e.g. releasing resources
The try-finally Statement
16
try {
// Do some work that can cause an exception
} finally {
// This block will always execute
}

try-finally – Example
17
static void testTryFinally() {
System.out.println("Code executed before try-finally.");
try {
String str = sc.nextLine();
System.out.println("Parsing was successful.");
return; // Exit from the current method
System.out.println("Parsing failed!");
} finally {
System.out.println("This cleanup code is always executed.");
}
System.out.println("This code is after the try-finally block.");
}

How Do Exceptions Work?
18
try Run this code
catch
Execute this code when there
is an exception
finally Always run this code

 Exceptions are thrown (raised) by the throw keyword
 Used to notify the calling code in case of an error
or unusual situation
 When an exception is thrown:
 The program execution stops
 The exception travels over the stack
 Until a matching catch block is reached to handle it
 Unhandled exceptions display an error message
Throwing Exceptions
21

 Throwing an exception with an error message:
 Exceptions can accept message and cause:
 Note: if the original exception is not passed, the initial cause of
the exception is lost
Using throw Keyword
22
throw new IllegalArgumentException("Invalid amount!");
try {
…
} catch (SQLException sqlEx) {
throw new IllegalStateException("Cannot save invoice.", sqlEx);
}

 Caught exceptions can be re-thrown again:
Re-Throwing Exceptions
23
try {
System.out.println("Parse failed!");
throw ex; // Re-throw the caught exception
}

Throwing Exceptions – Example
24
public static double sqrt(double value) {
if (value < 0)
throw new IllegalArgumentException(
"Sqrt for negative numbers is undefined!");
return Math.sqrt(value);
}
public static void main(String[] args) {
try {
sqrt(-1);
} catch (IllegalArgumentException ex) {
System.err.println("Error: " + ex.getMessage());
ex.printStackTrace();
}
}

 Catch blocks should:
 Begin with the exceptions lowest in the hierarchy
 Continue with the more general exceptions
 Otherwise a compilation error will occur
 Each catch block should handle only these exceptions
which it expects
 If a method is not competent to handle an exception, it should
leave it unhandled
 Handling all exceptions disregarding their type is a popular
bad practice (anti-pattern)!
Using catch Block
27

 When an application attempts to use null in a case where
an object is required – NullPointerException
 An array has been accessed with an illegal index –
ArrayIndexOutOfBoundsException
 An index is either negative or greater than the size of
the string – StringIndexOutOfBoundsException
 Attempts to convert a inappropriate string to one of the
numeric types - NumberFormatException
Choosing the Exception Type (1)
28

 When an exceptional arithmetic condition has occurred –
ArithmeticException
 Attempts to cast an object to a subclass of which it is not an
instance – ClassCastException
 A method has been passed an illegal or inappropriate
argument - IllegalArgumentException
Choosing the Exception Type (2)
29

 When raising an exception, always pass to the constructor a
good explanation message
 When throwing an exception always pass a good description
of the problem
 The exception message should explain what causes the problem
and how to solve it
 Good: "Size should be integer in range [1…15]"
 Good: "Invalid state. First call Initialize()"
 Bad: "Unexpected error"
 Bad: "Invalid argument"
Exceptions – Best Practices (1)
30

 Exceptions can decrease the application performance
 Throw exceptions only in situations which are really exceptional
and should be handled
 Do not throw exceptions in the normal program control flow
 JVM could throw exceptions at any time with no way to
predict them
 E.g. StackOverflowError
Exceptions – Best Practices (2)
31

 Custom exceptions inherit an exception class
(commonly – Exception)
 Thrown just like any other exception
Creating Custom Exceptions
33
public class TankException extends Exception {
public TankException(String msg) {
super(msg);
}
}
throw new TankException("Not enough fuel to travel");

 …
 …
 …
Summary
34
 Exceptions provide a flexible error
handling mechanism
 Unhandled exceptions cause error
messages
 try-finally ensures a given code
block is always executed
 Even when an exception is thrown

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12. Java Exceptions and error handling

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