Exceptions and Assertions

What is an Exception?

● Exceptional event

● Error that occurs during runtime

● Cause normal program flow to be disrupted

● Examples

• – Divide by zero errors
• – Accessing the elements of an array beyond its range
• – Invalid input
• – Hard disk crash
• – Opening a non-existent file
• – Heap memory exhausted

What Happens When an Exception Occurs?

● When an exception occurs within a method, the method creates an exception object and hands it off to the runtime system

– Creating an exception object and handing it to the runtime system is called “throwing an exception”

– Exception object contains information about the error, including its type and the state of the program when the error occurred

Benefits of Java Exception Handling Framework

● Separating Error-Handling code from “regular” business logic code

● Propagating errors up the call stack

● Grouping and differentiating error types

The finally Keyword

● Block of code is always executed despite of different scenarios:

– Forced exit occurs using a return, a continue or a break statement

– Normal completion

– Caught exception thrown

● Exception was thrown and caught in the method

– Uncaught exception thrown

● Exception thrown was not specified in any catch block in the method

Throwing Exceptions: The throw Keyword

● Java allows you to throw exceptions (generate exceptions)

throw <exception object>;

● An exception you throw is an object

– You have to create an exception object in the same way you create any other object

● Example:

throw new ArithmeticException(“testing...”);

Exception Class Hierarchy

The Error and Exception Classes

Throwable class

• Root class of exception classes
• Immediate subclasses
• Error
• Exception

Exception class

– Conditions that user programs can reasonably deal with

– Usually the result of some flaws in the user program code

– Examples

• Division by zero error
• Array out-of-bounds error

Error class

– Used by the Java run-time system to handle errors occurring in the run-time environment

– Generally beyond the control of user programs

– Examples

• Out of memory errors
• Hard disk crash

Checked and Unchecked Exceptions

● Checked exception

• – Java compiler checks if the program either catches or lists the occurring checked exception
• – If not, compiler error will occur

● Unchecked exceptions

• – Not subject to compile-time checking for exception handling
• – Built-in unchecked exception classes
• Error
• RuntimeException
• Their subclasses
• – Handling all these exceptions may make the program cluttered and may become a nuisance

Creating Your Own Exception Class

● Steps to follow

• – Create a class that extends the RuntimeException or the Exception class
• – Customize the class
• Members and constructors may be added to the class

What are Assertions?

● Allow the programmer to find out if an assumption was met

• – Example: month

● Extension of comments wherein the assert statement informs the person reading the code that a particular condition should always be satisfied

• – Running the program informs you if assertions made are true or not
• – If an assertion is not true, an AssertionError is thrown

● User has the option to turn it off or on at runtime