Java Exception Handling

Exception Handling

• An event that occurs during program execution which disrupts the normal flow is called an exception
• When an exception occurs, an exception object is created
• The exception object will contain information about the situation and the state of the program at that time
• Throwing an exception refers to the creation of the exception object with the handoff to the Java runtime system

Checked Exceptions

• Checked exceptions have to be handled to compile
• These exceptions typically represent common issues the program would be expected to recover from
• FileNotFoundException is a checked exception
• The program should not crash if a given file is not found when a FileNotFoundException occurs
• The program can prompt for a valid file name, or an end program gracefully on a FileNotFoundException
• EOFException is a checked exception
• EOFException is an end-of-file exception
• EOFException is a normal exception when a program is reading a file with an unknown number of values
• The program can handle this exception and continue on instead of crashing when EOFException occurs
• The program must address potential Input/Output Exceptions when using an Input/Output stream to compile

Unchecked Exceptions

• Unchecked exceptions do not have to be addressed to compile
• Unchecked Exceptions should not occur
• These exceptions can indicate an error in the logic of the code
• An example of an unchecked exception is ArrayIndexOutOfBoundsException
• The compiler is usually unable to anticipate logical errors
• Programs can use an array without specifying that the method may cause an ArrayIndexOutOfBoundsException
• The compiler does not force the programmer to account for unchecked exceptions in order to compile

Exceptions vs Errors

• Exceptions and errors are similar in concept
• Exceptions and errors are a part of the same hierarchy of objects
• Exceptions are situations that the program can generally be expected to recover from
• Errors are situations the program may not be able to recover from

Exception Handling

• Programs must include a throws clause or a try-catch structure for the exception, if a checked exception can occur during the execution of a program

Try-Catch Structure

• The general structure begins by declaring a try{}, which contains some Java statements that cause an exception
• Below the try{}, declare a catch(){}, which contains some Java statements that run if the exception occurs
• Below the catch(){}, declare finally{}, which contains Java statements that run after the try and catch statements

Execution of a Try Statement

• The program executes each statement in the try-block
• If no exception occurs, the program moves to the finally-block and executes the corresponding statements, then the program moves to the next statement to execute after the try statement
• If an expected exception listed is thrown, the program executes the catch-block, then executes statements listed in the finally-block

FileNotFoundException

• ReadBinary.java can throw a FileNotFoundException
• If an IOException occurs, the program terminates abnormally as the IOException is thrown
• If the 'myData.ser' file is not found, then a FileNotFoundException is thrown
• Since FileNotFoundException is a child class of IOException the program will terminate and the call trace appears

HandleFileNotFound.java

• This program's constructor accepts a String
• This program accepts exception for invalid Social Security number
• Extends Exception, so the exception is checked
• getSin() getter for Social Security number

Handling an Exception

• ReadBinary.java reads only 5 integers
• The program can be modified to read an arbitrary number of integers
• If ReadBinary attempts to execute readInt() and there are no more integers to be read, then an EOFException occurs
• Java docs will specify which exceptions a given method may throw
• For: https://docs.oracle.com/javase/7/docs/api/java/io/DataInputStream.html#re adInt()
• readInt() may throw IOException or EOFException

ReadFromBinaryUntilEndOfFile.java

• Try-catch statement for checking file-not-found exceptions

Handling EndOfFile

• Reads integers until there are none left
• Catches the end of the file and IOExceptions and presents a message

Custom Exceptions

• Exceptions are part of a hierarchy of objects
• Exception and Error classes are children of the Throwable class
• RunTimeException class is a child of Exception
• It is not generally recommended to create new exceptions unless necessary
• The Throwable class has many subclasses
• An exception already exists that will meet the programmer's needs

Custom Exceptions Defined

• A custom exception is a class that extends either Exception or RuntimeException
• Extend Exception when a programmer needs a checked exception
• Extend RuntimeException if a programmer requires an unchecked exception

Custom Exception - InvalidSIN

• Code can throw an exception if an employee has an invalid SIN
• Programs use the existing InvalidArgumentException, and it is possible to create one
• INVALID SIN exception would handle above custom exception case

Learn about exception handling in Java, including checked exceptions like FileNotFoundException and EOFException. Understand how to handle these exceptions to prevent program crashes. Explore strategies for graceful error recovery and user-friendly error messages.