Java Exceptions: Types and Hierarchy

• An exception in Java is an unexpected event or error during program execution that disrupts the normal flow
• Java provides a structured mechanism to handle errors gracefully instead of crashing
• Exceptions are represented as objects from the java.lang package

Exception Hierarchy

• The exception hierarchy in Java starts with the Throwable class, which is a base class for all errors and exceptions
• Throwable has two subclasses: Error and Exception

Error

• Errors represent serious problems that applications should not attempt to handle, such as OutOfMemoryError
• Errors typically indicate issues with the JVM or system resources

Exception

• Represents conditions that applications should catch and handle
• Subtypes include checked and unchecked

Checked Exceptions

• Must be handled at compile-time

Unchecked Exceptions

• Handled at runtime and represent programming logic errors

Types of Exceptions

Checked Exceptions

• These must be either caught or declared in the method signature
• The Java compiler ensures that developers handle these exceptions explicitly
• Occur due to external factors beyond the program's control, like file system access or network operations
• Handle using a try-catch block or propagate them with the throws keyword
• Examples include IOException, SQLException, ClassNotFoundException, and FileNotFoundException

IOException

• Occurs when there’s an issue with input/output operations

SQLException

• Occurs when interacting with databases

ClassNotFoundException

• Thrown when trying to load a class dynamically, but the class is not found in the classpath

FileNotFoundException

• A subclass of IOException that occurs when a file is not found

Why Use Checked Exceptions?

• Forces developers to anticipate and handle errors during compile time
• Ensures robust and reliable code

Unchecked Exceptions

• Are not checked at compile-time
• Usually occur due to programming logic errors
• They are subclasses of RuntimeException
• Represent bugs or issues in the program's logic that should be fixed during development
• Do not need to be declared in the throws clause or caught explicitly
• Examples include NullPointerException, ArithmeticException, ArrayIndexOutOfBoundsException, and IllegalArgumentException

NullPointerException

• Thrown when attempting to call a method on a null object reference

ArithmeticException

• Thrown when an illegal arithmetic operation occurs, like division by zero

ArrayIndexOutOfBoundsException

• Thrown when accessing an invalid index in an array

IllegalArgumentException

• Thrown when an invalid argument is passed to a method

Why Use Unchecked Exceptions?

• Simplify coding by not requiring explicit handling for logic-related errors
• Must be avoided by ensuring good programming practices

Errors

• Represent serious problems that are not meant to be caught or handled by applications
• Typically caused by issues external to the application, like system resource limitations or JVM malfunctions
• Examples include OutOfMemoryError, StackOverflowError, and VirtualMachineError

OutOfMemoryError

• Thrown when the JVM runs out of memory

StackOverflowError

• Thrown when a program recurses too deeply, exhausting the stack memory

VirtualMachineError

• Represents errors in the Java Virtual Machine
• Errors are not recoverable
• Applications generally terminate when an error occurs

How to Handle Exceptions

• Java provides several mechanisms for handling exceptions

Try-Catch Block

• Used to handle exceptions gracefully
• Code that might throw an exception is placed in the try block
• The exception is caught in the catch block

Finally Block

• Contains code that is always executed, regardless of whether an exception occurs or not
• Often used for cleanup operations, like closing files or releasing resources

Throw and Throws Keywords

• throw is used to explicitly throw an exception in the code
• throws declares exceptions that a method can throw

Custom Exceptions

• Custom Exceptions are defined by extending the Exception or RuntimeException class

Best Practices for Exception Handling

• Always catch specific exceptions rather than using a general Exception type
• Improves readability and precision
• Avoid empty catch blocks that simply ignore exceptions
• Log or handle them appropriately
• Always clean up resources in a finally block or use try-with-resources
• Use checked exceptions for recoverable errors
• Avoid overusing exceptions
• Exceptions should not be used for normal program flow

Common Scenarios for Exceptions

File Handling

• FileNotFoundException can occur
• Solution: Validate file existence before opening

Null Pointer

• NullPointerException can occur
• Solution: Check for null values before dereferencing objects

Array Access

• ArrayIndexOutOfBoundsException can occur
• Solution: Validate array bounds before accessing

Explore Java exceptions, their types, and hierarchy. Learn about checked and unchecked exceptions and error handling in Java. Understand the Throwable class and its subclasses: Error and Exception.