Exception Handling

In Java, Exception Handling is a way to handle errors that occur while a program is running. When something goes wrong, like dividing by zero or trying to open a file that doesn't exist, Java throws an exception. If we don't handle these exceptions, our program might crash. Exception handling helps us manage these errors in a controlled way so that our program can either fix the issue, show an error message, or continue running.

What is an Exception?

An exception is an unexpected event that occurs during the execution of a program and disrupts its normal flow. Java has a special way of handling these exceptions to keep the program from crashing.

Basic Exception Handling Keywords

Java provides four main keywords to handle exceptions:

try: Wraps the code that might throw an exception.
catch: Catches and handles the exception if it occurs.
finally: Runs code after try-catch, no matter what. Often used to clean up resources (like closing files).
throw: Used to explicitly throw an exception in the code.

Basic Structure of Exception Handling

Here's the basic syntax:

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try {
    // Code that might throw an exception
} catch (ExceptionType e) {
    // Code to handle the exception
} finally {
    // Code that always executes, whether or not an exception occurred
}

Example of Exception Handling

Imagine we want to divide two numbers. If we try to divide by zero, an exception will be thrown. Here’s how we can handle it:

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public class ExceptionExample {
    public static void main(String[] args) {
        try {
            int number1 = 10;
            int number2 = 0;
            int result = number1 / number2; // This line will throw an exception
            System.out.println("Result: " + result);
        } catch (ArithmeticException e) {
            System.out.println("Cannot divide by zero!");
        } finally {
            System.out.println("This block always runs.");
        }
    }
}

In the above example:

The try block contains code that might throw an exception (dividing by zero).
The catch block catches the ArithmeticException and handles it by printing a message.
The finally block always runs, whether or not an exception occurred. It’s useful for cleaning up resources, like closing files or databases.

Types of Exceptions in Java

There are two main types of exceptions:

Checked Exceptions: Exceptions that are checked at compile-time. Examples include IOException, SQLException.
Unchecked Exceptions: Exceptions that occur at runtime and are not checked at compile-time. Examples include ArithmeticException, NullPointerException, ArrayIndexOutOfBoundsException.

Using Multiple catch Blocks

If we want to handle different types of exceptions in different ways, we can use multiple catch blocks:

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try {
    int[] numbers = {1, 2, 3};
    System.out.println(numbers[5]); // ArrayIndexOutOfBoundsException
    int result = 10 / 0;            // ArithmeticException
} catch (ArrayIndexOutOfBoundsException e) {
    System.out.println("Index is out of bounds.");
} catch (ArithmeticException e) {
    System.out.println("Cannot divide by zero.");
}

In the above example, the program will catch and handle different exceptions separately.

Why Use Exception Handling?

Improves Program Stability: Prevents the program from crashing.
Provides Error Messages: Helps developers understand what went wrong.
Resource Management: Allows for proper closing of resources like files and databases.

Custom Exceptions

A Custom Exception is a user-defined exception that you create by extending the Exception class (for checked exceptions) or the RuntimeException class (for unchecked exceptions). You can use custom exceptions to provide clear error messages for situations specific to your program, making it easier to debug.

Why Use Custom Exceptions?

In Java, sometimes the built-in exceptions (like NullPointerException or ArithmeticException) aren’t enough to handle specific errors in your program. In these cases, you can create your own Custom Exceptions to make your code easier to read and understand. Custom exceptions allow you to define meaningful error messages and handle situations unique to your application. Custom exceptions help

Clearly indicate what went wrong in a specific context.
Improve code readability and maintainability.
Create specific error messages that make debugging easier.
Handle unique errors that standard exceptions don't cover.

How to Create a Custom Exception

Creating a custom exception involves:

Creating a new class that extends Exception (for a checked exception) or RuntimeException (for an unchecked exception).
Defining a constructor for your custom exception. This can include custom error messages to make debugging easier.

Here's an example of a custom exception:

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// Step 1: Create the custom exception by extending Exception
public class InvalidAgeException extends Exception {
    // Step 2: Define a constructor with a custom message
    public InvalidAgeException(String message) {
        super(message);
    }
}

In the above example, InvalidAgeException is a custom exception that takes a custom error message as a parameter.

When to Use Custom Exceptions

Use custom exceptions when you need to signal specific errors in your application that aren’t covered by standard Java exceptions.
Custom exceptions should be meaningful and reflect the issue they represent (e.g., InsufficientFundsException for a banking app).