1.4.4 Exception Handling Explained

Exception handling is a crucial aspect of Java programming that allows you to manage errors and unexpected situations gracefully. By understanding exception handling, you can write robust and reliable Java SE 11 applications.

Key Concepts

1. Exceptions

An exception is an event that occurs during the execution of a program, disrupting the normal flow of instructions. Exceptions can be caused by various factors such as invalid input, file not found, or network issues.

2. Exception Hierarchy

Exceptions in Java are organized in a hierarchy, with Throwable as the root class. The Throwable class has two main subclasses: Error and Exception. Errors represent serious problems that a reasonable application should not try to catch, while exceptions represent conditions that a program might want to catch and handle.

3. Checked and Unchecked Exceptions

Exceptions are categorized into two types: checked and unchecked. Checked exceptions are those that must be either caught or declared in the method signature using the throws keyword. Unchecked exceptions, which are subclasses of RuntimeException, do not require explicit handling.

4. Try-Catch Block

The try-catch block is used to handle exceptions. The code that might throw an exception is placed inside the try block, and the catch block contains the code to handle the exception if it occurs.

5. Finally Block

The finally block is used to execute code that must run regardless of whether an exception was thrown or caught. It is often used for cleanup operations such as closing files or releasing resources.

6. Throw and Throws Keywords

The throw keyword is used to manually throw an exception, while the throws keyword is used in a method signature to declare that the method might throw one or more exceptions.

Explanation and Examples

Try-Catch Block Example

Consider the following code snippet:

        try {
            int result = 10 / 0; // This will throw an ArithmeticException
        } catch (ArithmeticException e) {
            System.out.println("An error occurred: " + e.getMessage());
        }
    

In this example, the code inside the try block attempts to divide by zero, which throws an ArithmeticException. The catch block catches this exception and prints an error message.

Finally Block Example

Consider the following code snippet:

        FileInputStream file = null;
        try {
            file = new FileInputStream("file.txt");
            // Code to read from the file
        } catch (FileNotFoundException e) {
            System.out.println("File not found: " + e.getMessage());
        } finally {
            if (file != null) {
                try {
                    file.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        }
    

In this example, the finally block ensures that the file is closed regardless of whether an exception was thrown or caught.

Throw and Throws Example

Consider the following code snippet:

        public void validateAge(int age) throws IllegalArgumentException {
            if (age < 18) {
                throw new IllegalArgumentException("Age must be at least 18");
            }
        }
        
        public static void main(String[] args) {
            try {
                validateAge(15);
            } catch (IllegalArgumentException e) {
                System.out.println(e.getMessage());
            }
        }
    

In this example, the validateAge method throws an IllegalArgumentException if the age is less than 18. The main method catches this exception and prints the error message.

Analogies

Think of exception handling as a safety net in a circus act. Just as a safety net catches a performer if they fall, a try-catch block catches exceptions and prevents the program from crashing. The finally block is like the cleanup crew that ensures the stage is cleared regardless of whether the act was successful or not.

By mastering exception handling, you can create more resilient and user-friendly Java SE 11 applications.