Java Exception Handling Introduction

An exception is what happens when something goes wrong while a program runs, like tripping on a rock while walking. Handling it means planning what to do instead of falling.

In this page:

What is an Exception?
Checked vs Unchecked Exceptions
Why Handle Exceptions?
Common Exception Types
The try-catch-finally Concept

Syntax

java

try {
    // code that may throw
} catch (ExceptionType e) {
    // handle
}

What is an Exception?

An exception is an object Java creates and throws when something disrupts a program's normal flow, like dividing by zero or accessing a null reference.

Instead of letting the program crash outright, Java lets you catch that object and decide how to respond.

Example: What is an Exception?

java

public class Main {
	public static void main(String[] args) {
		try {
			int result = 10 / 0; // throws ArithmeticException
		} catch (ArithmeticException e) {
			System.out.println("Caught: " + e.getMessage());
		}
	}
}

Checked vs Unchecked Exceptions

Checked exceptions (like IOException) must be declared or caught at compile time because the compiler forces you to acknowledge the failure is possible; unchecked exceptions (like NullPointerException) extend RuntimeException and can surface without any such warning.

Example: Checked vs Unchecked Exceptions

java

public class Main {
	public static void main(String[] args) {
		try {
			throw new RuntimeException("Unchecked"); // no need to declare it
		} catch (RuntimeException e) {
			System.out.println(e.getMessage());
		}
	}
}

Why Handle Exceptions?

Handling exceptions keeps one failure from silently corrupting program state or crashing the whole application. It also lets you give the user a meaningful message instead of a raw stack trace, or fall back to a safe default and keep running.

Example: Why Handle Exceptions?

java

// Define the class Main
public class Main {
	// Program entry point: the JVM starts running here
	public static void main(String[] args) {
		// Try code that might throw an exception
		try {
			// Declare arr and set it to new int[2]
			int[] arr = new int[2];
			// Print a line to the console
			System.out.println(arr[5]);
		// Handle ArrayIndexOutOfBoundsException if it was thrown
		} catch (ArrayIndexOutOfBoundsException e) {
			// Print a line to the console
			System.out.println("Handled gracefully instead of crashing");
		}
	}
}

Common Exception Types

Some of the exceptions you'll hit constantly include NullPointerException (calling a method on null), ArrayIndexOutOfBoundsException (invalid array index), and ArithmeticException (like dividing an int by zero).

Recognizing these by name speeds up debugging considerably.

Example: Common Exception Types

java

// Define the class Main
public class Main {
	// Program entry point: the JVM starts running here
	public static void main(String[] args) {
		// Try code that might throw an exception
		try {
			// Declare s and set it to null
			String s = null;
			s.length();
		// Handle NullPointerException if it was thrown
		} catch (NullPointerException e) {
			// Print a line to the console
			System.out.println("Caught a NullPointerException");
		}
	}
}

The try-catch-finally Concept

The try block holds code that might fail, catch defines how to respond if it does, and finally runs regardless of whether an exception occurred — typically used to release resources like open files or network connections.

Example: The try-catch-finally Concept

java

// Define the class Main
public class Main {
	// Program entry point: the JVM starts running here
	public static void main(String[] args) {
		// Try code that might throw an exception
		try {
			// Declare result and set it to 10 / 0
			int result = 10 / 0;
		// Handle ArithmeticException if it was thrown
		} catch (ArithmeticException e) {
			// Print a line to the console
			System.out.println("Error occurred");
		// Always runs, whether or not an exception occurred
		} finally {
			// Print a line to the console
			System.out.println("Runs regardless");
		}
	}
}

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Java Exception Handling Introduction

What is an Exception?

Checked vs Unchecked Exceptions

Why Handle Exceptions?

Common Exception Types

The try-catch-finally Concept

Chapter Quiz — Complete all 8 topics to unlock

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