---
title: "Exception Handling and Threading"
description: Checked and unchecked exceptions, try-catch-finally, thread lifecycle, and synchronization.
---
## Exceptions

An exception is an event that disrupts the normal flow of a program.

### Types

- **Checked exceptions** — Verified at compile time (e.g., `IOException`, `SQLException`).
- **Unchecked exceptions** — Occur at runtime (e.g., `ArithmeticException`, `NullPointerException`).
- **Errors** — Serious problems not meant to be caught (e.g., `OutOfMemoryError`).

### Handling Exceptions

```java
try {
    // code that may throw an exception
} catch (ExceptionType e) {
    // handle the exception
} finally {
    // always executes, cleanup code
}
```

- `throw` — Explicitly throw an exception: `throw new Exception("message");`
- `throws` — Declare that a method may throw an exception: `void method() throws IOException { }`

## Threading

### Thread Lifecycle

A thread passes through these states: **New**, **Runnable**, **Running**, **Blocked/Waiting**, and **Terminated**.

### Creating Threads

1. **Extending `Thread`** — Override the `run()` method.
2. **Implementing `Runnable`** — Implement the `run()` method and pass to a `Thread` constructor.

### Synchronization

The `synchronized` keyword prevents multiple threads from accessing shared resources simultaneously.

### Inter-Thread Communication

- `wait()` — Causes the current thread to wait until notified.
- `notify()` — Wakes up a single waiting thread.
- `notifyAll()` — Wakes up all waiting threads.

## Example

```java
// ExceptionThreadDemo.java
// Demonstrates exception handling and threading in Java.

// Exception handling example
class ExceptionDemo {
	public static void main(String[] args) {
		try {
			int a = 5 / 0; // This will cause ArithmeticException
		} catch (ArithmeticException e) {
			// Handle exception
			System.out.println("Exception caught: " + e);
		} finally {
			// Finally block always executes
			System.out.println("Finally block executed");
		}
	}
}

// Thread example
class MyThread extends Thread {
	public void run() {
		// Print thread name
		System.out.println("Thread running: " + getName());
	}
}

public class ExceptionThreadDemo {
	public static void main(String[] args) {
		// Create and start thread
		MyThread t = new MyThread();
		t.start();
	}
}
```

> Full source: [`examples/12-exceptions-threads/ExceptionThreadDemo.java`](https://github.com/University-Of-Sri-Jayewardenepura/Java/blob/main/examples/12-exceptions-threads/ExceptionThreadDemo.java)