Reminder: there are two ways of creating threads:
- Create a class that extends the Thread class
- Create a class that implements the Runnable interface and then create a Thread object by passing a Runnable object in the constructor.

We will use the Passport Renew scenario to demostrate both cases.

To better organize multiple threads it is possible to collect them into a group. The ThreadGroup class does exactly this. We will elaborate more on the ThreadGroup later.

In the example below we just provide a constructor that allows a thread to join a thread group.
And as soon as one constructor is provided by a developer, default constructor will not be created automatically by Java. If you plan using a default constructor, you need to provide it in the class as it is illustrated in this example.


package week6.threads;



//In this example we create MyThread by extending the Thread class

public class MyThread extends Thread {

	// provide the constructor for joining the thread group

	public MyThread(ThreadGroup group, String threadName) {

		super(group, threadName);

	}

	// providing default constructor; we must do this because 

	// after one constructor was created, default constructor is not created automatically

	public MyThread() {

		super();

	}

	// Thread must have the run() method,

	// which is the implementation of the Runnable interface

	public void run() {

		boolean passportArrived = false;

		System.out.println("Renew the passport!");

		for (; !passportArrived;) {

			// periodically check the mailbox event

			System.out.println("Check your mailbox");

			// if arrived set passportArrived=true

			// if not wait for some time before checking again

			try {

				Thread.sleep(2000); // this thread will sleep (do nothing) for 2 seconds

			} catch (Exception e) {

				// never happen

			}



		}

	}

	/**

	 * Test the class MyThread

	 * The thread will display the message in the Console every 2 seconds

	 * To stop the program - click on the red box in the Eclipse

	 * @param args

	 */

	public static void main(String args[]) {

		// create a thread group

		ThreadGroup group = new ThreadGroup("parent");

		MyThread myThread = new MyThread(group, "threadSample");

		myThread.start(); // call the run() method

		// do something else!

	}



}



// The following two classes create threads by implementing the Runnable interface



package its.day14.threads;



import java.util.Scanner;



/**

 * This thread periodically calls the checkMailbox() method till a user answer is yes

 */ 

public class MyRunnableMailbox implements Runnable {

	private boolean passportArrived = false;

	private Scanner sc = new Scanner(System.in);



	public void run() {

		System.out.println("Renew your passport!");

		for (; !passportArrived;) {

			// periodically check the mailbox event

			// if arrived (the answer is yes) set passportArrived=true

			checkPassportArrived();

			// if not wait for some time before checking again

			try {

				Thread.sleep(9000); // this thread will sleep (do nothing) for 9 seconds

			} catch (Exception e) {

				// never happen

			}

		}



	}

       /**

        * Use Scanner to get an answer from a user

        */

	public void checkPassportArrived() {

		System.out.println("Did your passport arrive?");

		String answer = sc.nextLine();

		if (answer.toLowerCase().startsWith("y")) {

			passportArrived = true; // this will stop running

		}

	}

	/**

	 * Test the thread

	 * @param args

	 */

	public static void main(String args[]) {

		// create a runnable object first

		MyRunnableMailbox myRunnableMailbox = new MyRunnableMailbox();

		// then create a thread based on this runnable object

		Thread mailboxThread = new Thread(myRunnableMailbox);

		mailboxThread.start(); // call the run() method

		// do something else!

	}

}



package its.day14.threads;



import java.util.Scanner;



//This example illustrates the method of creating threads with the Runnable interface

/**

 * This thread periodically calls the calculate() method till the number becomes 40

 */ 

public class MyRunnableCalculator implements Runnable {

	private int number = 0;

	private int maxNumber = 40;

	public void run() {

		System.out.println("Renew the passport!");

		for (; number < maxNumber;) {

			// periodically check the mailbox event

			// if arrived set passportArrived=true

			calculate();

			// if not wait for some time before checking again

			try {

				Thread.sleep(3000); // this thread will sleep (do nothing) for 3 seconds

			} catch (Exception e) {

				// never happen

			}

		}



	}



	public void calculate() {

		number = number + 4;

		System.out.println("The number is "+number);

		System.out.println("Do not forget to check your mailbox!");

	}

	/**

	 * Test by running two threads: mailbox and calculator

	 * @param args

	 */

	public static void main(String args[]) {

		// create a runnable object first

		MyRunnableCalculator myRunnableCalculator = new MyRunnableCalculator();

		// then create a thread based on this runnable object

		Thread calculatorThread = new Thread(myRunnableCalculator);

		calculatorThread.start(); // call the run() method

		// do something else: check mailbox and click on the Console to answer the question

		MyRunnableMailbox myRunnableMailbox = new MyRunnableMailbox();

		// then create a thread based on this runnable object

		Thread mailboxThread = new Thread(myRunnableMailbox);

		mailboxThread.start(); // call the run() method

	}

}

Was it clear so far?

Assignments:
1. Open Eclipse and create a new project: week6.threads.
2. Open this project and under the src create a new package its.day14.threads.
3. Right mouse click on the package name and create a new class MyThread.
4. Type into Eclipse the content of the class MyThread provided above. Please avoid copy/paste and still continue typing sources. This gives you comparable experience with the programmers who typed thousands of them from scratch.
Right mouse click on the source page and Run � As Java Application. The thread will display the message in the Console every 2 seconds. To stop the program - click on the red box in the Eclipse
5. Do similar things for MyRunnableMailbox and MyRunnableCalculator. Create a class by typing in Eclipse and Run � As Java Application. The test for MyRunnableCalculator will be running two threads: mailbox and calculator, this is a bit more fun, but the next one � SimpleGame - will be even better.

package week6.threads; //In this example we create MyThread by extending the Thread class public class MyThread extends Thread { // provide the constructor for joining the thread group public MyThread(ThreadGroup group, String threadName) { super(group, threadName); } // providing default constructor; we must do this because // after one constructor was created, default constructor is not created automatically public MyThread() { super(); } // Thread must have the run() method, // which is the implementation of the Runnable interface public void run() { boolean passportArrived = false; System.out.println("Renew the passport!"); for (; !passportArrived;) { // periodically check the mailbox event System.out.println("Check your mailbox"); // if arrived set passportArrived=true // if not wait for some time before checking again try { Thread.sleep(2000); // this thread will sleep (do nothing) for 2 seconds } catch (Exception e) { // never happen } } } /** * Test the class MyThread * The thread will display the message in the Console every 2 seconds * To stop the program - click on the red box in the Eclipse * @param args */ public static void main(String args[]) { // create a thread group ThreadGroup group = new ThreadGroup("parent"); MyThread myThread = new MyThread(group, "threadSample"); myThread.start(); // call the run() method // do something else! } } <hr> // The following two classes create threads by implementing the Runnable interface package its.day14.threads; import java.util.Scanner; /** * This thread periodically calls the checkMailbox() method till a user answer is yes */ public class MyRunnableMailbox implements Runnable { private boolean passportArrived = false; private Scanner sc = new Scanner(System.in); public void run() { System.out.println("Renew your passport!"); for (; !passportArrived;) { // periodically check the mailbox event // if arrived (the answer is yes) set passportArrived=true checkPassportArrived(); // if not wait for some time before checking again try { Thread.sleep(9000); // this thread will sleep (do nothing) for 9 seconds } catch (Exception e) { // never happen } } } /** * Use Scanner to get an answer from a user */ public void checkPassportArrived() { System.out.println("Did your passport arrive?"); String answer = sc.nextLine(); if (answer.toLowerCase().startsWith("y")) { passportArrived = true; // this will stop running } } /** * Test the thread * @param args */ public static void main(String args[]) { // create a runnable object first MyRunnableMailbox myRunnableMailbox = new MyRunnableMailbox(); // then create a thread based on this runnable object Thread mailboxThread = new Thread(myRunnableMailbox); mailboxThread.start(); // call the run() method // do something else! } } <hr> package its.day14.threads; import java.util.Scanner; //This example illustrates the method of creating threads with the Runnable interface /** * This thread periodically calls the calculate() method till the number becomes 40 */ public class MyRunnableCalculator implements Runnable { private int number = 0; private int maxNumber = 40; public void run() { System.out.println("Renew the passport!"); for (; number < maxNumber;) { // periodically check the mailbox event // if arrived set passportArrived=true calculate(); // if not wait for some time before checking again try { Thread.sleep(3000); // this thread will sleep (do nothing) for 3 seconds } catch (Exception e) { // never happen } } } public void calculate() { number = number + 4; System.out.println("The number is "+number); System.out.println("Do not forget to check your mailbox!"); } /** * Test by running two threads: mailbox and calculator * @param args */ public static void main(String args[]) { // create a runnable object first MyRunnableCalculator myRunnableCalculator = new MyRunnableCalculator(); // then create a thread based on this runnable object Thread calculatorThread = new Thread(myRunnableCalculator); calculatorThread.start(); // call the run() method // do something else: check mailbox and click on the Console to answer the question MyRunnableMailbox myRunnableMailbox = new MyRunnableMailbox(); // then create a thread based on this runnable object Thread mailboxThread = new Thread(myRunnableMailbox); mailboxThread.start(); // call the run() method } }