Java

Path de los códigos:

Variable definida con:
{{#vardefine:codespath|/home/adelo/1-disco_local/.archivos_programas/eclipse-workspace/}}

Para usar la variable:
{{#var:codespath}}

1 Instalación de Java
2 Eclipse
- 2.1 Installing Eclipse
- 2.2 Combinaciones de teclas
3 NetBeans
4 Jar File
- 4.1 Creating a Jar
- 4.2 Exploring the Jar File
5 Connecting to a MySQL database using Java
6 Epoch time
7 Threads
8 Expanding our program across multiple files (Classes)
- 8.1 Passing Object References

Instalación de Java

Installing the Default OpenJDK

http://ubuntuhandbook.org/index.php/2018/11/how-to-install-oracle-java-11-in-ubuntu-18-04-18-10/

sudo apt update

sudo apt install default-jdk

Oracle Java Development Kit JDK

https://en.wikipedia.org/wiki/Java_Development_Kit

https://www.oracle.com/technetwork/java/javase/overview/index.html

Es apropiado instalar Oracle JDK. Para GUI Programming, por ejemplo, se necesitan librerías que se encuentran en Oracle JDK.

Hay diferentes Edition:

Java Platform, Standard Edition (Java SE) (ésta es la que generalmente necesitamos)
Java Platform, Enterprise Edition (Java EE)
Java Platform, Micro Edition (Java ME)
...

Installing Oracle JDK on Ubuntu

Installing Oracle JDK (Java EE) on Ubuntu 18.04: http://ubuntuhandbook.org/index.php/2018/11/how-to-install-oracle-java-11-in-ubuntu-18-04-18-10/

En una ocasion, al instalar java-10-oracle la instalación de NetBeans no funcionó correctamente. Esto creo que se debía a que Java-10-oracle era muy reciente para la fecha. Fue por tanto necesario instalar java-8-oracle y configurar la ruta del JDK editando el archivo /usr/local/netbeans-8.2/etc/netbeans.conf

Installing Oracle JDK on Windows

Installing Oracle JDK (Java EE) on Windows:

Descargar Java SE de la página oficial de Oracle:

https://www.oracle.com/technetwork/java/index.html

https://www.oracle.com/technetwork/java/javase/downloads/index.html

Instalación común sin ninguna remarca.
Los archivos se guardan generalmente by default en: C:\Program Files\Java
- Cuando instalé NetBeans, éste detectó automáticamente la ruta del JDK - JDK for the NetBeans IDE:

C:\Program Files\Java\jdk1.8.0_181

Check your installed java version

java  -version
javac -version

The fallowing command should tell you what is currently providing the Java virtual machine (java) and the Java compiler (javac): https://askubuntu.com/questions/150057/how-can-i-tell-what-version-of-java-i-have-installed

file /etc/alternatives/java /etc/alternatives/javac

Eclipse

Installing Eclipse

Para estar seguro de que estamos instalando la última versión, es apropiado descargar el paquete desde la página oficial, y no instalar la versión que se encuentra en los repositorios de Ubuntu a través de apt-get (pues estar versiones generalmente están desactualizadas).

Página oficial de Eclipse: http://www.eclipse.org/downloads/

En la siguiente página se explica como instalar eclipse en Ubuntu 16.04: http://ubuntuhandbook.org/index.php/2016/01/how-to-install-the-latest-eclipse-in-ubuntu-16-04-15-10/

Para instalarlo hay que simplemente ejecutar el instalador y seguir las instrucciones (como si se tratara de una instalación en Windows).

Luego, para agregar el ejecutable a la lista de comandos, he agregado la siguiente línea en el .bashrc:

PATH=$PATH:/home/adelo/1-system/.1-conf-system/1-archivos_programas-ubuntu/eclipse/java-photon/eclipse:

Combinaciones de teclas

Para que coloque los import (luego de haber ingresado un comando que necesite una librería en particular): Ctrl Shif o
Run: Ctrl 11
Comentar: Ctrl 7
Rastrea la variable seleccionada: CTRL k

NetBeans

https://netbeans.org/

https://en.wikipedia.org/wiki/NetBeans

Installing NetBeans on Ubuntu 18.04

https://www.itzgeek.com/how-tos/linux/ubuntu-how-tos/how-to-install-netbeans-ide-on-ubuntu-18-04-ubuntu-17-10.html

Se puede descargar NetBeans manualmente de la página oficial: https://netbeans.org/downloads/

o a través de wget:

wget http://download.netbeans.org/netbeans/8.2/final/bundles/netbeans-8.2-linux.sh

Luego:

sudo bash netbeans-8.2-linux.sh

Durante la instalación es importante seleccionar el directorio en donde se encuentra Java, en mi caso:

/usr/lib/jvm/java-8-oracle
/usr/lib/jvm/java-10-oracle

Con java-10-oracle la instalación no funcionó correctamente. Fue por tanto necesario instalar java-8-oracle y configurar la ruta del JDK editando el archivo /usr/local/netbeans-8.2/etc/netbeans.conf

Uninstall NetBeans

https://askubuntu.com/questions/76908/how-to-uninstall-netbeans

Sólo hay que ejecutar el script uninstall.sh in /usr/local/netbeans-x.x

bash uninstall.sh

Installing NetBeans on Windows 10

Descargar NetBeans de la página oficial: https://netbeans.org/downloads/
Cuando instalé NetBeans, éste detectó automáticamente la ruta del JDK - JDK for the NetBeans IDE:

C:\Program Files\Java\jdk1.8.0_181

Debido a que instalé la versión que tiene todos los componentes, se instaló también el GlassFish Server Open Source Edition. No sé para que es sirve y si sea apropiado instalarlo.

The Darcula plugin

In NetBeans IDE 8.2 you can now go to:

Tools > Plugins > Available Plugins > and install the Darcula LAF for NetBeans plugin directly from there.
En las versiones 9 y 10 el Plugin no se encuentra disponible por defecto. Es entonces necesario adicionarlo a la lista de "Available Plugins" de la siguiente forma:

https://stackoverflow.com/questions/52688439/changing-theme-in-apache-netbeans-9-0

Jar File

A Jar File (acronym for Java Archive) is format used for archiving java files (merging several java files into one)

The Jar File, provides a method to package our software to give it to the users.

The Jar File can be Runnable. That is, when we double click it, our program will run.

A typical extension to files that we have always seen when working in a Windows environment is the .exe file extension. These files are a single binary executable file that when double clicked will open the program for the user.

In the Java world however things are a little different. There is no native .exe file in Java and we must use the .jar file extension instead. A jar file works exactly the same as a .exe file and when double clicked will run the program.

Jar files come in two different varieties:

Simple archive folder
Runnable archive folder

A jar file is an acronym for Java Archive. Simple archive folder is a filetype similar to that of the .zip extension. If we want, we can open up a jar file using a program such as WinZip or WinRar and check the contents inside of the jar.

The second variant of this is the runnable archive folder. This type of file is exactly the same as the archive folder, just with one additional file. This extra file is used to define what happens when the user double clicks on the icon. In our case, we will point to the main method in our class file that we want to run. This will kick start the program to run when it is double clicked.

Creating a Jar

Open up your Eclipse and navigate to the project you are currently working on. Note this will only work correctly with an application that has a GUI for the user to see. In this case the project we are looking at is called Sample Program 1.

Right click on the project name, and a menu will open.

In the menu that opens, click on the Export button.

In the window that opens you will see that two different types of JAR file are available:
- If you wanted to simply package your project to give to someone, you could use the JAR file.
- In our case, we are interested in the Runnable JAR file, so we will click on this option and then click Next.

You will then be passed to a second window where you will be able to specify the details for the runnable Jar file you are about to create:
- The first of these is the launch configuration. This is the most important step to ensure that your Jar file will open the project when it is double clicked.
  - Select the name of the class you want to run, inside of the project that you have been working on. In this case, you can see that the Main class is selected which is inside of the project titled SampleProgram1 that we are working in.
- The second step is then to select a location where we would like to export the Jar file to.
- After you have finished, click Finish at the bottom of the page.

A small warning message may pop up, but this is ok. Click Ok to ignore this message.

Exploring the Jar File

Now that we have created the Jar file. If we double click on the file our program will run. However if we want, we can take a look inside of the Jar file. This is easily done by right clicking on the Jar file that we have created and clicking “Open with WinRAR” or whichever program you currently have installed on your system for viewing Zip files.

After WinRAR has opened you will be able to see the contents of the Jar file. In this case you can see the class file from the project that we have been developing. In addition to this, you will see an additional folder titled META-INF. This is the folder where all of the configuration for the Jar file are held. We will go into this folder and take a look around.

Inside of this folder, you will see a file titled MANIFEST.MF. This is the main configuration file for the Jar file we have created. If we open up this file, we can see the details of what is called when we double click on the Jar file to view it.

There is very little content that is needed to make the Jar file run. Inside of this file, you can see that the Main-Class attribute is set to the name of the class file that we want to run when the Jar is double clicked. In the case of this project, the class file that we want to run is called Main.class, so the title Main is used as the value for this attribute. If your class was called MyProgram.class, this would be set to MyProgram.

Connecting to a MySQL database using Java

La conección a la base de datos MySQL la haremos a través de Connector/J. Esta librería no se encuentra por defecto en las librerías de Java y debemos por lo tanto agregarla a nuestro proyecto Java.

Entonces, para realizar la conección debemos:

Creación de la base de datos MySQL

Luego de haber instalado y configurado MySQL (Instalación de MySQL) debemos crear la base de datos a la cual queremos conectarnos con Java. Para más detalles ver Database#MySQL

En este caso vamos a crear la base de datos "graham" y la tabla "samplelogin".

CREATE DATABASE graham;

USE graham;

CREATE TABLE samplelogin(
   id	 	INT(10)		NOT NULL AUTO_INCREMENT,
   username 	VARCHAR(15)	NOT NULL,
   password 	VARCHAR(15)	NOT NULL,
   PRIMARY KEY(id)
);

INSERT INTO samplelogin VALUES('001','Adelo Vieira','a2930');
INSERT INTO samplelogin VALUES('002','Rachel Smith','r1039');

Creating an Eclipse Java Project to use Connector/J

Before we make a connection to the database we first must create a new Eclipse project.

Creación de un proyecto Java en Eclipse:

File > New > Java Project: enter Project name
- Esto creará una carpeta con el nombre del proyecto y dentro de ésta la carpeta srs
- ...

Agregar la librería externa Connector/J a nuestro proyecto

Connector/J

MySQL Connector/J (Desarrollado por MySQL) is the official JDBC driver for MySQL (MySQL is now owned by Oracle Corporation).

We will need this additional library to allow us to make a connection from Java to the MySQL database. Java does not come with this library by default so we need to include this ourselves.

Para descargar Connector/J: https://dev.mysql.com/downloads/connector/j/

You will then be brought to a page where you will be asked if you would like to signup or login. This step is not needed, at the end of the page you will see a link that says "No thanks, just start my download". Click this link to skip ahead and begin the download.

Inside this folder you will see lots of different files, but we only need one file from this folder. The file we are interested in is the mysql-connector-java-5.1.44-bin.jar file. This is a single jar file that contains all the compiled source code we need to connect to a database.

Making a reference to Connector/J

Now that we have a new project, we can begin the process of adding the new library into the eclipse project. Right click on the project name and click "Properties". This will open a window showing you all of the project specific settings for the application you are developing.
Inside the properties dialog that opens, click on the Java Build Path option.
Once you have opened the Java Build Path option, next click the Libraries tab. This will show you a list of additional libraries you have already referenced. But because this is a new project, you will not see any libraries yet!
On the right hand side of this window click "Add External Jar" and a dialog window will open. Use this window to locate the mysql-connector-java-5.1.44-bin.jar file that you extracted earlier.
After you have found the jar file, you will see a new entry will be made under the libraries tab. This is telling you that when you go to compile the software you are building, it will also look for the .jar file you have just pointed to.
Click Ok to finish this step.
When you have finished making a reference to the library you will see that inside of your project under the Referenced Libraries menu, there will be a reference to the JAR file that you just pointed to in the previous step. You are now ready to start adding some code into your project to connect to the database.

Creación de la Clase Java en donde se hará la conexión

Luego de haber creado el proyecto Java, debemos crear una clase de la siguiente forma:

src folder (clic d) > New > Class: enter name and Click OK!

One thing to keep in mind when working with this code is that it is tempting to allow Eclipse to automatically settle of the imports for you. This can cause issues however, because some libraries in Java have the same name as the ones inside the Connector/J library. To prevent any problems, manually add the imports yourself to the top of your program:

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.Statement;
import java.sql.ResultSet;
import java.sql.SQLException;

All of the code that we are working on is inside of a try catch statement as we do not know for sure what may happen to the database connection. If the database connection suddenly dropped it would crash our program. To prevent this, we can use a try catch statement and catch the error if anything goes wrong and allow our program to fail gracefully rather than come to a complete crash from an error.

La conexión con la base de datos la haremos a través de DriverManager.getConnection, método que se encuentra en la Librería Connector/J.

MyDBConnection.java

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.Statement;
import java.sql.ResultSet;
import java.sql.SQLException;

public class MyDBConnection{
	
	public MyDBConnection() throws SQLException{
		
		// La conexi'on con la base de datos se hace a trav'es de esta orden:
           	Connection conn = DriverManager.getConnection("jdbc:mysql://localhost:3306/graham?"
    					             	       +"user=root&password=a1640774200");
		
		// Luego de establecer la conexi'on podemos realizar un query sobre la base de datos:
        	Statement stmt = conn.createStatement();
        	ResultSet rs = stmt.executeQuery("select * from samplelogin;");
			
	}

}

En el ejemplo anterior hemos realizado la conexión con la base de datos SQL «graham» y hemos almacenado el resultado de la query en la variable rs. Sin embago, no hemos desplegado el resultado de la query ni hemos hecho nada útil con ella. A continuación se muestran algunas clases que conectan con la base de datos y muestran ejemplos más elaborados.

Creación de la clase ShowingData.java

ShowingData.java

import java.sql.DriverManager;
import java.sql.Connection;
import java.sql.Statement;
import java.sql.ResultSet;
import java.sql.SQLException;

import javax.swing.JFrame;
import javax.swing.JTable;
import javax.swing.JScrollPane;
import javax.swing.JTextField;


public class ShowingData extends JFrame  {
	
    public ShowingData(){
    
    	setSize(500,500);
    	setVisible(true);
    	
    	Connection conn = null;
    	Statement stmt = null;
    	ResultSet rs = null;
    	
    	try {
    	    // La conexi'on con la base de datos se hace a trav'es de esta orden:
    	    conn = DriverManager.getConnection("jdbc:mysql://localhost:3306/graham?"
    	    				        +"user=root&password=a1640774200");

    	    // Luego de establecer la conexi'on podemos realizar un query sobre la base de datos:
    	    stmt = conn.createStatement();
    	    rs = stmt.executeQuery("select * from samplelogin;");

    	    // El reultado del query se encuentra en el objeto "rs" creado arriba.
    	    // Ahora queremos desplegar la data almacenada en "rs".
    	    // En este ejemplo vamos a desplegar la data en un JTable.
    	    
    	    // Lo primero que hay que hacer es crear una variable de tipo String (una matriz en este caso)
    	    // El n'umero de columnas debe, por supuesto, ser igual al n'umero de columnas de la tabla
    	    // almacenada en "rs". El n'umero de filas debe ser igual o mayor al n'umero de filas de la tabla.
     	    String[][] data = new String[100][3];
     	    
     	    // Ahora vamos a ingresar la data que se encuentra en el objeto "rs" a la matriz "data" creada arriba.
     	    // Para esto vamos a realizar un ciclo que va a recorrer el objeto "rs" e ingresar los valores a "data"
    	    int counter = 0;
    	    while(rs.next()){
    	    	String id = rs.getString("id");      
    	       
    	    	data[counter][0] = id;
    	      
    	        String un = rs.getString("username");
    	    	data[counter][1] = un;    	      
    	        
    	        String pw = rs.getString("password");
    	    	data[counter][2] = pw;
    	        
    	        counter = counter+ 1;
    	    }
    	    
    	    // Ahora que ya tenemos la data almacenada en la matriz "data", podemos desplegarla en una JTable.
    	    // S'ólo debemos antes crear un vector String que contenga el nombre de las columnas:
    	    String[] colNames = {"id" , "username" , "password"};
    	    
    	    // Ahora s'i podemos desplegar la data correctamente en el JTable:
    	    JTable table = new JTable(data , colNames);
    	    
    	    // Tenemos tambi'en que agregamos un JScrollPane a nuestra tabla para poder desplazarnos en ella:
    	    JScrollPane sr = new JScrollPane(table);
    	    
    	    // Por 'ultimo, agregamos nuestra Jtable al JFrame:
    	    this.add(sr);
    	        
    	} 
    	
    	// Handle any errors:
    	catch (SQLException ex) {
    	    System.out.println("SQLException: " + ex.getMessage());
    	    System.out.println("SQLState: " + ex.getSQLState());
    	    System.out.println("VendorError: " + ex.getErrorCode());
    	}
    	
    	
    }
    
	public static void main(String[] args) {
		// TODO Auto-generated method stub
			new ShowingData();
	}

}

Creación de la clase Register.java

En este caso vamos a crear la clase Register.java, la cual permite ingresar datos a una base de datos MySQL utilizando la librería Connector/J.

Register.java

import java.awt.GridLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;

import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JOptionPane;
import javax.swing.JTextField;


public class Register extends JFrame implements ActionListener{
	
	// make the fields global so we can see them!!
	JTextField username = null;
	JTextField password = null;
	
	public Register(){		

		setSize(400,400);
		setVisible(true);
		this.setLayout(new GridLayout(3,1));

		JLabel un = new JLabel("Username:");
		this.add(un);
		username = new JTextField(20);
		this.add(username);
		
		JLabel pw = new JLabel("Password:");
		this.add(pw);
		password = new JTextField(20);
		this.add(password);
		
		JButton register = new JButton("Register");
		
		register.addActionListener(this);
		
		this.add(register);
		
		
	}
	
	public static void main(String[] args) {
		// TODO Auto-generated method stub
		new Register();
	}
	
	public void registerNewUser(){
		try {
			Class.forName("com.mysql.jdbc.Driver").newInstance();
		}
		
		catch(Exception e ){}
		
		Connection conn = null;
		Statement stmt = null;
		ResultSet rs = null;
		
		try {
			// En esta linea debemos ajustar: 
			// El IP (127.0.0.1 si es localhost) y el Port (generalmente el 3306) en donde corre MySQL.
			// El nombre de la base de datos a la cual queremos conectarnos: graham en nuestro ej.
			// El usuario y la clave de MySQL.
			conn = DriverManager.getConnection("jdbc:mysql://127.0.0.1:3306/graham?user=root&password=a1640774200");
	    	
			// Do something with the Connection
			stmt = conn.createStatement();
			
			// or alternatively, if you don't know ahead of time that
			// the query will be a SELECT...
			String un = username.getText();
			String pw = password.getText();
			
			// En esta linea debemos ajustar el nombre de la base de datos y la tabla
			// en donde queremos agragar datos. En nuestro ejemplo: graham, samplelogin.
			if (stmt.execute("INSERT INTO `graham`.`samplelogin` (`username`, `password`) VALUES ('"+un+"', '"+pw+"');")) {
			}
		}
		
		// loop over results
		catch (SQLException ex) {
			// handle any errors
			System.out.println("SQLException: " + ex.getMessage());
			System.out.println("SQLState: " + ex.getSQLState());
			System.out.println("VendorError: " + ex.getErrorCode());
		}
	}
	
	@Override
	public void actionPerformed(ActionEvent e) {
		// TODO Auto-generated method stub
		registerNewUser();
	}
}

Epoch time

Threads

La siguiente es la referencia colocada por el Prof. para este tema: https://docs.oracle.com/javase/tutorial/essential/concurrency/

Threads are used to allow our program to do more than one thing at any one time.

We picture a process which has two tasks that aren't necessarily dependent on each other. They can occur at the same time, each handled by a "thread"

So, a process (think program) can contain a number of different threads. As the program is running, the processor switches between each of the threads allowing each of them to have some CPU time to run.

For dual (or more) core machines, each thread can be executed on a separate core, so no switching is neceessary. The threads truly run concurrently.

In the previous picture you can also see that Thread number 2 has died off and thread 1 continued on. This is ok.

Both processes and threads provide an execution environment, but creating a new thread requires fewer resources than creating a new process.

Threads exist within a process — every process has at least one. Threads share the process's resources, including memory and open files.

Every application has at least one thread — or several, if you count "system" threads that do things like memory management and signal handling. From the programmer’s point of view, you start with just one thread, called the main thread. This thread has the ability to create additional threads

The difficulty with threads

Threads can be difficult to work with because their behavior can be unpredictable.
The order in which they are running may change unless synchronized.

Some methods available in the Thread class

// Changes the name of this thread to be equal to the argument name.
setName(String name) 

// Changes the priority of this thread. Valid values are 1 through 10.
setPriority(int newPriority)

// Causes the currently executing thread to sleep (temporarily cease execution) for the specified number of milliseconds
sleep(long millis)

// Causes this thread to begin execution; the Java Virtual Machine calls the run method of this thread.
start()

// Causes the current thread to block (pause) until the specified thread terminates.
Thread.join()

// Causes the current thread to block until the specified thread terminates or the specified number of milliseconds passes
Thread.join(long millis)

Creating Threads

We'll see that (just like everything in Java) a thread is just an object. We can create them, use them, etc...

When we want to create threads, two options are available to us:

Extend the thread class, or
We can implement the Runnable interface

Extend the thread class:

By doing like this we have access to all the methods in the thread class.

We need to extend the thread class and add a run method (Instead of a main method, threads run the run() method):

MyThread.java:

public class MyThread extends Thread {

	public void run(){
		// This is the code that is executed when the thread is started
		// Este es s'olo un c'odigo ejemplo. Aqu'i podr'ia haber cualquier c'odigo
		for (int i=0 ; i < 100000000 ; i++){
			System.out.println(this.getName() + ": " + i);
			
		}
		
	}
	
}

Then, when we want to use our thread (in our standard class: MyProgram.java in our example), we create an instance of the thread, then start it.

MyThread t = new MyThread();
t.start();

Implement the Runnable interface:

To do this we just start by making a regular class, but implement the runnable interface.

After this, we need to add a run() method to the class (Instead of a main method, threads run the run() method):

MyRunnable.java

public class MyRunnable implements Runnable {
	
	public void run(){
		
		for (int i=0 ; i < 100000000 ; i++){
			System.out.println("My Tread implementing the Runnable interface: " + i);
			
		}
		
	}
	
}

Then, in our standard class (MyProgram.java in our example), when we want to use our thread, we just need to create an instance of the thread in case we extended the thread class (as we said above), then start it. However, if we implemented the Runnable interface:

We need to create an instance of the Runnable Tread we created:

MyRunnable r = new MyRunnable();

After this, we need to create a Thread object and pass the Runnable Tread object we created to it, and then start it:

Thread tr = new Thread(r);
tr.start();

Whatever code is inside the run method will be executed when the thread is started.

For instance, if we would like the thread to sleep for a certain amount of time, we can call:

try{ 
     myThread.sleep(10000);
}catch (InterruptedException e){
     e.printStackTrace();
}

A continuación («MyProgram.java») es la «standad class» en donde vamos a utilizar las «Threads» («MyThread.java» and «MyRunnable.java») creadas arriba:

MyProgram.java:

import javax.swing.JFrame;

public class MyProgram extends JFrame{
	
	public MyProgram(){

		setSize(500,500);
		setVisible(true);
		
//		Object created extending the thread class:
		MyThread t = new MyThread();
		t.setName("t1");
		t.setPriority(10);
		t.start();

//		Object created implementing the runnable interface:		
		MyRunnable r = new MyRunnable();
		Thread tr = new Thread(r);
		tr.start();
		
	}
	public static void main(String[] args) {
		new MyProgram();
	}

}

En el ejemplo anterior hemos creado un JFrame que cuando se abre se ejecuta también el Thread.

Cuando se ejecuta MyProgram.java se debe notar que la ejecución no sigue el orden del código. Es decir, en nuestro ejemplo nuestras Threads sólo imprimen un valores en pantalla. Note que no necesariamente se debe terminar la impresión de todos los valores de la primera Thread en el código antes de comenzar la segunda. Esta característica impredecible de las Threads fue mencionada en The difficulty with threads

A continuación se muestra un ejemplo un poquito más elaborado en el cual la Thread se ejecuta una vez que se hace clic en un botón desplegado en el JFrame. Para esto se implementa el ActionListener

GUIThread.java:

import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

import javax.swing.JButton;
import javax.swing.JFrame;

public class GUIThread extends JFrame implements ActionListener{

	public GUIThread(){
		
		setSize(300,300);
		setVisible(true);
		
		JButton btn = new JButton("Button");
		btn.addActionListener(this);
		this.add(btn);
		
	}
	
	public static void main(String[] args) {
	    // TODO Auto-generated method stub
            new GUIThread();
	}
	
	@Override
	public void actionPerformed(ActionEvent arg0) {
		// TODO Auto-generated method stub
		System.out.println("Button");
		
		MyThread x = new MyThread();
		x.start();
			
	}

}

Daemon thread

Daemon thread in Java are those thread which runs in background and mostly created by JVM* for performing background task like Garbage collection and other house keeping tasks. Difference between Daemon and Non Daemon(User Threads) is also an interesting multi-threading interview question, which asked mostly on fresher level java interviews.

(*) A Java virtual machine (JVM) is an abstract computing machine that enables a computer to run a Java program.

In one line main difference between daemon thread and user thread is that as soon as all user thread finish execution java program or JVM terminates itself, JVM doesn't wait for daemon thread to finish the execution. As soon as last non daemon thread finished JVM terminates no matter how many Daemon thread exist or running inside JVM.

Another difference between daemon thread and user thread is that Daemon Thread are treated differently than User Thread when JVM terminates, finally blocks are not called, Stacks are not unwounded and JVM just exits.

Thread inherits its daemon nature from the Thread which creates it (i.e. parent Thread). If the main thread is a non daemon thread, any other thread created from it will remain non-daemon until explicitly made daemon by calling setDaemon(true).

Thread.setDaemon(true) makes a Thread daemon but it can only be called before starting Thread in Java. It will throw IllegalThreadStateException if corresponding Thread is already started and running.

Daemon Threads are suitable for doing background jobs like housekeeping, Though I have yet to use it for any practical purpose in application code. Let us know if you have used daemon thread in your java application for any practical purpose.

Ejemplos clásicos para comprender el concepto de Thread

Los siguientes problemas fueron abordados por el Prof. para explicar el concepto the Thread.

Dining philosophers problem

https://en.wikipedia.org/wiki/Dining_philosophers_problem

Atomicity (database systems)

https://en.wikipedia.org/wiki/Atomicity_(database_systems)

Expanding our program across multiple files (Classes)

Although we have developing all of our code inside a single java file, it is common practice to break the application down into smaller individual classes.

We will only ever have one main method. In this case, our Login.java is the file that will have the main method. No other class should have a main method!

Para integrar distintas «class» lo único que tenemos que hacer es tener los archivos en el mismo directorio y crear una instancia de la «class». En el siguiente ejemplo hemos creado una instancia de la «class» ShosingData en Login.java. En este caso, si se cumple el IF, se ejecutará «ShowingData.java».

Fracmento de /home/adelo/1-disco_local/.archivos_programas/eclipse-workspace/DBtest/src/Login.java

if(un.equals("Nathalie")){
        ShowingData d = new ShowingData();
}

Passing Object References

Ahora que nuestro programa está formado por diferentes archivos (diferentes class), veremos que en ocasiones, cuando creamos una «instance» de una class (Ej: «BasicShowData.java») dentro de, por ejemplo, «BasicLogin.java», podría ser necesario que «BasicShowData.java» colecte información de «BasicLogin.java». To do this, we will pass a reference across from the BasicLogin.java page to the «BasicShowData.java» page that will allow us to call back to the «BasicLogin.java» and access any variables or methods that it may have.

Note que inside «», when we click the "Login" button, the actionListener method is then called and the BasicShowData class is just being created. Instead of just creating the «BasicShowData()» class, we are going to pass a reference to «this» current class that we are in (BasicLogin.java class). En otras palabras, cuando en «BasicLogin.java» creamos la instance de «BasicShowData()» vamos a pasar una referencia de la clase donde estamos («BasicLogin.java») a través de «this»:

In BasicLogin.java:

BasicShowData d = new BasicShowData(this);

Now, in the «BasicShowData class», we must change the default constructor to catch «this» reference:

public BasicShowData(BasicLogin ref) {

De esta forma, la instancia de «BasicShowData class» puede acceder to all the variables and fields of the current class («BasicLogin.java»).

/home/adelo/1-disco_local/.archivos_programas/eclipse-workspace/PassingObjectReferences/src/BasicLogin.java

import java.awt.GridLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JTextField;

public class BasicLogin extends JFrame implements ActionListener{
	JTextField username = null;
	JTextField password = null;
	
	public BasicLogin(){
		
		setSize(300,300);
		setVisible(true);
		
		this.setLayout(new GridLayout(3,2));
		
		JLabel un = new JLabel("Username");
		this.add(un);
		
		username = new JTextField(20);
		this.add(username);
		
		JLabel pw = new JLabel("Password");
		this.add(pw);
		password = new JTextField(20);
		this.add(password);
		
		JButton button = new JButton("Login!");
		button.addActionListener(this);
		this.add(button);
		
		validate();
		repaint();
	}
	public static void main(String[] args){
		
		new BasicLogin();

	}
	@Override
	public void actionPerformed(ActionEvent arg0){
		// TODO Auto-generated method stub
		BasicShowData d = new BasicShowData(this);
	}

}

/home/adelo/1-disco_local/.archivos_programas/eclipse-workspace/PassingObjectReferences/src/BasicShowData.java

import javax.swing.JFrame;
import javax.swing.JLabel;

public class BasicShowData extends JFrame {

	public BasicShowData(BasicLogin ref) {
		
		String username = ref.username.getText();
		
		setSize(300,300);
		
		setVisible(true);
		
		JLabel tx = new JLabel("They logged in as: " + username);
		this.add(tx);
	}
}