Hola mundo desde CMD, sin IDE compilado con Javac y Java

Creación del HolaMundo.java

En la clase de hoy, vamos a realizar nuestro primer programa. El Hola Mundo, destaca por ser el programa con el que normalmente todos los que nos adentramos en el mundo de la programación o a un lenguaje de programación especifico comenzamos. Por lo que, también podemos decir que es el más simple y también el famoso.

Como excepción y para entender como el proceso de compilación de un programa, no usaremos un IDE. Así veremos además mejor las diferencias entre trabajar con un IDE a directamente en línea de comandos. A modo de introducción para quién no sepa lo que es un IDE (Integrated Development Environment o en castellano Entorno de Desarollo Integrado), podemos decir que dicho software nos hará la vida más fácil en la creación de nuestros programas en un lenguaje de programación. También hay que destacar que normalmente sí que usaremos un IDE para desarrollar nuestros programas.

Estamos empezando a programar y estamos en un túnel, hay cosas que por el momento tendréis que asumir que son así sin más.Poco a poco, más adelante empezaremos a salir de dicho túnel y empezaréis a “ver la luz” gracias a que explicaremos todos los conceptos con más detalle. Pero sí lo realizáramos ahora,  sería mucho más difícil de asimilar las clases.

Una cosa importante antes de nada, es que debemos tener el programa escrito igual que los ejemplos, sino puede que no se ejecute en Java. Una de las características de Java es que es Key Sensitive, por lo tanto y como ejemplo, no es lo mismo HolaMundo que holamundo.

Para esté primer programa, iremos al Notepad y crearemos el archivo como HolaMundo.java .Una vez guardado, escribiremos lo siguiente sin los números de alado. Está PROHIBIDO PEGARLO, una de las normas de cuando se aprender a programar.

Objetivo del programa Hola Mundo:

Mostrar un Hola Mundo en la pantalla de línea de comandos.

Observaciones parte 1:

Una condición para poder ejecutar el programa que vamos a crear es que, el archivo de Notepad, se deberá llamar igual que el nombre que asignemos en la public class (con las mismas mayúsculas y minúsculas). En esté caso, por lo tanto, el nombre que pondremos al guardar el archivo de Notepad será HolaMundo.java en dicho archivo, guardaremos el contenido de la Parte 1. Si os fijáis, la public class tiene el mismo nombre HolaMundo.

Lo guardaremos en el escritorio.

Todo programa en Java necesita una class dentro de la cual irá el método main (lo veremos en el siguiente paso). En este caso y al ser una clase publica, se ejecutará sin restricciones en cualquier otra clase o paquete o subclase (se verá más adelante). La clase se declara mediante la línea public class HolaMundo.

Parte 1:

1. public class HolaMundo {
2.
3. }

Resultado:

Observaciones parte 2:

Bien una vez hemos creado la public class, vamos a crear el método principal (main). El método main, es el punto de entrada a un programa en Java (lo veremos más adelante a detalle).

Parte 2:

 1. public class HolaMundo {
 2.    public static void main(String[] args) {
 3.
 4.    }
 5. }

Resultado:

Observaciones parte 3:

El tercer paso, finalmente, añadiremos un mensaje de sistema que imprima Hola Mundo. Esto se realizará, mediante a la clase estática System.out (salida de sistema) y el método println, que nos imprimirá el texto que añadamos dentro del paréntesis y de las comillas, acompañado de un salto de línea de hay vienen las dos letras finales ln (Line New).

Parte 3:

 1. public class HolaMundo {
 2.    public static void main(String[] args) {
 3.        System.out.println("Hola Mundo");
 4.    }
 5. }

Resultado:

Compilación del HolaMundo.java

Bien ahora ya tenemos nuestro primer programa hecho vamos a explicar el proceso de compilación de java.


Nosotros hemos creado el programa y lo hemos guardado con extensión .java con el nombre de HolaMundo.java ahora tenemos que compilarlo para que pase de alto , a bytecode cambiando su extensión a .class y transformandose a bajo nivel. Una vez compilado lo abriremos y lo ejecutaremos con la JVM (Java Virtual Machine) en el sistema operativo que deseemos (lo que se conoce como WORA: Write One Run Anywhere).

Bien todo ello es mucho más fácil si lo vemos con el ejemplo de HolaMundo.java . Hemos creado el programa, ahora debemos compilarlo como se ve en la imagen de abajo.

Un ejemplo para ya estar seguros de que entendemos el concepto de compilación es el siguiente:

Tenemos una barra de pan (el archivo .java) que aún no es comestible (ejecutable por la JVM) la queremos cocinar (o compilar si pensamos en el programa en Java) una vez compilada la barra ya está cocida (en el caso del programa cambia extensión de .java a .class) y ya la podemos comernosla (ejecutar desde el JVM para que el programa se ejecute).

Para compilarlo, vamos pulsamos Inicio>Sistema de Windows>Ejecutar o la combinación de teclas Windows+R y escribimos cmd y pulsamos aceptar o intro.

Una vez abierta la línea o interprete de comandos, tendremos que ir a la ruta donde está el programa en HolaMundo.

Si os da error es porque nono tenemos configuradas las variables de entorno (si las queréis configurar explicó como en el artículo ¿Que es el JRE? ¿y JDK? en el apartado Variables de Entorno).  Si no las queremos configurar podemos ir desde el CMD, cambiar la ruta al directorio bin (donde se encuentra el JAVAC.EXE) y hacer un javac con la ruta donde tenemos el HolaMundo.java. De tal manera, que estaríamos en mí caso por ejemplo situariamos el cmd con un cd a la ruta C:\Program Files\Java\jdk1.8.0_144\bin y nos quedaría ejecutar un JAVAC C:\Users\David\Desktop/HolaMundo.java

Ejecución del HolaMundo.class

Bien una vez compilado, si volvemos al escritorio, podemos comprobar que se nos ha creado una nueva copia del programa con extensión .class

Ya hemos hecho el proceso de compilación por lo tanto hemos pasado la compilación, como se ha efectuado dicha compilación correctamente nos ha creado el programa HolaMundo.class y solo nos falta el tercer y último paso ejecutarlo con la JVM.

O también desde el interprete de comandos, haciendo otra vez un dir vemos que tenemos el archivo HolaMundo.class . Por lo tanto, ahora solo debemos ejecutar este archivo con el comando java y SIN poner la extensión. Ejemplo java HolaMundo.

Posibles errores al compilar:

El javac nos indica con ^ que ha encontrado un error en la public class. En este caso, en particular, si nos fijamos en el nombre del public class y el del programa HolaMundo.java no son el mismo. La public class, debería de coincidir con el nombre del programa y para ello tenemos que substituir HolaMundo con la H inicial en mayúscula en la public class.

En este segundo caso, el System.out.println, no tiene la inicial en mayúscula, y por lo tanto, la ^ se para este punto. Si substituimos la s minúscula por una en mayúscula System.out.prinln el error se arreglará.

Por lo tanto, hay que destacar la característica de que Java es un lenguaje de programación Key Sensitive y cualquier mayúscula mal puesta o a la inversa nos puede provocar problemas a la hora de compilar un programa.

Os dejamos las soluciones aquí, por si queréis revisarlas, ante cualquier problema:
HolaMundo

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