Published Book on Amazon

	All of IOT Starting with the Latest Raspberry Pi from Beginner to Advanced – Volume 1
	All of IOT Starting with the Latest Raspberry Pi from Beginner to Advanced – Volume 2

출판된 한글판 도서

	최신 라즈베리파이(Raspberry Pi)로 시작하는 사물인터넷(IOT)의 모든 것 – 초보에서 고급까지 (상)
	최신 라즈베리파이(Raspberry Pi)로 시작하는 사물인터넷(IOT)의 모든 것 – 초보에서 고급까지 (하)

Original Book Contents

23.7.3  Writing and Running Program

23.7.3.1    Writing Program Source Code

A typical Java program usually has the following form. Class is defined in the first line of program, and "main" function corresponds to the body of program.

Here we will use a simple printing program. We will write and use the following program code. The class name was defined as "HelloWorld".

The created program is saved as a file. A source program written in Java language is stored in a file where file name is same as class name and extension is ".java". We will store it in the "~/program_test/ HelloWorld.java" file. Now, if you check the folder contents where the file is located, it will show the results as follows.

pi@raspberrypi ~ $ cd program_test

pi@raspberrypi ~/program_test $ ls –l

-rw-r--r-- 1 pi pi  152 May  9 13:21 HelloWorld.java

23.7.3.2    Compiling and Running Program

●   Compiling program and creating class file

To run a program that has been completely writed, you must compile the program and create a class file. To do this, use "javac" command.

[Command Format]

[Command Overview]               

■    This compiles a program and creates a class file.

■    User privilege           -- Normal user.

[Detail Description]

■    None

[Used Example]

Let's compile the program that is written. If there is an error in the program, error will be displayed as follows.

pi@raspberrypi ~/program_test $ javac HelloWorld.java

HelloWorld.java:5: error: unclosed string literal

        java.lang.System.out.println("Hello World.);

                                     ^

HelloWorld.java:5: error: ';' expected

        java.lang.System.out.println("Hello World.);

                                                    ^

HelloWorld.java:6: error: illegal start of expression

        java.lang.System.out.println("This is test program.");

            ^

HelloWorld.java:6: error: ';' expected

        java.lang.System.out.println("This is test program.");

                 ^

4 errors

If there is no error in the developed program code, no message is output and a class file is created. The following is confirming the result.

pi@raspberrypi ~/program_test $ ls –l

-rw-r--r-- 1 pi pi  448 May  9 13:09 HelloWorld.class

-rw-r--r-- 1 pi pi  152 May  9 13:21 HelloWorld.java

●   Executing class program

Use the following command to execute with class file.

[Command Format]

[Command Overview]               

■    This runs a program using class file.

■    User privilege           -- Normal user.

[Detail Description]

■    None

[Used Example]

The class file created above is "HelloWorld.class". Now let's run the program.

pi@raspberrypi ~/program_test $ java HelloWorld

Hello World.

This is test program. 

You can see that the program is normally operated and printing commands are normally processed.