Java 17 Recipes

1-3. Testing Java
 Problem
You want to compile Java from the command line.
 Solution
Use the little application shown in Listing 
1-1
. You can use a smart editor like Notepad++ 
or a different text editor in this example.
Figure 1-2. Testing Java version
ChApteR 1 GettInG StARted wIth JAvA 17

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6
To create the source code for your first Java program, you must do the following.
1. Declare a class named Hello.
2. Declare public static void main(String args[]) or String... args 
as the main method.
3. Enter the System.out.println(“Hello World”) command to display 
“Hello World” in the command prompt window.
Listing 1-1. Hello.java
public class Hello {
public static void main(String args[]){
System.out.println("Hello World");
}
}
Now, save the file as Hello.java in your working folder. Open the command window. 
After changing to your work directory, enter the following to compile the application.
javac Hello.java
It should return the prompt without saying anything. It also means that you have 
correctly updated the Path system variable. If you want to know more about what the 
javac compiler is doing, type -verbose between javac and the name of the file (javac
-verbose Hello.java). You see a file named Hello.class in your work directory.
Enter the following in the command prompt to run the application (see Figure 
1-3
).
java Hello
Figure 1-3. Testing a Java class
ChApteR 1 GettInG StARted wIth JAvA 17

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Note All the code described in this book is available for download from github 
.com/apress/java17-recipes. In this case, you can simply type the code; 
in others, you don’t need to retype it. It is important that you improve your 
programming skills using the eclipse environment, which is a famous free and 
open source Java Ide, containing a workspace for developing applications, that 
you can download at 
www.eclipse.org
. You can find the examples in folders 
with the same names as the corresponding chapters.
 How It Works
Java is an object-oriented programming language based on classes and objects with 
attributes and methods. A class is necessary for creating objects. To create a Java 
program (i.e., a .class file), you must create a .java file. A .java file is a readable text file, 
while a .class file is a binary file, which is a file containing Java bytecode that can be 
executed on the Java Virtual Machine (JVM), and that is produced by a Java compiler 
from the .java file.
Tip You must compile source code. Source code is kept in files with a .java 
suffix, so your operating system’s file and directory path notation are appropriate. 
One executes a class. A class is an abstract concept in the language, so its dot- 
notation becomes appropriate (i.e., it is necessary to write a dot (.) after the name 
of the instance of a class followed by the method to be used). Keep this distinction 
in mind to help yourself remember when to use which notation.