HelloWorld.java.javac.java.vim and emacs, which are also available on macOS and Windows. Free macOS editors include BBEdit and TextEdit, which is also available for Windows computers. Notepad++ is also a popular Windows text editor. There are also Integrated Development Environments (IDEs) that include an editor, compiler, and other tools, such as Eclipse, Netbeans, IntelliJ, jGrasp, etc.HelloWorld program, it must be stored in a file named HelloWorld.java.publicJava class named ClassName must be saved in a text file named exactly ClassName.java. There can be only one public class per file.HelloWorld class were named helloworld.java or Helloworld.java, that would cause an error. Java would be looking for a file named HelloWorld.java.javac. It is included in the JDK. To compile HelloWorld.java, you would simply type the following command on the MacOS or Linux or Windows command line, where ’$’ represents the command prompt:
$ javac HelloWorld.java
javac will create a Java bytecode file named HelloWorld.class—a file that has the same name as the source file but with the extension class instead of java. By default, the bytecode file will be placed in the same directory as the source file. After compiling, the directory would look something like this:
HelloWorld.class
HelloWorld.java
javac detects errors in the Java code, a list of error messages will be printed on the console —e.g.:
HelloWorld.java:11: error: ';' expected
System.out.println(greeting) // Output statement
^
1 error
HelloWorld.class bytecode file, it can be executed using the java command, also included in the JDK:
$ java HelloWorld
Hello World!
.class extension with the java command, just the file’s name.
java command works as follows. It first loads the Java Virtual Machine (JVM) into the computer’s main memory. The JVM then loads the Java bytecode into memory and runs it./** File: HelloWorldCanvas program */
import javax.swing.JFrame; // Import class names
import java.awt.Graphics;
import java.awt.Canvas;
public class HelloWorldCanvas extends Canvas // Class header
{
// Start of body
public void paint(Graphics g) // The paint method
{
g.drawString("Hello, World!", 10, 10);
} // End of paint
public static void main(String[] args) // The main() method
{ HelloWorldCanvas c = new HelloWorldCanvas();
JFrame f = new JFrame();
f.add(c);
f.setSize(150,150);
f.setVisible(true);
} // End of main
} // End of HelloWorldCanvas
HelloWorldCanvas program (Figure 1.6.5) are identical: $ javac HelloWorldCanvas.java
$ java HelloWorldCanvas
JFrame.
