documentclass[10pt,pdftex]{article}

usepackage[utf8]{inputenc}
usepackage[T1]{fontenc}
usepackage{lmodern}
usepackage{listings}

begin{document}

lstset{language=Java, columns=fullflexible, keepspaces}

begin{lstlisting}
import javax.swing.JFrame;  // Importing class JFrame
import javax.swing.JLabel;  // Importing class JLabel
public class HelloWorld {
    public static void main(String[] args) {
        JFrame frame = new JFrame();            // Creating frame
        frame.setTitle("Hi!");                  // Setting title frame
        frame.add(new JLabel("Hello, world!")); // Adding text to frame
        frame.pack();                           // Setting size to smallest
        frame.setLocationRelativeTo(null);      // Centering frame
        frame.setVisible(true);                 // Showing frame
    }
}
end{lstlisting}

end{document}

documentclass[10pt,pdftex]{article}

 usepackage[utf8]{inputenc}
 usepackage[T1]{fontenc}
 usepackage{lmodern}
 usepackage{listings}

 begin{document}

 lstdefinelanguage[]{Java}{morecomment=[l]{//}}[]
 lstset{language=Java, columns=spaceflexible, keepspaces}

 begin{lstlisting}
 import javax.swing.JFrame;  // Importing class JFrame
 import javax.swing.JLabel;  // Importing class JLabel
 public class HelloWorld {
     public static void main(String[] args) {
         JFrame frame = new JFrame();            // Creating frame
         frame.setTitle("Hi!");                  // Setting title frame
         frame.add(new JLabel("Hello, world!")); // Adding text to frame
         frame.pack();                           // Setting size to smallest
         frame.setLocationRelativeTo(null);      // Centering frame
         frame.setVisible(true);                 // Showing frame
     }
 }
 end{lstlisting}

 end{document}