Kingwood Computer Programming

Learn to Code

Why Learn Java?

► Unlike languages such as C or C++, java comes with everything you need to build gui programs right out-of-the-box.

Here's an example program that shows a rotating, 5-sided polygon. Observe that only about 80 lines of code are required to create this application. Here's the source and a screenshot of the running application (file: test.java):

import java.awt.event.ActionListener;
import java.awt.event.ActionEvent;
import javax.swing.JPanel;
import javax.swing.JFrame;
import javax.swing.Timer;
import java.awt.Graphics;

public class test extends JPanel
{
   public test()
   {
      mTimer = new Timer(
         100, new ActionListener()
         {
            public void actionPerformed(ActionEvent e)
            {
               if(++mDegrees == 360) mDegrees = 0;
               repaint();
            }
         }
      );

      mTimer.start();
   }

   public void paintComponent(Graphics g)
   {
      super.paintComponent(g);

      int w = getWidth();
      int h = getHeight();

      int size = (int)(.45 * Math.min(w, h) + 0.5);

      int xFirst = 0, yFirst = 0, xOld = 0, yOld = 0;

      for(int i = 0, degrees = mDegrees; i < N_SIDES; i ++, degrees += STEP)
      {
         double radians = Math.toRadians(degrees);

         int x = (int)(w / 2 + size * Math.cos(radians) + 0.5);
         int y = (int)(h / 2 + size * Math.sin(radians) + 0.5);

         if(i == 0)
         {
            xFirst = x;
            yFirst = y;
         }
         else
         {
            g.drawLine(x, y, xOld, yOld);

            if(i + 1 == N_SIDES)
               g.drawLine(x, y, xFirst, yFirst);
         }

         xOld = x;
         yOld = y;
      }
   }

   private Timer mTimer;
   private int   mDegrees;

   private static final int N_SIDES = 5;
   private static final int STEP    = (360 / N_SIDES);

   public static void main(String[] args)
   {
      JFrame j = new JFrame("test");

      j.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
      j.add(new test());
      j.setSize(500, 500);

      j.setVisible(true);
   }
}

Results:

Source Code

► Java has been around for more than 20 years and as a consequence there are many, many libraries, tools and packages already written to address common needs.

► Java is popular. As of July, 2021, the TIOBE Index of programming popularity was:

C
Java
Python
C++
C#

► Java is compiled down to bytecode class files and runs on the Java Virtual Machine. There are a number of other languages that also operate on the JVM and interacting with them from java is easy and straightforward.

► ONLINE RESOURCES

► ONLINE JAVA COMPILERS

► BOOKS

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