Getting Started (Review)

The above image shows a JFrame that contains a JPanel. The panel has a white background (set in the constructor). Here is the program:

import javax.swing.*;
import java.awt.* ;
import java.lang.Math ;

class SnowFlakePanel extends JPanel
{

  public SnowFlakePanel()
  {
    setPreferredSize( new Dimension(400, 400) );
    setBackground( Color.WHITE );    
  }  
 
  public void paintComponent ( Graphics gr )
  { 
    int width  = getWidth();
    int height = getHeight();
    
    super.paintComponent( gr );
    gr.setColor( Color.BLUE );
    gr.drawLine( 0, 0, width-1, height-1 );
  }
}

public class SnowFlakeClear
{
   public static void main ( String[] args )
   {
      JFrame frame = new JFrame( "Snowflake Clear" );
      frame.setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE );
      frame.getContentPane().add( new SnowFlakePanel() );
      frame.pack();
      frame.setVisible( true );
   }
}

Copy the program into a programming editor and save to a file SnowFlakeClear.java. If you compile and run it, you should get the above figure. (Or, if you want, save each class to a separate file.)

Details: (Skip if you don't need a review.)

JFrame frame = new JFrame( "Snowflake Clear" );

This create a new JFrame, which is the window which contains the graphics for the program. The JFrame has a title bar (as specified in the constructor) snd small buttons for minimizing, maximizing, and closing.

frame.setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE );

This says what to do when the small close button is clicked. In this case, the program will exit.

frame.getContentPane().add( new SnowFlakePanel() );

The content pane is the part of the frame that holds graphics. getContentPane() returns a reference to it. Then add() can add and object to it. The object added here is a subclass of JPanel defined in our code.

frame.pack();

This adjusts the size of the frame to fit the panel that is in it. If you forget this your code will run but you will see nothing.

frame.setVisible( true );

This asks for the frame and its contents to be displayed. Again, if you forget to include this, you will see nothing.