Question about JScrollPane

[hei1233212000]

I don't know why I cannot create a scroll baron a JPanel when I inport a image which is larger than the frame and the panel.

image2 is a image with size ( 700, 525 ) which is larger than the frame( 400, 300 ) and panel. So, I expect the scroll bar will automatically created but it did't.

What wrong in my code?
Thank you. :)

Test.java

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

public class Test extends JFrame
{
	private myPanel panel;
	private JScrollPane spanel;
	private JButton button1;
	private JButton button2;
	private JPanel topPanel;
	
	public Test()
	{
		super( "Test" );
		setLayout( new BorderLayout() );
		
		button1 = new JButton( "Button1" );
		button2 = new JButton( "Button2" );
		panel = new myPanel();

		topPanel = new JPanel();
		topPanel.add( button1 );
		topPanel.add( button2 );

		add( topPanel, BorderLayout.NORTH );
		add( new JScrollPane( panel ), BorderLayout.CENTER );
		
		setMinimumSize( new Dimension( 400, 300 ) );
	}
	
	public static void main( String args[] )
	{
		Test myTest = new Test();
		myTest.setVisible( true );
	}
}

myPanel.java

import java.awt.*;
import java.awt.image.*;
import java.io.*;
import javax.imageio.ImageIO;
import javax.swing.*;

public class myPanel extends JPanel
{
	private BufferedImage image1;
	private BufferedImage image2;
	
	public myPanel()
	{
	     try 
	     {                
			image1 = ImageIO.read( new File( "doc/resources/inherit.gif"   ) );
			image2 = ImageIO.read( new File( "doc/resources/Car.jpg" ) );
	     } 
	     catch ( IOException ex ) 
	     {
		   // do something
	     }
	}
	
	public void paintComponent( Graphics g )
	{
		super.paintComponent( g );
		
		g.drawImage( image2, 0, 0, null );
		g.drawLine( 0, 0, 100, 100);
	}
}

[DarrylBurke]

Your panel doesn't have a preferredSize, so the scroll pane doesn't know it should be sized to accommodate the image. One approach would be to override getPreferredSize() to return the size of the image.

@Override
public Dimension getPreferredSize() {
   if (image == null) {
      return super.getPreferredSize();
   }
   return new Dimension(image2.getWidth(), image2.getHeight());
}

Also note that by convention, Java class names start with an uppercase letter. MyPanel, not myPanel.

db

[hei1233212000]

it works, thank you.
but when I modify my program, another problem comes out.

I add a method called changePanelPreferredSize() to change the preferred size of the panel.
Then, I add an actionListener to the button to execute the changePanelPreferredSize().

I expected the scroll bar will be created after I press the button. However, it didn't unless I resize the frame manually.

what is the problem now?
Thank you for your help.

P.S: Actually, what I am going to do is:
import a image by pressing a button to select a image file( using fileChooser that I made it already but I didn't show it out for more readable ) and display the image on the panel, also the scroll bar will be created if the image size is larger than the panel.

my new code:
I highlighted the new parts

Test.java

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;

public class Test extends JFrame implements ActionListener
{
	private myPanel panel;
	private JButton buttonChange;
	private JButton button2;
	private JPanel topPanel;
	
	public Test() 
	{
		super( "Test" );
		setLayout( new BorderLayout() );
		
		buttonChange = new JButton( "Change" );
		button2 = new JButton( "Button2" );
		panel = new MyPanel();

		[COLOR="Red"]buttonChange.addActionListener( this );[/COLOR]
		
		topPanel = new JPanel();
		topPanel.add( buttonChange );
		topPanel.add( button2 );

		add( topPanel, BorderLayout.NORTH );
		add( new JScrollPane( panel ), BorderLayout.CENTER );
		
		setMinimumSize( new Dimension( 400, 300 ) );
	}
	
	[COLOR="red"]@Override
	public void actionPerformed( ActionEvent actionEvent )
	
	{
		panel.changePanelPreferredSize();
	}[/COLOR]
	
	public static void main( String args[] )
	{
		Test myTest = new Test();
		myTest.setVisible( true );
	}

}

MyPanel.java

import java.awt.*;
import java.awt.image.*;
import java.io.*;
import javax.imageio.ImageIO;
import javax.swing.*;

public class MyPanel extends JPanel
{
	private BufferedImage image1;
	private BufferedImage image2;
	
	public myPanel()
	{
		try 
		{                
			image1 = ImageIO.read( new File( "doc/resources/inherit.gif" ) );
			image2 = ImageIO.read( new File( "doc/resources/Car.jpg" ) );
			
		} 
		catch ( IOException ex ) 
		{
			// do something
		}
	}
	
	[COLOR="red"]public void changePanelPreferredSize()
	{
		setPreferredSize( new Dimension( image2.getWidth(), image2.getHeight() ) );
	}[/COLOR]
	
	public void paintComponent( Graphics g )
	{
		super.paintComponent( g );
		
		g.drawImage( image2, 0, 0, null );
	}
}

[hei1233212000]

I solved it by adding setSize()
However, I don't know the reason.....

public void changePanelPreferredSize()
	{
		setPreferredSize( new Dimension( image2.getWidth(), image2.getHeight() ) );
               [COLOR="Red"] setSize( new Dimension( image2.getWidth(), image2.getHeight() ) );[/COLOR]
	}

[DarrylBurke]

revalidate() the scroll pane.

db