A JScrollPane provides a scrollable view of a component. When screen real estate is limited, use a scroll pane to display a component that is large or one whose size can change dynamically. Here is a snapshot of an application that uses a customized scroll pane to view a large photograph:
The scroll pane also has two scroll bars, a row header, a column header, and four corners, three of which have been customized.

This program establishes the scroll pane's client when creating the scroll pane:
// where the member variables are declared
private ScrollablePicture picture;
...
        // where the GUI is created
        picture = new ScrollablePicture( ... );
        JScrollPane pictureScrollPane = new JScrollPane(picture);
You can change a scroll pane's client dynamically by calling the setViewportView method. Note that JScrollPane has no corresponding getViewportView method, so you should cache the client object in a variable if you need to refer to it later.
When the user manipulates the scroll bars in a scroll pane, the area of the client that is visible changes accordingly. This picture shows the relationship between the scroll pane and its client and indicates the classes that the scroll pane commissions to help:
A scroll pane uses a JViewport instance to manage the visible area of the client. The viewport is responsible for computing the bounds of the current visible area, based on the positions of the scroll bars, and displaying it.A scroll pane uses two separate instances of JScrollBar for the scroll bars. The scroll bars provide the interface for the user to manipulate the visible area. The following figure shows the three areas of a scroll bar: the knob, the buttons, and the track.
When the user moves the knob on the vertical scroll bar up and down, the visible area of the client moves up and down. Similarly, when the user moves the knob on the horizontal scroll bar to the right and left, the visible area of the client moves back and forth accordingly. The position of the knob relative to its track is proportionally equal to the position of the visible area relative to the client. In the Java Look & Feel and some others, the size of the knob gives a visual clue as to how much of the client is visible.By clicking a button, the user can scroll by a unit increment. By clicking within the track, the user can scroll by a block increment. Information about unit and block increments is in Implementing a Scrolling-Savvy Client.
Typical programs don't directly instantiate or call methods on a viewport or scroll bar. Instead, programs achieve their scrolling behavior using the JScrollPane API and the API discussed in Implementing a Scrolling-Savvy Client. Some scrolling-savvy components such as JList, JTable, and JTree also provide additional API to help you affect their scrolling behavior.

Setting the Scroll Bar Policy

On startup, the scroll pane in the ScrollDemo application has two scroll bars. If you make the window as large as your screen, both scroll bars disappear because they are no longer needed. If you then shrink the height of the window without changing its width, the vertical scroll bar reappears. Further experimentation will show that in this application both scroll bars disappear and reappear as needed. This behavior is controlled by the scroll pane's scroll bar policy, Actually, it's two policies: you specify the policy for each scroll bar separately.
ScrollDemo doesn't explicitly set the scroll pane's sroll bar policies--it uses the default. But you can set the policies when you create the scroll pane or change them dynamically.
Of the constructors provided by JScrollPane, these two let you set the scroll bar policies when you create the scroll pane:
JScrollPane(Component, int, int)
JScrollPane(int, int)
The first int specifies the policy for the vertical scroll bar, the second specifies the policy for the horizontal scroll bar. You can also set the policies dynamically with the setHorizontalScrollBarPolicy andsetVerticalScrollBarPolicy methods. With both the constructors and the methods, use one of the following constants defined in the ScrollPaneConstants interface (which is implemented by JScrollPane):
PolicyDescription
VERTICAL_SCROLLBAR_AS_NEEDED 
HORIZONTAL_SCROLLBAR_AS_NEEDED		The default. The scroll bar appears when the viewport is smaller than the client and disappears when the viewport is larger than the client.
VERTICAL_SCROLLBAR_ALWAYS 
HORIZONTAL_SCROLLBAR_ALWAYS		Always display the scroll bar. The knob disappears if the viewport is large enough to show the whole client.
VERTICAL_SCROLLBAR_NEVER 
HORIZONTAL_SCROLLBAR_NEVER		Never display the scroll bar. Use this option if you don't want the user to directly control what part of the client is shown. Perhaps you have an application that requires all scrolling to occur programmatically.

Providing Custom Decorations

The area drawn by a scroll pane consists of up to nine parts: the center, four sides, and four corners. The center is the only component that is always present in all scroll panes. Besides scroll bars, the sides can contain column and row headers. A corner component is visible only if both sides that intersect at that corner contain visible components.
As shown in the figure, the scroll pane in ScrollDemo has custom row and column headers. Additionally, because all four sides are populated, all four corners are present. The program customizes three of the corners--two just fill their area with the same color as the Rules, and the other contains a toggle button. The fourth corner, the lower right corner, is the default provided by the scroll pane. Notice that because the row and column headers are always present in this example, that the toggle button is also always present.
If a corner contains a control that the user needs access to all the time, make sure the sides that intersect at the corner are always present. For example, if this application placed the toggle in the lower right corner where the scroll bars intersect, then the toggle would disappear if the user resized the window and even one of the scroll bars disappeared.
The scroll pane's row and column headers are provided by a custom JComponent subclass, Rule, that draws a ruler in centimeters or inches. Here's the code that creates and sets the scroll pane's row and column headers:
//...where the member variables are defined:
private Rule columnView;
private Rule rowView;
...
    //...where the GUI is initialized:
    ImageIcon david = new ImageIcon("images/youngdad.jpeg");
    ...
    // Create the row and column headers
    columnView = new Rule(Rule.HORIZONTAL, true);
    columnView.setPreferredWidth(david.getIconWidth());
    rowView = new Rule(Rule.VERTICAL, true);
    rowView.setPreferredHeight(david.getIconHeight());
    ...
    pictureScrollPane.setColumnHeaderView(columnView);
    pictureScrollPane.setRowHeaderView(rowView);
    ...
You can use any component for a scroll pane's row and column headers. The scroll pane puts the row and column headers in JViewPorts of their own. Thus, when scrolling horizontally, the column header follows along, and when scrolling vertically, the row header follows along.As a JComponent subclass, our custom Rule class puts its rendering code in its paintComponent method. Careful scrutiny of the code reveals that special effort is taken to draw only within the current clipping bounds. Your custom row and column headers should do the same to ensure speedy scrolling.
You can also use any component for the corners of a scroll pane. ScrollDemo illustrates this by putting a toggle button in the upper left corner, and custom Corner objects in the upper right and lower left corners. Here's the code that creates the Corner objects and calls setCorner to place them:
// Create the corners.
JPanel buttonCorner = new JPanel();
isMetric = new JToggleButton("cm", true);
isMetric.setFont(new Font("SansSerif", Font.PLAIN, 11));
isMetric.setMargin(new Insets(2,2,2,2));
isMetric.addItemListener(new UnitsListener());
buttonCorner.add(isMetric); //Use the default FlowLayout
...
// Set the corners.
pictureScrollPane.setCorner(JScrollPane.UPPER_LEFT_CORNER, 
                            buttonCorner);
pictureScrollPane.setCorner(JScrollPane.LOWER_LEFT_CORNER,
                            new Corner());
pictureScrollPane.setCorner(JScrollPane.UPPER_RIGHT_CORNER,
                            new Corner());
Remember that the size of each corner is determined by the size of the sides intersecting there. For some components you must take care that the specific instance of the component fits in its corner. For example, the program sets the font and margins on the toggle button so that it fits within the space established by the headers. It's not an issue with the Corner class because that class colors its entire bounds, whatever they happen to be, with a solid color.As you can see from the code, constants indicate the corner positions. This figure shows the constant for each position:
The constants are defined in the ScrollPaneConstants interface, which JScrollPane implements.

Implementing a Scrolling-Savvy Client

To customize the way that a client component interacts with its scroll pane, you can make the component implement the Scrollable interface. By implementing Scrollable, a client can specify both the size of the viewport used to view it and the amount to scroll for clicks on the different controls on a scroll bar.
Here again are the three control areas of a scroll bar: the knob, the buttons, and the track.
You might have noticed when manipulating the scroll bars in ScrollDemo that clicking the buttons scrolls the image to a tick boundary. You might also have noticed that clicking in the track scrolls the picture by a "screenful". More generally, the button scrolls the visible area by a unit increment and the track scrolls the visible area by a block increment. The behavior you see in the example is not the scroll pane's default behavior, but is specified by the client in its implementation of the Scrollable interface.The client for the ScrollDemo program is ScrollablePicture. ScrollablePicture is a subclass of JLabel that provides implementations of all five Scrollable methods:
  • getScrollableBlockIncrement
  • getScrollableUnitIncrement
  • getPreferredScrollableViewportSize
  • getScrollableTracksViewportHeight
  • getScrollableTracksViewportWidth
ScrollablePicture implements the Scrollable interface primarily to affect the unit and block increments. However, it must provide implementations for all five methods. So it provides reasonable defaults for the other three methods that you might want to copy for your scrolling-savvy classes.The scroll pane calls the client's getScrollableUnitIncrement method whenever the user clicks one of the buttons on the scroll bar. This method returns the number of pixels to scroll. An obvious implementation of this method returns the number of pixels between tick marks on the header rulers. But ScrollablePicture does something different: It returns the value required to position the image on a tick mark boundary. Here's the implementation:
public int getScrollableUnitIncrement(Rectangle visibleRect,
                                      int orientation,
                                      int direction) {
    //get the current position
    int currentPosition = 0;
    if (orientation == SwingConstants.HORIZONTAL)
        currentPosition = visibleRect.x;
    else
        currentPosition = visibleRect.y;

    //return the number of pixels between currentPosition
    //and the nearest tick mark in the indicated direction
    if (direction < 0) {
        int newPosition = currentPosition -
                         (currentPosition / maxUnitIncrement) *
                          maxUnitIncrement;
        return (newPosition == 0) ? maxUnitIncrement : newPosition;
    } else {
        return ((currentPosition / maxUnitIncrement) + 1) * 
                 maxUnitIncrement - currentPosition;
    }
}
If the image is already on a tick mark boundary, this method returns the number of pixels between ticks. Otherwise, it returns the number of pixels from the current location to the nearest tick.Likewise, the scroll pane calls the client's getScrollableBlockIncrement method each time the user clicks on the track. Here's ScrollablePicture's implementation of this method:
public int getScrollableBlockIncrement(Rectangle visibleRect,
                                       int orientation,
                                       int direction) {
    if (orientation == SwingConstants.HORIZONTAL)
        return visibleRect.width - maxUnitIncrement;
    else
        return visibleRect.height - maxUnitIncrement;
}
This method returns the height of the visible rectangle minus a tick mark. This behavior is typical. A block increment should be slightly smaller than the viewport to leave a little of the previous visible area for context. For example, a text area might leave one or two lines of text for context and a table might leave a row or column (depending on the scroll direction).

Sizing a Scroll Pane

Unless you explicitly set a scroll pane's preferred size, the scroll pane computes it based on the preferred size of its nine components (the viewport, and, if present, the two scroll bars, the row and column headers, and the four corners). The largest factor, and the one most programmers care about, is the size of the viewport used to display the client.
If the client is not scrolling-savvy, then the scroll pane sizes itself so that the client displays at its preferred size. For typical unsavvy clients, this makes the scroll pane redundant. That is, the scroll pane has no scroll bars because the client's preferred size is big enough to display the entire client. In this case, if the client doesn't change size dynamically, you should probably limit the size of the scroll pane by setting its preferred size or the preferred size of its container.
If the client is scrolling-savvy, then the scroll pane uses the value returned by the client's getPreferredScrollableViewportSize method to compute the size of its viewport. Implementations of this method generally report a preferred size for scrolling that's smaller than the component's standard preferred size. For example, by default, the value returned by JList's implementation of getPreferredScrollableViewportSize is just big enough to display eight rows.

Source Code

ScrollDemo.java

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

public class ScrollDemo extends JPanel {
    private Rule columnView;
    private Rule rowView;
    private JToggleButton isMetric;
    private ScrollablePicture picture;

    public ScrollDemo() {
        // Start loading the image icon now.
        ImageIcon david = new ImageIcon("youngdad.jpeg");

        // Create the row and column headers.
        columnView = new Rule(Rule.HORIZONTAL, true);
        columnView.setPreferredWidth(david.getIconWidth());
        rowView = new Rule(Rule.VERTICAL, true);
        rowView.setPreferredHeight(david.getIconHeight());

        // Create the corners.
        JPanel buttonCorner = new JPanel();
        isMetric = new JToggleButton("cm", true);
        isMetric.setFont(new Font("SansSerif", Font.PLAIN, 11));
        isMetric.setMargin(new Insets(2,2,2,2));
        isMetric.addItemListener(new UnitsListener());
        buttonCorner.add(isMetric); //Use the default FlowLayout

        // Set up the scroll pane.
        picture = new ScrollablePicture(david, columnView.getIncrement());
        JScrollPane pictureScrollPane = new JScrollPane(picture);
        pictureScrollPane.setPreferredSize(new Dimension(300, 250));
        pictureScrollPane.setViewportBorder(
                BorderFactory.createLineBorder(Color.black));

        pictureScrollPane.setColumnHeaderView(columnView);
        pictureScrollPane.setRowHeaderView(rowView);

        pictureScrollPane.setCorner(JScrollPane.UPPER_LEFT_CORNER, 
                                    buttonCorner);
        pictureScrollPane.setCorner(JScrollPane.LOWER_LEFT_CORNER,
                                    new Corner());
        pictureScrollPane.setCorner(JScrollPane.UPPER_RIGHT_CORNER,
                                    new Corner());

        // Put it in this panel.
        setLayout(new BoxLayout(this, BoxLayout.X_AXIS));
        add(pictureScrollPane);
        setBorder(BorderFactory.createEmptyBorder(20,20,20,20));
    }

    class UnitsListener implements ItemListener {
        public void itemStateChanged(ItemEvent e) {
            if (e.getStateChange() == ItemEvent.SELECTED) {
                // Turn it to metric.
                rowView.setIsMetric(true);
                columnView.setIsMetric(true);
            } else {
                // Turn it to inches.
                rowView.setIsMetric(false);
                columnView.setIsMetric(false);
            }
            picture.setMaxUnitIncrement(rowView.getIncrement());
        }
    }

    public static void main(String s[]) {
        JFrame frame = new JFrame("ScrollDemo");
        frame.addWindowListener(new WindowAdapter() {
            public void windowClosing(WindowEvent e) {
                System.exit(0);
            }
        });
 
        frame.setContentPane(new ScrollDemo());
        frame.pack();
        frame.setVisible(true);
    }
}

ScrollablePicture.java

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

public class ScrollablePicture extends JLabel implements Scrollable {

    private int maxUnitIncrement = 1;

    public ScrollablePicture(ImageIcon i, int m) {
        super(i);
        maxUnitIncrement = m;
    }

    public Dimension getPreferredScrollableViewportSize() {
        return getPreferredSize();
    }

    public int getScrollableUnitIncrement(Rectangle visibleRect,
                                          int orientation,
                                          int direction) {
        //Get the current position.
        int currentPosition = 0;
        if (orientation == SwingConstants.HORIZONTAL)
            currentPosition = visibleRect.x;
        else
            currentPosition = visibleRect.y;

        //Return the number of pixels between currentPosition
        //and the nearest tick mark in the indicated direction.
        if (direction < 0) {
            int newPosition = currentPosition - 
                             (currentPosition / maxUnitIncrement) *
                              maxUnitIncrement;
            return (newPosition == 0) ? maxUnitIncrement : newPosition;
        } else {
            return ((currentPosition / maxUnitIncrement) + 1) *
                   maxUnitIncrement - currentPosition;
        }
    }

    public int getScrollableBlockIncrement(Rectangle visibleRect,
                                           int orientation,
                                           int direction) {
        if (orientation == SwingConstants.HORIZONTAL)
            return visibleRect.width - maxUnitIncrement;
        else
            return visibleRect.height - maxUnitIncrement;
    }

    public boolean getScrollableTracksViewportWidth() {
        return false;
    }

    public boolean getScrollableTracksViewportHeight() {
        return false;
    }

    public void setMaxUnitIncrement(int pixels) {
        maxUnitIncrement = pixels;
    }
}

Corner.java

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

public class Corner extends JComponent {
    public void paintComponent(Graphics g) {
        // Fill me with dirty brown/orange.
        g.setColor(new Color(230, 163, 4));
        g.fillRect(0, 0, getWidth(), getHeight());
    }
}

Rule.java

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

public class Rule extends JComponent {
    public static final int INCH = Toolkit.getDefaultToolkit().
            getScreenResolution();
    public static final int HORIZONTAL = 0;
    public static final int VERTICAL = 1;
    public static final int SIZE = 35;

    public int orientation;
    public boolean isMetric;
    private int increment;
    private int units;

    public Rule(int o, boolean m) {
        orientation = o;
        isMetric = m;
        setIncrementAndUnits();
    }

    public void setIsMetric(boolean isMetric) {
        this.isMetric = isMetric;
        setIncrementAndUnits();
        repaint();
    }

    private void setIncrementAndUnits() {
        if (isMetric) {
            units = (int)((double)INCH / (double)2.54); // dots per centimeter
            increment = units;
        } else {
            units = INCH;
            increment = units / 2;
        }
    }

    public boolean isMetric() {
        return this.isMetric;
    }

    public int getIncrement() {
        return increment;
    }

    public void setPreferredHeight(int ph) {
        setPreferredSize(new Dimension(SIZE, ph));
    }

    public void setPreferredWidth(int pw) {
        setPreferredSize(new Dimension(pw, SIZE));
    }

    public void paintComponent(Graphics g) {
        Rectangle drawHere = g.getClipBounds();

        // Fill clipping area with dirty brown/orange.
        g.setColor(new Color(230, 163, 4));
        g.fillRect(drawHere.x, drawHere.y, drawHere.width, drawHere.height);

        // Do the ruler labels in a small font that's black.
        g.setFont(new Font("SansSerif", Font.PLAIN, 10)); 
        g.setColor(Color.black);

        // Some vars we need.
        int end = 0;
        int start = 0;
        int tickLength = 0;
        String text = null;
    
        // Use clipping bounds to calculate first tick and last tick location.
        if (orientation == HORIZONTAL) {
            start = (drawHere.x / increment) * increment;
            end = (((drawHere.x + drawHere.width) / increment) + 1)
                  * increment;
        } else {
            start = (drawHere.y / increment) * increment;
            end = (((drawHere.y + drawHere.height) / increment) + 1)
                  * increment;
        }

        // Make a special case of 0 to display the number
        // within the rule and draw a units label.
        if (start == 0) {
            text = Integer.toString(0) + (isMetric ? " cm" : " in");
            tickLength = 10;
            if (orientation == HORIZONTAL) {
                g.drawLine(0, SIZE-1, 0, SIZE-tickLength-1);
                g.drawString(text, 2, 21);
            } else {
                g.drawLine(SIZE-1, 0, SIZE-tickLength-1, 0);
                g.drawString(text, 9, 10);
            }
            text = null;
            start = increment;
        }

        // ticks and labels
        for (int i = start; i < end; i += increment) {
            if (i % units == 0)  {
                tickLength = 10;
                text = Integer.toString(i/units);
            } else {
                tickLength = 7;
                text = null;
            }

            if (tickLength != 0) { 
                if (orientation == HORIZONTAL) {
                    g.drawLine(i, SIZE-1, i, SIZE-tickLength-1);
                    if (text != null)
                        g.drawString(text, i-3, 21);
                } else {
                    g.drawLine(SIZE-1, i, SIZE-tickLength-1, i);
                    if (text != null)
                        g.drawString(text, 9, i+3);
                }
            }
        }
    }
}
Maintained by John Loomis, last updated 15 June 2000