Moving the visible area of a JScrollPane to a specific position

Question

I am trying to simulate the movement of a debugging cursor using java. I am having problem to get the viewable area of the JScrollPane to the right position.

Here is

Answer 1

This is simple example, using a JList and JScrollPane's rowHeader support.

The magic basically happens here...

int index = list.getSelectedIndex();
index++;
if (index >= list.getModel().getSize()) {
    index = 0;
}
list.setSelectedIndex(index);
Rectangle cellBounds = list.getCellBounds(index, index);
list.scrollRectToVisible(cellBounds);

Basically, we ask the view to calculate the Rectangle which is represented by the selected index and simply ask the component to scroll so that rectangle is visible

import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.Container;
import java.awt.EventQueue;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.Rectangle;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.io.BufferedReader;
import java.io.File;
import java.io.FileReader;
import java.io.IOException;
import javax.swing.DefaultListModel;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JList;
import javax.swing.JPanel;
import javax.swing.JScrollPane;
import javax.swing.JViewport;
import javax.swing.UIManager;
import javax.swing.UnsupportedLookAndFeelException;
import javax.swing.event.ListSelectionEvent;
import javax.swing.event.ListSelectionListener;

public class Test {

    public static void main(String[] args) {
        new Test();
    }

    public Test() {
        EventQueue.invokeLater(new Runnable() {
            @Override
            public void run() {
                try {
                    UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
                } catch (ClassNotFoundException | InstantiationException | IllegalAccessException | UnsupportedLookAndFeelException ex) {
                    ex.printStackTrace();
                }

                JFrame frame = new JFrame("Testing");
                frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                frame.add(new TestPane());
                frame.pack();
                frame.setLocationRelativeTo(null);
                frame.setVisible(true);
            }
        });
    }

    public class TestPane extends JPanel {

        private JList list;

        public TestPane() {
            setLayout(new BorderLayout());
            DefaultListModel model = new DefaultListModel();
            try (BufferedReader br = new BufferedReader(new FileReader(new File("src/test/Test.java")))) {
                String text = null;
                while ((text = br.readLine()) != null) {
                    model.addElement(text);
                }
            } catch (IOException exp) {
                exp.printStackTrace();
            }
            list = new JList(model);
            list.setSelectedIndex(0);
            JScrollPane sp = new JScrollPane(list);
            sp.setRowHeaderView(new Header(list));

            add(sp);

            JButton next = new JButton("Next");
            next.addActionListener(new ActionListener() {
                @Override
                public void actionPerformed(ActionEvent e) {
                    int index = list.getSelectedIndex();
                    index++;
                    if (index >= list.getModel().getSize()) {
                        index = 0;
                    }
                    list.setSelectedIndex(index);
                    Rectangle cellBounds = list.getCellBounds(index, index);
                    list.scrollRectToVisible(cellBounds);
                }
            });

            add(next, BorderLayout.SOUTH);
        }

    }

    protected class Header extends JPanel {

        private JList list;

        public Header(JList list) {
            this.list = list;
            list.addListSelectionListener(new ListSelectionListener() {
                @Override
                public void valueChanged(ListSelectionEvent e) {
                    repaint();
                }
            });
        }

        @Override
        protected void paintComponent(Graphics g) {
            super.paintComponent(g);
            Container parent = list.getParent();
            if (parent instanceof JViewport) {
                JViewport viewport = (JViewport) parent;
                Graphics2D g2d = (Graphics2D) g.create();
                int selectedRow = list.getSelectedIndex();
                if (selectedRow >= 0) {
                    Rectangle cellBounds = list.getCellBounds(selectedRow, selectedRow);
                    cellBounds.y -= viewport.getViewPosition().y;
                    g2d.setColor(Color.RED);
                    g2d.fillRect(0, cellBounds.y, getWidth(), cellBounds.height);
                }
                g2d.dispose();
            }
        }

    }

}