Java Swing How to - Create a JScrollPane that displays its component centered

Question

We would like to know how to create a JScrollPane that displays its component centered.
Answer

import java.awt.BorderLayout;
import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.geom.AffineTransform;
/*from   www . java2  s. c  om*/
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.JScrollPane;
import javax.swing.JSpinner;
import javax.swing.SpinnerNumberModel;

public class Main extends JFrame {
  public Main() {
    MyCanvas thingy = new MyCanvas();
    setLayout(new BorderLayout());
    JPanel panel = new JPanel();
    panel.add(thingy);
    JScrollPane scroll = new JScrollPane();
    scroll.setViewportView(panel);
    add(scroll, BorderLayout.CENTER);
    SpinnerNumberModel spmodel = new SpinnerNumberModel(thingy.getThingySize(),
        10.0, 2000.0, 10.0);
    spmodel.addChangeListener(e->thingy.setThingySize((Double) spmodel.getNumber()));
    add(new JSpinner(spmodel), BorderLayout.NORTH);
  }
  public static void main(String[] args) {
    new Main().start();
  }
  private void start() {
    setDefaultCloseOperation(EXIT_ON_CLOSE);
    pack();
    setVisible(true);
  }
}

class MyCanvas extends JPanel {
  double size = 20.0;
  @Override
  public Dimension getPreferredSize() {
    int isize = (int) this.size;
    return new Dimension(isize, isize);
  }
  @Override
  protected void paintComponent(Graphics g) {
    super.paintComponent(g);
    int[] x = { 0, 100, 100, 0, 0, 75, 75, 125, 25, 50 };
    int[] y = { 0, 10, 10, 100, 25, 125, 75, 75, 510, 50 };
    Graphics2D g2d = (Graphics2D) g;
    AffineTransform at0 = g2d.getTransform();
    g2d.scale(size / 100, size / 100);
    g.drawPolyline(x, y, x.length);
    g2d.setTransform(at0);
  }
  public void setThingySize(double size) {
    this.size = size;
    revalidate();
    repaint();
  }
  public double getThingySize() {
    return this.size;
  }
}