Swing

//http://terai.xrea.jp/data/swing/Sample.java
public void zooming(double zoom) {
  double origzoom = this.zoom;
  this.zoom = zoom;
  Point p = spCanvas.getViewport( ).getViewPosition( ); //左上の座標
  canvas.setPreferredSize(
    new Dimension((int)( PWIDTH * zoom ) , (int)( PHEIGHT * zoom )));
  canvas.revalidate( );
  // 左上の位置を拡大縮小した大きさでの位置にあわせる
  //<ins>
  Point pt = new Point(
    (int)( p.x * zoom / origzoom ) , (int)( p.y * zoom / origzoom ));
  canvas.scrollRectToVisible(new Rectangle(pt, spCanvas.getViewport( ).getSize()));
  //</ins>
  spCanvas.getViewport( ).setViewPosition(pt);
}

import java.awt.*;
import javax.swing.*;
public class Test{
  public static void main(String[] args) {
    //<XXX>
    createAndShowGUI();
    //</XXX>
    //EventQueue.invokeLater(new Runnable() {
    //  public void run() {
    //    createAndShowGUI();
    //  }
    //});
  }
  public static void createAndShowGUI() {
    JFrame frame = new JFrame();
    //<XXX>
    frame.setVisible(true);
    frame.setSize(300,200); //or frame.pack();
    frame.setLocationRelativeTo(null);
    //</XXX>
    frame.setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
    frame.getContentPane().add(new JScrollPane(new JTree()));
    //frame.setSize(320,240); //or frame.pack();
    //frame.setLocationRelativeTo(null);
    //frame.setVisible(true);
  }
}