Java by API/javax.swing.event/HierarchyListener

implements HierarchyListener

import java.awt.BorderLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.event.HierarchyEvent;
import java.awt.event.HierarchyListener;
import javax.swing.JButton;
import javax.swing.JComponent;
import javax.swing.JFrame;
import javax.swing.JSplitPane;
public class MainClass {
  public static void main(String args[]) throws Exception {
    JFrame vFrame = new JFrame("Vertical Split");
    vFrame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    JComponent leftButton = new JButton("Left");
    JComponent rightButton = new JButton("Right");
    final JSplitPane splitPane = new JSplitPane(JSplitPane.VERTICAL_SPLIT);
    splitPane.setOneTouchExpandable(true);
    splitPane.setLeftComponent(leftButton);
    splitPane.setRightComponent(rightButton);
    
    ActionListener oneActionListener = new ActionListener() {
      public void actionPerformed(ActionEvent event) {
        splitPane.resetToPreferredSizes();
      }
    };
    ((JButton) rightButton).addActionListener(oneActionListener);
    
    ActionListener anotherActionListener = new ActionListener() {
      public void actionPerformed(ActionEvent event) {
        splitPane.setDividerLocation(10);
        splitPane.setContinuousLayout(true);
      }
    };
    
    ((JButton) leftButton).addActionListener(anotherActionListener);
    HierarchyListener hierarchyListener = new HierarchyListener() {
      public void hierarchyChanged(HierarchyEvent e) {
        long flags = e.getChangeFlags();
        if ((flags & HierarchyEvent.SHOWING_CHANGED) == HierarchyEvent.SHOWING_CHANGED) {
          splitPane.setDividerLocation(.75);
        }
      }
    };
    splitPane.addHierarchyListener(hierarchyListener);
    vFrame.add(splitPane, BorderLayout.CENTER);
    vFrame.setSize(300, 150);
    vFrame.setVisible(true);
  }
}