private final class MyViewerTreeListener implements ITreeViewerListener
{
	public void treeCollapsed(TreeExpansionEvent treeexpansionevent)
	{

		// Do something with before collapsing,
		// probably using the info on the Element.
		// For example:
		Object objectCollapsed = treeexpansionevent.getElement();
		System.out.println(object.toString());

		Display.getCurrent().asyncExec(new Runnable() {
			@Override
			public void run() {
				// Do something with AFTER collapsing,
				// probably using the info on the Element.
				// Remember that to access local variable
				// you have to deine it final in the host
				// class/function. In this example it would apply
				// to treeexpansionevent or objectCollapsed
				...
			}//- run()
		}//- Runnable -anonymous class-
	}//- treeCollapsed() --
}//- MyTreeViewerTreeListener 

package org.eclipse.jface.viewers;


/*** BaseTreeViewerTreeListener 
* This class will be extended, and 2 methods overridden :
* -void beforeTreeCollapsedEventHandler( TreeExpansionEvent treeexpansionevent );
* -void afterTreeCollapsedEventHandler( TreeExpansionEvent treeexpansionevent );
*
*/
public class BaseTreeViewerListener implements ITreeViewerListener
{
	public /* if possible private */
         void treeCollapsed( final TreeExpansionEvent treeexpansionevent )
	{
		beforeTreeCollapsedEventHandler( treeexpansionevent );

		Display.getCurrent().asyncExec(new Runnable() {
			@Override
			public void run() {
				afterTreeCollapsedEventHandler( treeexpansionevent );
			}//- run()
		}//- Runnable -anonymous class-
	}//- treeCollapsed() --
}//- BaseTreeViewerTreeListener 

package mypackage;

import org.eclipse.jface.viewers.BaseTreeViewerTreeListener;
import org.eclipse.jface.viewers.TreeExpansionEvent; // btw, to call expansion event when it handles collapseing event too... :O


/*** MyTreeViewerTreeListener 
* Overrides the 2 methods:
* -void beforeTreeCollapsedEventHandler( TreeExpansionEvent treeexpansionevent );
* -void afterTreeCollapsedEventHandler( TreeExpansionEvent treeexpansionevent );
*
*/
public class MyTreeViewerListener extends BaseTreeViewerListener
{
	/***
	*
	*/
	void beforeTreeCollapsedEventHandler( TreeExpansionEvent treeexpansionevent )
	{
		// Do something before collapsing,
		// probably using the info on the Element.
		// For example:
		Object objectCollapsed = treeexpansionevent.getElement();
		System.out.println(object.toString());

	}//- beforeTreeCollapsedEventHandler() - 

	/***
	*
	*/
	void afterTreeCollapsedEventHandler( TreeExpansionEvent treeexpansionevent );
	{
		// Do something with before collapsing,
		// probably using the info on the Element.
		// For example:
		// I check what is the new element on top, because Eclipse TreeViewer
		// may have changed it scrolling up in order not to have empty
		// rows at the bottom.
		// 

		System.out.println(object.toString());
	}//- afterTreeCollapsedEventHandler()

}//- MyTreeViewerTreeListener