import org.eclipse.swt.layout.RowLayout;
import org.eclipse.swt.widgets.Display;
import org.eclipse.swt.widgets.Shell;


public class Main {

	public static void main(String[] args) {
		Main main = new Main();
		main.init();
	}

	private void init() {

		Display display = new Display();
		Shell shell = new Shell(display);
		shell.setLayout(new RowLayout());
		MyCombo myCombo = new MyCombo(shell);
		
		shell.pack();
		shell.open();
		while (!shell.isDisposed()) {
			if (!display.readAndDispatch()) {
				display.sleep ();
			}
		}
		display.dispose();
	}
}

import org.eclipse.swt.SWT;
import org.eclipse.swt.events.ModifyEvent;
import org.eclipse.swt.events.ModifyListener;
import org.eclipse.swt.widgets.Combo;
import org.eclipse.swt.widgets.Shell;


public class MyCombo {
	private Combo combo;
	private String[] items;

	
	
	public MyCombo(Shell shell) {
		combo = new Combo(shell, SWT.NONE);
		initItems();
		combo.setItems(items);
		combo.addModifyListener(new MyComboModifyListener());
	}
	
	private void initItems() {
		items = new String[] {"aaa", "aab", "aac", "abb", "abc", "bbb", "bbc", "bbd"};
	}
	
	
	private class MyComboModifyListener implements ModifyListener {
		public void modifyText(ModifyEvent event) {
			filter();
		}
	}
	
	
	private void filter() {
		for (int i = 0; i < items.length; i++) {
			if (items[i].startsWith(combo.getText()) && !containsItem(items[i])) {
				combo.add(items[i]);
			} else if (!items[i].startsWith(combo.getText()) && containsItem(items[i])) {
				combo.remove(items[i]);
			}
		}
		if (combo.getItemCount() > 0) {
			combo.setListVisible(true);
			combo.forceFocus();			// <----- this doesn't seem to work
		}
	}
	
	
	private boolean containsItem(String item) {
		for (int i = 0; i < combo.getItems().length; i++) {
			if (combo.getItem(i).equals(item)) {
				return true;
			}
		}
		return false;
	}
}