import org.eclipse.core.commands.Command;
import org.eclipse.core.commands.ParameterizedCommand;
import org.eclipse.e4.core.commands.ECommandService;
import org.eclipse.e4.core.commands.EHandlerService;
import org.eclipse.e4.core.di.annotations.Execute;
import org.eclipse.e4.core.services.log.Logger;

@SuppressWarnings("restriction")
public class PreferencesHandler {

	@Execute
	void execute(ECommandService commandService, EHandlerService handlerService, Logger logger) {

		
		Command command = commandService.getCommand("org.eclipse.ui.window.preferences");

		// check if the command is defined
		System.out.println(command.isDefined());

		// activate handler, assumption: the AboutHandler() class exists already
		handlerService.activateHandler("my.fancy.app.ui.app.command.preferences", 
		    new PreferencesHandler());

		// prepare execution of command
		ParameterizedCommand cmd =
		  commandService.createCommand("org.eclipse.ui.window.preferences", null);

		// check if the command can get executed
		if (handlerService.canExecute(cmd)){
		  // execute the command
		  handlerService.executeHandler(cmd);
		} else {
			System.err.println("NEIN");
		}
	}
}

handlerService.canExecute(cmd)