EDIT

package eu.etaxonomy.taxeditor.handler.defaultHandler;

import java.util.UUID;

import org.eclipse.core.commands.AbstractHandler;

import org.eclipse.core.commands.ExecutionEvent;

import org.eclipse.core.commands.ExecutionException;

import org.eclipse.core.commands.ParameterType;

import org.eclipse.core.commands.common.NotDefinedException;

import org.eclipse.jface.viewers.ISelection;

import org.eclipse.jface.viewers.IStructuredSelection;

import org.eclipse.ui.handlers.HandlerUtil;

import eu.etaxonomy.taxeditor.model.MessagingUtils;

public abstract class DefaultOpenHandlerBase <T> extends AbstractHandler {

    @Override

    public Object execute(ExecutionEvent event) throws ExecutionException {

        String commandId = event.getCommand().getId();

        String uuidParameterId = commandId+".uuid";

        //check if uuid parameter is set

        if(event.getParameter(uuidParameterId)!=null){

            Object object = event.getObjectParameterForExecution(uuidParameterId);

            ParameterType parameterType;

            try {

                parameterType = event.getCommand().getParameterType(uuidParameterId);

                if(parameterType.isCompatible(object)){

                    T entity = getEntity((UUID) object);

                    open(event, entity);

                }

            } catch (NotDefinedException e) {

                MessagingUtils.error(DefaultOpenHandlerBase.class, "Error while opening entity!", e);

            }

        }

        //if not try current selection

        else{

            ISelection selection = HandlerUtil.getCurrentSelection(event);

            if(selection instanceof IStructuredSelection){

                open(event, (T) ((IStructuredSelection) selection).getFirstElement());

            }

        }

        return null;

    }

    protected abstract T getEntity(UUID uuid);

    protected abstract void open(ExecutionEvent event, T entity);

}