集合处理工具

/**
     * 按给定对象属性为参数ls分组整理
     * @param ls 集合
     * @param propertyName 对象中的某个属性
     * @return HashMap(key=propertyValue,Value=ArrayList)
     * @throws IllegalAccessException
     * @throws InvocationTargetException
     * @throws NoSuchMethodException
     */
    @SuppressWarnings("unchecked")
    public static <T,E> HashMap<T,List<E>> groupByProperty(List<E> ls,String propertyName) throws IllegalAccessException, InvocationTargetException, NoSuchMethodException{
        HashMap<T,List<E>> result=new HashMap<T,List<E>>();
        List<E> list=null;
        for (Iterator<E> iter = ls.iterator(); iter.hasNext();) {
            E element = iter.next();
            T proValue=(T)PropertyUtils.getProperty(element, propertyName);
            if(proValue==null)
                throw new NullPointerException("propertyValue is null"); 
            if(result.containsKey(proValue)){
                list=(List<E>) result.get(proValue);
            }else{
                list=new ArrayList<E>();
                result.put(proValue, list);
            }
            list.add(element);
        }
        return result;
    }

    /**
     * 提取集合中的对象的属性,组合成List.
     * 
     * @param collection 来源集合.
     * @param propertyName 要提取的属性名.
     */
    @SuppressWarnings({ "unchecked", "rawtypes" })
    public static List fetchElementPropertyToList(final Collection collection, final String propertyName) throws Exception {
        List list = new ArrayList();
        for (Object obj : collection) {
            list.add(PropertyUtils.getProperty(obj, propertyName));
        }

        return list;
    }