6.判断list集合中元素是否相等（无序）

public class ListUtils {

    /**
     * 判断集合元素是否相等（无序）
     */
    public static boolean isEqualCollection(Collection a, Collection b){

        if (a.size() !=b.size()) {  // size是最简单的相等条件
            return false;
        }
        Map mapa = getCardinalityMap(a);
        Map mapb = getCardinalityMap(b);

        // 转换map后，能去掉重复的，这时候size就是非重复项，也是先决条件
        if (mapa.size() !=mapb.size()) {
            return false;
        }
        Iterator it =mapa.keySet().iterator();
        while (it.hasNext()) {
            Object obj = it.next();
            // 查询同一个obj，首先两边都要有，而且还要校验重复个数，就是map.value
            if (getFreq(obj,mapa) != getFreq(obj, mapb)) {
                return false;
            }
        }
        return true;
    }


    public static Map getCardinalityMap(Collection coll) {
        Map count = new HashMap();
        Iterator it = coll.iterator();
        for (it.hasNext()) {
            Object obj =it.next();
            Integer c =(Integer) count.get(obj);
            if (c == null)
                count.put(obj, 1);
            else {
                count.put(obj, new Integer(c.intValue() + 1));
            }
        }
        return count;
    }

    private static final int getFreq(Object obj, Map freqMap) {
        Integer count =(Integer) freqMap.get(obj);
        if (count != null) {
            return count.intValue();
        }
        return 0;
    }

}