如何计算ArrayList中的重复元素？

List<String> list = new ArrayList<String>();    list.add("a");    list.add("b");    list.add("c");    list.add("a");    list.add("a");    list.add("a");int countA=Collections.frequency(list, "a");int countB=Collections.frequency(list, "b");int countC=Collections.frequency(list, "c");