Stream流操作

    public static void main(String[] args) {
        CardVo c1 = new CardVo();
        c1.setInCardNo("a");
        CardVo c2 = new CardVo();
        c2.setInCardNo("b");
        CardVo c3 = new CardVo();
        c3.setInCardNo("c");
        CardVo c4 = new CardVo();
        c4.setInCardNo("a");

        List<CardVo> list = new ArrayList<>();
        list.add(c1);
        list.add(c2);
        list.add(c3);
        list.add(c4);

        //查找list重复元素
        List<String> nos = list.stream()
                .map(CardVo::getInCardNo)
                .collect(Collectors.toMap(e -> e, e -> 1, Integer::sum))
                .entrySet()
                .stream()
                .filter(e -> e.getValue() > 1)
                .map(Map.Entry::getKey)
                .collect(Collectors.toList());
        System.out.println(nos);
    }