最近接触了一些stream的东西，顺便写了一些例子，保存下来，代码如下：

 static class Town{
        int area = 0;
        String name;

        public int getArea() {
            return area;
        }

        public void setArea(int area) {
            this.area = area;
        }

        public String getName() {
            return name;
        }

        public void setName(String name) {
            this.name = name;
        }
    }

    public static void main(String[] args) {
        List<Town> townList = new ArrayList<>();
        for (int i = 0;i<10;i++){
            Town temp = new Town();
            temp.setArea(2);
            townList.add(temp);
        }

        //求和
        long areaSum = townList.stream().mapToInt(i->{
            //这里可以写一些业务处理逻辑
            i.getArea();
            return i.getArea();
        }).sum();
        System.out.println(areaSum);

        //另一种写法
        long areaSum1 = townList.stream().mapToInt(Town::getArea).sum();
        System.out.println(areaSum1);

        System.out.println(Stream.of("hello", "world", "ww", "java").
                collect(Collectors.toMap(o->{
                    //可以写自己的业务逻辑
                    o.toString();
                    return o.toLowerCase();
                },o->o.toUpperCase())));

        //简单写法
        System.out.println(Stream.of("hello", "world", "ww", "java").
                collect(Collectors.toMap(String::toLowerCase,String::toUpperCase)));


        //分组
        Map<Integer,List<Town>> map = townList.stream().
                collect(Collectors.groupingBy(t->t.getArea()));
        System.out.println(map);

    }

今天又遇到了一个复杂类型集合去重和字符串拼接，先记下来：

//查询优惠券关联的活动
            List<Map<String,Object>> actions = dal_mkt_cou_type.getListAutoType("getActByCoupon",info);

String infomation = actions.stream()
                        .filter(distinctByKey(i->i.get("action_id")))
                        .map(p -> {
                            StringBuffer str = new StringBuffer();
                            str.append(p.get("action_id"))
                                    .append(",")
                                    .append(p.get("action_name"));
                            return str.toString();
                        }).collect(Collectors.joining(";", "【", "】"));


 public <T> Predicate<T> distinctByKey(Function<? super T, ?> keyExtractor) {
        Set<Object> seen = ConcurrentHashMap.newKeySet();
        return t -> seen.add(keyExtractor.apply(t));
    }