每日学习--java日期

1.BigDecimal.setScale()

详解：http://blog.youkuaiyun.com/guoyankun/article/details/12558261
double value=2222.22888888;
BigDecimal(value).setScale(2,BigDecimal.ROUND_HALF_UP) 可以得到2222.23

2.mysql数据库查询显示的日期格式

date_format(addtime,'%Y-%m-%d')='2015-05-02'
date_format(addtime,'%Y-%m')='2015-05'

3.mysql将多行数据合并为一行

select group_concat(list_name) from aa_list

group_concat有长度限制
详解：http://blog.youkuaiyun.com/yyzhq/article/details/12848327
concat:用来连接字段，或者字符。但是只要被连接的字段中有一个值为null，则concat得到的就是null
concat_ws:

select concat_ws(',','11','22','33');
//11,22,33 
select concat_ws(',','11','22',NULL);
//11,22

4.java获取本周的monday / sunday

public String getInfo(String time,int k) throws ParseException{
                    //time='2015-03-02'  传输进的日期
                    //k==1  return本周周一
                    //k==2  return本周周日
                    //k==3  return本月的天数
                    String monday="";
                    SimpleDateFormat sdf=new SimpleDateFormat("yyyy-MM-dd");
                    Date date=sdf.parse(time);
                    Calendar cal=Calendar.getInstance();
                    cal.setTime(date);
                    if(k==1){
                        cal.add(Calendar.DAY_OF_MONTH, -1);
                        cal.set(Calendar.DAY_OF_WEEK, Calendar.MONDAY);
                        monday=sdf.format(cal.getTime());
                    }else if(k==2){
                        cal.set(Calendar.DAY_OF_WEEK, Calendar.SUNDAY);
                        cal.add(Calendar.WEEK_OF_YEAR, 1);
                        monday=sdf.format(cal.getTime());
                    }else if(k==3){
                        monday=cal.getActualMaximum(Calendar.DAY_OF_MONTH)+"";
                    }
                    //System.out.println(monday);
                    return monday;
                }

5.List遍历

for(int i=0;i<a.size();i++){
    System.out.println(a.get(i).id);
}