java小技巧

String 转 Date

String classCode = RequestHandler.getString(request, "classCode");
SimpleDateFormat sdf = new SimpleDateFormat("yyyy-mm-dd hh:mm:ss");
DicClassInfo classInfo=new DicClassInfo();
classInfo.setStartDate(sdf.parse(startDate));

int型除法保留两位小数/求百分比

int openCount = temp.getIsOpenCount();
int passCount = temp.getIsPassCount();
double k = (double)passCount/openCount*100;
java.math.BigDecimal   big   =   new   java.math.BigDecimal(k);
String  l =big.setScale(2,java.math.BigDecimal.ROUND_HALF_UP).doubleValue() +"%";

float保留两位小数

//1
float f = 34.232323;
BigDecimal b = new BigDecimal(f);
float f1 = b.setScale(2, BigDecimal.ROUND_HALF_UP).floatValue();
// b.setScale(2, BigDecimal.ROUND_HALF_UP) 表明四舍五入，保留两位小数 

//2
float scale = 34.236323;
DecimalFormat fnum = new DecimalFormat("##0.00");
String dd=fnum.format(scale);
System.out.println(dd);

//3
float a = 123.2334f;
float b = (float)(Math.round(a*100))/100;

BigDecimal保留两位小数

//金额保留小数后两位
BigDecimal  b  =   invoiceTemp.getInvoicePrice();
BigDecimal setScale = b.setScale(2,BigDecimal.ROUND_DOWN);
invoiceTemp.setInvoicePrice(setScale);

参数定义

ROUND_CEILING 向正无穷方向舍入 

ROUND_DOWN 向零方向舍入 

ROUND_FLOOR 向负无穷方向舍入 

ROUND_HALF_DOWN 向（距离）最近的一边舍入，除非两边（的距离）是相等,如果是这样，向下舍入, 例如1.55 保留一位小数结果为1.5 

ROUND_HALF_EVEN 向（距离）最近的一边舍入，除非两边（的距离）是相等,如果是这样，如果保留位数是奇数，使用ROUND_HALF_UP
，如果是偶数，使用ROUND_HALF_DOWN 

ROUND_HALF_UP 向（距离）最近的一边舍入，除非两边（的距离）是相等,如果是这样，向上舍入,
1.55保留一位小数结果为1.6 

ROUND_UNNECESSARY 计算结果是精确的，不需要舍入模式 

ROUND_UP 向远离0的方向舍入

Action文件mapping.findForward()动态传参至Struts-config.xml

//后台方法
return mapping.findForward(outSystemSign);

//配置文件无需判断，直接接收，直接写明映射内容即可
<forward name="qgtzsbcjh" path="/WEB-INF/jsp/outsystem/kpflowchart/qgtzsbcjh.jsp"/>
<forward name="gusuwx" path="/WEB-INF/jsp/outsystem/kpflowchart/gusuwx.jsp"/>    

isBlank、isEmpty、isNull

org.apache.commons.lang.StringUtils类提供了String的常用操作,最常用判空如下

• StringUtils.isEmpty(String str)

//判断某字符串是否为空，标准是 str==null 或 str.length()==0
System.out.println(StringUtils.isEmpty(null));        //true
System.out.println(StringUtils.isEmpty(""));          //true
System.out.println(StringUtils.isEmpty("   "));       //false
System.out.println(StringUtils.isEmpty("dd"));        //false

• StringUtils.isNotEmpty(String str) 等价于 !isEmpty(String str),判断是否不为空也不为null，在要替换字段为空串时，应使用isNotNull，isNotEmpty会自动跳过
• StringUtils.isBlank(String str)，StringUtils.isNotBlank(String str) 等价于 !isBlank(String str)

//判断某字符串是否为空或长度为0或由空白符(whitespace) 构成System.out.println(StringUtils.isBlank(null));     //true
System.out.println(StringUtils.isBlank(""));          //true
System.out.println(StringUtils.isBlank("   "));       //true
System.out.println(StringUtils.isBlank("dd"));        //false   

时间段控制（00:00:00—23:59:59）

if(null!=startDate){
    int len = startDate.length();
    switch(len){
        case(10):startDate = startDate + " 00:00:00.000";break;
        case(19):startDate = startDate + ".000";break;
    　　 default:if(len>10){startDate = startDate.substring(0,10)+"00:00:00.000";}break;
    }
}

if(null!=endDate){
    int len = endDate.length();
    switch(len){
        case(10):endDate = endDate + " 23:59:59.999";break;
        case(19):startDate = startDate + ".999";break;
        default:if(len>10){startDate = startDate.substring(0,10)+"23:59:59.999";}break;
    }
}

计算时间差

String date = "2010-01-01 11:11:11";
SimpleDateFormat df = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");    String nowDate = df.format(new Date());
Date d1 = df.parse(date);
Date d2 = df.parse(nowDate);
long between = (d2.getTime() - d1.getTime())/1000; //时间差（秒）

获取指定时间

//当前月第一天
Calendar c = Calendar.getInstance();
c.setTime(new Date());
c.set(5, 1);
Date startMonthDate =c.getTime()；
String startMonthStr = dateToString(startMonthDate , "yyyy-MM-dd 00:00:00");

//当前月最后一天
Calendar calendar = Calendar.getInstance();
calendar.setTime(startMonthDate );
calendar.add(2, n);
Date endMonthDate=calendar .getTime()；
String endMonthStr = dateToString(endMonthDate, "yyyy-MM-dd 00:00:00");

//七天前
Calendar calendar = Calendar.getInstance();
calendar.set(Calendar.DAY_OF_YEAR, calendar.get(Calendar.DAY_OF_YEAR) - 7);
Date earlySevenDate = calendar.getTime();
String earlySevenStr = dateToString(earlySevenDate, "yyyy-MM-dd 00:00:00");

//当天
String todayStr = dateToString(new Date(), "yyyy-MM-dd 00:00:00");

//Date date, int n   date时间加n天
Calendar calendar = Calendar.getInstance();
calendar.setTime(date);
calendar.add(5, n);
Date nDayLaterDate = calendar.getTime();
String earlySevenStr = dateToString(nDayLaterDate, "yyyy-MM-dd 00:00:00");

//Date date, int n   date时间加n月
Calendar calendar = Calendar.getInstance();
calendar.setTime(date);
calendar.add(2, n);
Date nMonthLaterDate = calendar.getTime();
String earlySevenStr = dateToString(nMonthLaterDate, "yyyy-MM-dd 00:00:00");

public static String dateToString(Date date, String pattern) {
        if (date != null) {
            SimpleDateFormat sdf = new SimpleDateFormat(pattern);
            return sdf.format(date);
        } else {
            return "";
        }
}

//Calendar的add()方法：1是对年份操作，2是对月份操作，3是对星期操作，5是对日期操作，11是对小时操作，12是对分钟操作，13是对秒操作，14是对毫秒操作。

String数组、List转换

// List转String数组
List<String> list = new ArrayList<String>();
list.add("a1");
list.add("a2");
String[] toBeStored = list.toArray(new String[list.size()]);    

for(String s : toBeStored) {
　　System.out.println(s);
}  

//String数组转List
String[] arr = new String[] {"1", "2"};
List<String> list = Arrays.asList(arr);  

excel导出合并单元格（行）、设置样式   

<%@page import="org.apache.poi.hssf.util.Region"%>
<%@page import="org.apache.poi.hssf.usermodel.HSSFFont"%>
<%@page import="org.apache.poi.hssf.util.HSSFColor"%>

HSSFCellStyle cellStyle = wb.createCellStyle();//新样式//居中
cellStyle.setAlignment(HSSFCellStyle.ALIGN_CENTER); //居中
HSSFFont font=wb.createFont();
font.setColor(HSSFColor.RED.index);//字体颜色
font.setFontHeightInPoints((short)12);
font.setBoldweight(HSSFFont.BOLDWEIGHT_BOLD);//字体增粗
cellStyle.setFont(font);//把字体应用到当前的样式
cell.setCellStyle(cellStyle);
//合并第（0，0）单元格到第（0,13）单元格
sheet.addMergedRegion(new Region(0, (short)0, 0, (short)13));
if(0 != total){
　　cell.setCellValue("支付金额合计 : "+total+"元");
}

excel动态合并单元格（列）

short num = 1;  //列
String s = "";
//sheet.getLastRowNum()获取总行数
for (int k = 1; k <= sheet.getLastRowNum(); k++) {
  HSSFRow rows = sheet.getRow(k);
  HSSFCell cells = rows.getCell((short)2);
  if (cells.getStringCellValue().equals(s)) {
      sheet.addMergedRegion(new Region(num,(short)2,k,(short)2));
      HSSFRow rowsV = sheet.getRow(k);
      HSSFCell cellsV = rows.getCell((short)2);
      cellsV.setCellStyle(style);
  } else {
      num =(short) k;
}
  s = cells.getStringCellValue();
}