double精确位数摘记
double精确位数摘录
突然需要对double的数字做精确记录，go之发现网上给出了很好的答案，故摘录在此，谢谢网上的各处答疑。
出自如下2处 ：
1： http://topic.****.net/t/20030318/12/1544852.html
2： http://bbs.chinaunix.net/viewthread.php?tid=723528
一：利用NumberFormat
import java.text.*;
public class Test{
public static void main(String[]args){
float value=1.23456789f;
NumberFormat numFormat = NumberFormat.getNumberInstance();
numFormat.setMaximumFractionDigits(2);
String str = numFormat.format(value);
System.out.println(str);
}
}
说明下，默认的是会做四舍五入了。例如1.236 取2位得到的是 1.24 ，如果取0 则默认是取整操作了，另外传递负数也是取整，因为max（0,value）
二：利用BigDecimal
import java.math.BigDecimal; String str = String.valueOf((new BigDecimal(1.234567)).setScale(2,BigDecimal.ROUND_HALF_UP));
BigDecimal提供了如下几种round modes
// Rounding Modes
/**
* Rounding mode to round away from zero. Always increments the
* digit prior to a nonzero discarded fraction. Note that this rounding
* mode never decreases the magnitude of the calculated value.
*/
public final static int ROUND_UP = 0;
/**
* Rounding mode to round towards zero. Never increments the digit
* prior to a discarded fraction (i.e., truncates). Note that this
* rounding mode never increases the magnitude of the calculated value.
*/
public final static int ROUND_DOWN = 1;
/**
* Rounding mode to round towards positive infinity. If the
* {@code BigDecimal} is positive, behaves as for
* {@code ROUND_UP}; if negative, behaves as for
* {@code ROUND_DOWN}. Note that this rounding mode never
* decreases the calculated value.
*/
public final static int ROUND_CEILING = 2;
/**
* Rounding mode to round towards negative infinity. If the
* {@code BigDecimal} is positive, behave as for
* {@code ROUND_DOWN}; if negative, behave as for
* {@code ROUND_UP}. Note that this rounding mode never
* increases the calculated value.
*/
public final static int ROUND_FLOOR = 3;
/**
* Rounding mode to round towards {@literal "nearest neighbor"}
* unless both neighbors are equidistant, in which case round up.
* Behaves as for {@code ROUND_UP} if the discarded fraction is
* ≥ 0.5; otherwise, behaves as for {@code ROUND_DOWN}. Note
* that this is the rounding mode that most of us were taught in
* grade school.
*/
public final static int ROUND_HALF_UP = 4;
/**
* Rounding mode to round towards {@literal "nearest neighbor"}
* unless both neighbors are equidistant, in which case round
* down. Behaves as for {@code ROUND_UP} if the discarded
* fraction is {@literal >} 0.5; otherwise, behaves as for
* {@code ROUND_DOWN}.
*/
public final static int ROUND_HALF_DOWN = 5;
/**
* Rounding mode to round towards the {@literal "nearest neighbor"}
* unless both neighbors are equidistant, in which case, round
* towards the even neighbor. Behaves as for
* {@code ROUND_HALF_UP} if the digit to the left of the
* discarded fraction is odd; behaves as for
* {@code ROUND_HALF_DOWN} if it's even. Note that this is the
* rounding mode that minimizes cumulative error when applied
* repeatedly over a sequence of calculations.
*/
public final static int ROUND_HALF_EVEN = 6;
/**
* Rounding mode to assert that the requested operation has an exact
* result, hence no rounding is necessary. If this rounding mode is
* specified on an operation that yields an inexact result, an
* {@code ArithmeticException} is thrown.
*/
public final static int ROUND_UNNECESSARY = 7;
三 ：利用DecimalFormat
通过使用java.text package包中提供的类型，将数字类型装换成指定的格式。
http://www.javaalmanac.com/egs/index.html
// The 0 symbol shows a digit or 0 if no digit present
NumberFormat formatter = new DecimalFormat("000000");
String s = formatter.format(-1234.567); // -001235
// notice that the number was rounded up
// The # symbol shows a digit or nothing if no digit present
formatter = new DecimalFormat("##");
s = formatter.format(-1234.567); // -1235
s = formatter.format(0); // 0
formatter = new DecimalFormat("##00");
s = formatter.format(0); // 00
// The . symbol indicates the decimal point
formatter = new DecimalFormat(".00");
s = formatter.format(-.567); // -.57
formatter = new DecimalFormat("0.00");
s = formatter.format(-.567); // -0.57
formatter = new DecimalFormat("#.#");
s = formatter.format(-1234.567); // -1234.6
formatter = new DecimalFormat("#.######");
s = formatter.format(-1234.567); // -1234.567
formatter = new DecimalFormat(".######");
s = formatter.format(-1234.567); // -1234.567
formatter = new DecimalFormat("#.000000");
s = formatter.format(-1234.567); // -1234.567000
// The , symbol is used to group numbers
formatter = new DecimalFormat("#,###,###");
s = formatter.format(-1234.567); // -1,235
s = formatter.format(-1234567.890); // -1,234,568
// The ; symbol is used to specify an alternate pattern for negative values
formatter = new DecimalFormat("#;(#)");
s = formatter.format(-1234.567); // (1235)
// The ' symbol is used to quote literal symbols
formatter = new DecimalFormat("'#'#");
s = formatter.format(-1234.567); // -#1235
formatter = new DecimalFormat("'abc'#");
s = formatter.format(-1234.567); // -abc1235
再次对网上各位答仁表示感谢。