Java浮点数除法，整数除得⼩数，指定⼩数位数
在Java中如果除运算符“/”，在不加任何限制的情况下，两个整数相除，得到的是整数，⼩数点后的被舍弃。但是有些场景下我们需要拿到除得的⼩数，还要指定位数的⼩数。这时候有以下处理⽅法：
1.使⽤DecimalFormat来限定得到的⼩数位数
int pcm = 98;
int fcm = 11;
DecimalFormat df = new DecimalFormat("0.00");
double tmpVal = Double.parseDouble(df.format((double) pcm/(pcm+fcm)));
//get value 0.89
注意，它默认返回的是String，如果需要double/float要做⼀下转换。
2.直接使⽤Decimal运算
@Test
public void testDecimalOper(){
int pcm = 94;
int fcm = 11;
BigDecimal pcmbd = new BigDecimal(pcm);
BigDecimal fcmbd = new BigDecimal(fcm);
BigDecimal rate = new BigDecimal(0.00);
rate = pcmbd.divide(pcmbd.add(fcmbd), 2, RoundingMode.HALF_UP);
System.out.println(rate);//0.90
}
float/double在⼯程运算中使⽤的⽐较多，在商业计算中使⽤Decimal类型的⽐较多。（注：
在《Effective Java》这本书中也提到这个原则，float和double只能⽤来做科学计算或者是⼯程计算，
在商业计算中我们要⽤java.math.BigDecimal，另外，我们如果需要精确计算，要⽤String来够造BigDecimal。在《Effective Java》⼀书中的例⼦是⽤String来够造BigDecimal的。（注意：divide⽅法中推荐使⽤枚举RoundingMode.HALF_UP）
）
两种⽅式都可以。推荐使⽤第⼆种⽅式来处理精度和round mode的设置。
附BigDecimal rouding mode:
/**
* 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
bigdecimal除法保留小数* 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;

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