BigDecimal 进行四舍五入 四舍六入和保留两位小数

舍入模式向“最近的邻居”舍入，除非两个邻居是等距的，在这种情况下向上舍入。 如果丢弃的分数 ≥ 0.5，则与 ROUND_UP 一样； 否则，行为与 ROUND_DOWN 相同。 请注意，这是我们大多数人在小学时所教的舍入模式。

    /**
     * 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;

舍入模式向“最近的邻居”舍入，除非两个邻居是等距的，在这种情况下向下舍入。 如果丢弃的分数 > 0.5，则表现与 ROUND_UP 相同； 否则，行为与 ROUND_DOWN 相同。

    /**
     * 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;

保留小数位

返回一个 BigDecimal，其标度为指定值，其未标度值通过将此 BigDecimal 的未标度值乘以或除以适当的 10 次方来确定，以保持其整体值。 如果通过操作缩小比例，则必须将未缩放的值除（而不是相乘），并且值可能会改变； 在这种情况下，指定的舍入模式应用于除法。

    /**
     * Returns a {@code BigDecimal} whose scale is the specified
     * value, and whose unscaled value is determined by multiplying or
     * dividing this {@code BigDecimal}'s unscaled value by the
     * appropriate power of ten to maintain its overall value.  If the
     * scale is reduced by the operation, the unscaled value must be
     * divided (rather than multiplied), and the value may be changed;
     * in this case, the specified rounding mode is applied to the
     * division.
     *
     * <p>Note that since BigDecimal objects are immutable, calls of
     * this method do <i>not</i> result in the original object being
     * modified, contrary to the usual convention of having methods
     * named <tt>set<i>X</i></tt> mutate field <i>{@code X}</i>.
     * Instead, {@code setScale} returns an object with the proper
     * scale; the returned object may or may not be newly allocated.
     *
     * <p>The new {@link #setScale(int, RoundingMode)} method should
     * be used in preference to this legacy method.
     *
     * @param  newScale scale of the {@code BigDecimal} value to be returned.
     * @param  roundingMode The rounding mode to apply.
     * @return a {@code BigDecimal} whose scale is the specified value,
     *         and whose unscaled value is determined by multiplying or
     *         dividing this {@code BigDecimal}'s unscaled value by the
     *         appropriate power of ten to maintain its overall value.
     * @throws ArithmeticException if {@code roundingMode==ROUND_UNNECESSARY}
     *         and the specified scaling operation would require
     *         rounding.
     * @throws IllegalArgumentException if {@code roundingMode} does not
     *         represent a valid rounding mode.
     * @see    #ROUND_UP
     * @see    #ROUND_DOWN
     * @see    #ROUND_CEILING
     * @see    #ROUND_FLOOR
     * @see    #ROUND_HALF_UP
     * @see    #ROUND_HALF_DOWN
     * @see    #ROUND_HALF_EVEN
     * @see    #ROUND_UNNECESSARY
     */
    public BigDecimal setScale(int newScale, int roundingMode) {
        if (roundingMode < ROUND_UP || roundingMode > ROUND_UNNECESSARY)
            throw new IllegalArgumentException("Invalid rounding mode");

        int oldScale = this.scale;
        if (newScale == oldScale)        // easy case
            return this;
        if (this.signum() == 0)            // zero can have any scale
            return zeroValueOf(newScale);
        if(this.intCompact!=INFLATED) {
            long rs = this.intCompact;
            if (newScale > oldScale) {
                int raise = checkScale((long) newScale - oldScale);
                if ((rs = longMultiplyPowerTen(rs, raise)) != INFLATED) {
                    return valueOf(rs,newScale);
                }
                BigInteger rb = bigMultiplyPowerTen(raise);
                return new BigDecimal(rb, INFLATED, newScale, (precision > 0) ? precision + raise : 0);
            } else {
                // newScale < oldScale -- drop some digits
                // Can't predict the precision due to the effect of rounding.
                int drop = checkScale((long) oldScale - newScale);
                if (drop < LONG_TEN_POWERS_TABLE.length) {
                    return divideAndRound(rs, LONG_TEN_POWERS_TABLE[drop], newScale, roundingMode, newScale);
                } else {
                    return divideAndRound(this.inflated(), bigTenToThe(drop), newScale, roundingMode, newScale);
                }
            }
        } else {
            if (newScale > oldScale) {
                int raise = checkScale((long) newScale - oldScale);
                BigInteger rb = bigMultiplyPowerTen(this.intVal,raise);
                return new BigDecimal(rb, INFLATED, newScale, (precision > 0) ? precision + raise : 0);
            } else {
                // newScale < oldScale -- drop some digits
                // Can't predict the precision due to the effect of rounding.
                int drop = checkScale((long) oldScale - newScale);
                if (drop < LONG_TEN_POWERS_TABLE.length)
                    return divideAndRound(this.intVal, LONG_TEN_POWERS_TABLE[drop], newScale, roundingMode,
                                          newScale);
                else
                    return divideAndRound(this.intVal,  bigTenToThe(drop), newScale, roundingMode, newScale);
            }
        }
    }