BigDecimal.toDouble 如何工作？

Question

I am using BigDecimal and doubles in my math program. What decimal places does this method take? Is it fast? Is this recommended over just using BigDecimal?

Answer 1

Q: How does BigDecimal.toDouble work?

Multiplications and additions. Take a look at the source code . (You can find it by googling for "java.math.BigDecimal source".)

Q: What decimal places does this method take?

Here is what the javadoc states:

"Converts this BigDecimal to a double. This conversion is similar to the narrowing primitive conversion from double to float as defined in section 5.1.3 of The Java™ Language Specification: if this BigDecimal has too great a magnitude represent as a double, it will be converted to Double.NEGATIVE_INFINITY or Double.POSITIVE_INFINITY as appropriate."

5.1.3 of the JLS states:

"A narrowing primitive conversion from double to float is governed by the IEEE 754 rounding rules (§4.2.4). This conversion can lose precision, but also lose range, resulting in a float zero from a nonzero double and a float infinity from a finite double. A double NaN is converted to a float NaN and a double infinity is converted to the same-signed float infinity."

And 4.2.4 states:

"The Java programming language requires that floating-point arithmetic behave as if every floating-point operator rounded its floating-point result to the result precision. Inexact results must be rounded to the representable value nearest to the infinitely precise result; if the two nearest representable values are equally near, the one with its least significant bit zero is chosen. This is the IEEE 754 standard's default rounding mode known as round to nearest."

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So what does this mean?

1. If the number is representable, the BigDecimal.doubleValue() will give the double value that is closest to the Real number represented by the object.

2. If the number is not representable (ie it is too large or too small to be represented as a double ) then the method will give +INF or -INF.

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Q: Is it fast?

It is typically roughly as fast as converting a string representation of the number to a double . (Indeed, in some cases the conversion is actually done by converting the value to a string and then parsing it!)

Fast is a relative term.

Q: Is this recommended over just using BigDecimal?

Recommended for what purpose?

If you convert BigDecimal values to double values, then operations on the double values will be orders of magnitude faster than the analogous BigDecimal operations. But the results won't be as accurate. (Assuming that you specify the BigDecimal precision appropriately.)