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  Java提供的API类BigDecimal，用来对超过16位有效位的数进行精确的运算。双精度浮点型变量double可以处理16位有效数。在实际应用中，需要对更大或者更小的数进行运算和处理。(BigDecimal Java API classes provided for more than 16 significant bits of the number of precise calculations. Double-precision floating-point variables can handle double the number of 16-bit effective. In practice, the need for a larger or smaller number of computing and processing.)