java - 为什么 BigDecimal 构造函数实例具有不同的值？

Question

我不知道为什么会发生这种情况:

float flo = 196.7f;
BigDecimal bd = new BigDecimal(flo);
System.out.println(bd); //print 196.6999969482421875

BigDecimal bd2 = new BigDecimal(Float.toString(flo));
System.out.println(bd2); //print 196.7

Answer 1

您的两个问题在 Javadoc 中都有明确说明。

1. The results of this constructor can be somewhat unpredictable. One might assume that writing new BigDecimal(0.1) in Java creates a BigDecimal which is exactly equal to 0.1 (an unscaled value of 1, with a scale of 1), but it is actually equal to 0.1000000000000000055511151231257827021181583404541015625. This is because 0.1 cannot be represented exactly as a double (or, for that matter, as a binary fraction of any finite length). Thus, the value that is being passed in to the constructor is not exactly equal to 0.1, appearances notwithstanding.
2. The String constructor, on the other hand, is perfectly predictable: writing new BigDecimal("0.1") creates a BigDecimal which is exactly equal to 0.1, as one would expect. Therefore, it is generally recommended that the String constructor be used in preference to this one.