java - BigDecimal 不正确的舍入

Question

我有这个除法结果:

4893.305785123966785163027491292661448576748

我想四舍五入到 4893.30，但它总是返回 4893.31。但当第二位小数为 6 或更多时，我想四舍五入。

new BigDecimal("4893.305785123966785163027491292661448576748").setScale(2,BigDecimal.ROUND_HALF_DOWN)); to 4893.31 //BAD

new BigDecimal("4893.305").setScale(2,BigDecimal.ROUND_HALF_DOWN)); to 4893.30 //OK

new BigDecimal("4893.3055").setScale(2,BigDecimal.ROUND_HALF_DOWN)); to 4893.31 //BAD

回合如何进行？我只想考虑第二位小数的值。有什么想法吗？

Answer 1

"How does the round work? I want to take in consideration only the value of the second decimal."

听起来您想截断而不是舍入。

而不是使用 ROUND_HALF_DOWN :

Rounding mode to round towards "nearest neighbor"...

...相反，尝试使用 ROUND_DOWN :

Rounding mode to round towards zero. Never increments the digit prior to a discarded fraction (i.e., truncates)...

例如

    BigDecimal bd = new BigDecimal("4893.305785123966785163027491292661448576748");
    bd = bd.setScale(2, BigDecimal.ROUND_DOWN);
    System.out.println(bd); // 4893.30

更新回答问题编辑

如果您只想考虑某些小数位，那就是截断后跟四舍五入:

    BigDecimal bd = new BigDecimal("4893.305785123966785163027491292661448576748");
    bd = bd.setScale(3, BigDecimal.ROUND_DOWN);       // truncate to 4893.305
    bd = bd.setScale(2, BigDecimal.ROUND_HALF_DOWN);  // round to 2 decimal places
    System.out.println(bd);                           // 4893.30

    bd = new BigDecimal("4893.306785123966785163027491292661448576748");
    bd = bd.setScale(3, BigDecimal.ROUND_DOWN);       // truncate to 4893.306
    bd = bd.setScale(2, BigDecimal.ROUND_HALF_DOWN);  // round to 2 decimal places
    System.out.println(bd);                           // 4893.31