Java Math RoundingMode.HALF_EVEN 不同的结果

Question

看来 HALF_EVEN 舍入模式在 Java DecimalFormat 中的工作方式与 BigDecimal 不同。有没有办法使 DecimalFormat 保持一致？

// Using BigDecimal scale
System.out.println("Big decimal scale (HALF_EVEN) of 21.255 ==> " + new BigDecimal("21.255").setScale(2, RoundingMode.HALF_EVEN));
System.out.println("Big decimal scale (HALF_EVEN) of 21.265 ==> " + new BigDecimal("21.265").setScale(2, RoundingMode.HALF_EVEN));

// Using DecimalFormat
DecimalFormat cdf = new DecimalFormat("#,##0.00");
cdf.setRoundingMode(RoundingMode.HALF_EVEN); // default anyway
System.out.println("Decimal format (HALF_EVEN) of 21.255 ==> " + cdf.format(21.255));
System.out.println("Decimal format (HALF_EVEN) of 21.265 ==> " + cdf.format(21.265));

Output:
Big decimal scale (HALF_EVEN) of 21.255 ==> 21.26
Big decimal scale (HALF_EVEN) of 21.265 ==> 21.26
Decimal format (HALF_EVEN) of 21.255 ==> 21.25
Decimal format (HALF_EVEN) of 21.265 ==> 21.27

Answer 1

如评论中所述，如果您尝试:

Double double1 = new Double(21.255);
BigDecimal bigDecimal1 = new BigDecimal(double1);
System.out.println(bigDecimal1);  //21.254999999999999005240169935859739780426025390625

Double double2 = new Double(21.265);
BigDecimal bigDecimal2 = new BigDecimal(double2);
System.out.println(bigDecimal2); //21.2650000000000005684341886080801486968994140625

你会发现:

• double 21.255 比 21.255
小一点
• double 21.265 比 21.265
大一点

当使用 DecimalFormat 时，您可以将输入初始化为 BigDecimal 以避免丢失准确性:

System.out.println("Decimal format (HALF_EVEN) of 21.255 ==> " + cdf.format(new BigDecimal("21.255")));
//21.26
System.out.println("Decimal format (HALF_EVEN) of 21.265 ==> " + cdf.format(new BigDecimal("21.265")));
//21.26