Question

我写了这个来测试BigDecimal的实际操作，但发现RoundingMode.UNNECESSARY引发了异常。任何人都可以解释原因吗？

public class TestRounding2 {

    public static void main(String args[]) {

        Locale swedish = new Locale("sv", "SE");
        BigDecimal pp; //declare variable pp=pounds pence

        NumberFormat swedishFormat = NumberFormat.getCurrencyInstance(swedish);

        Scanner scan = new Scanner(System.in);
        System.out.println("ENTER POUNDS AND PENCE TO AT LEAST FIVE DECIMAL PLACES :");

        pp = scan.nextBigDecimal();

        BigDecimal pp1 = pp.setScale(2, RoundingMode.HALF_EVEN);
        System.out.println("HALF_EVEN: £ " + pp1.toString());
        System.out.println(swedishFormat.format(pp1));

        BigDecimal pp2 = pp.setScale(2, RoundingMode.FLOOR);
        System.out.println("FLOOR: £ " + pp2.toString());
        System.out.println(swedishFormat.format(pp2));

        BigDecimal pp3 = pp.setScale(2, RoundingMode.CEILING);
        System.out.println("CEILING £: " + pp3.toString());
        System.out.println(swedishFormat.format(pp3));

        BigDecimal pp4 = pp.setScale(2, RoundingMode.HALF_DOWN);
        System.out.println("HALF DOWN £: " + pp4.toString());
        System.out.println(swedishFormat.format(pp4));

        BigDecimal pp5 = pp.setScale(2, RoundingMode.HALF_UP);
        System.out.println("HALF UP: £ " + pp5.toString());
        System.out.println(swedishFormat.format(pp5));

        BigDecimal pp6 = pp.setScale(2, RoundingMode.UP);
        System.out.println("UP: £ " + pp6.toString());
        System.out.println(swedishFormat.format(pp6));

        BigDecimal pp7 = pp.setScale(2, RoundingMode.DOWN);
        System.out.println("DOWN: £ " + pp7.toString());
        System.out.println(swedishFormat.format(pp7));

        BigDecimal pp8 = pp.setScale(2, RoundingMode.UP);
        System.out.println("UP:  " + pp8.toString());
        System.out.println(swedishFormat.format(pp8));

    }
}

Answer 1

这是设计的。见javadoc：

舍入模式断言请求的操作具有精确性结果，因此不需要舍入。如果这种舍入模式是在产生不精确结果的操作上指定的，抛出{@code ArithmeticException}。

如果有要回合的话，可以使用此模式专门抛出异常。

实施例。下一个代码不会抛出异常。

    BigDecimal pp = new BigDecimal(7);
    pp.setScale(2, RoundingMode.UNNECESSARY);
    System.out.println(pp);

将7更改为小数会导致异常，因为它现在必须绕过它：

    BigDecimal pp = new BigDecimal(7.1);
    pp.setScale(2, RoundingMode.UNNECESSARY);
    // java.lang.ArithmeticException: Rounding necessary
    System.out.println(pp);

RoundingMode.UNNECESSARY抛出异常

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