java - 为什么 `%4.` 在我的号码中添加空格？

Question

我有这个代码:

private String padWithZeroRightToPeriod(String serverFormat, float unformattedNumber, int index) {
    int nDigits = getNumberOfDigitsAfterPeriod(serverFormat);
    String floatingFormat = "%4." + nDigits + "f";
    String formattedPrice = String.format(floatingFormat, unformattedNumber);

当调用unformattedNumber为846时，为什么结果是“846”(一个空格，然后是三个数字)？

%4. 是什么意思？

Answer 1

docs for String.format请您参阅this documentation on format strings ，其中表示:

The format specifiers for general, character, and numeric types have the following syntax:

%[argument_index$][flags][width][.precision]conversion

The optional argument_index is a decimal integer indicating the position of the argument in the argument list. The first argument is referenced by "1$", the second by "2$", etc.

The optional flags is a set of characters that modify the output format. The set of valid flags depends on the conversion.

The optional width is a positive decimal integer indicating the minimum number of characters to be written to the output.

The optional precision is a non-negative decimal integer usually used to restrict the number of characters. The specific behavior depends on the conversion.

The required conversion is a character indicating how the argument should be formatted. The set of valid conversions for a given argument depends on the argument's data type.

从您的示例输出来看，getNumberOfDigitsAfterPeriod 似乎正在返回 0，因此您的格式字符串为 %4.0f。所以:

• 它没有参数索引(4后面没有$)
• 它有一个宽度 (4)
• 它具有精度 (0)
• 当然它有一个转换 (f)

因此 846 输出为 "846" 因为宽度为 4。所讨论的宽度是输出的字符总数。这是一个不同的示例:Live Copy

public class Example {

    public static void main(String args[]) {
        System.out.println(String.format("%8.2f", 846.0));
        System.out.println(String.format("%8.2f", 42.4));
    }
}

输出:

  846.00
   42.40

请注意，每个字符长为八个字符。