记录对String.format(Formatter().format())方法的总结

String.format其实是调用的Formatter.format：

    public static String format(String format, Object... args) {
        return new Formatter().format(format, args).toString();
    }

第一个参数是格式化字符串，第二个参数是可变的被格式化的参数。

我们主要看的是格式化参数的说明：

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

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

这个参数是用来格式化通用的、字符串、数字类型，主要说一下每个参数的作用：

% 用于表示这后面将是一个格式化数据的模板，

argument_index$ 表示要作用在第几个参数上，注意一定要加上'$'，

flags 以下是flag的对照表，对不同的类型有不同的作用：

• The following table summarizes the supported flags. y means the flag is supported for the indicated argument types.

  Flag	General	Character	Integral	Floating Point	Date/Time	Description
  '-'	y	y	y	y	y	The result will be left-justified.
  '#'	y1	-	y3	y	-	The result should use a conversion-dependent alternate form
  '+'	-	-	y4	y	-	The result will always include a sign
  '  '	-	-	y4	y	-	The result will include a leading space for positive values
  '0'	-	-	y	y	-	The result will be zero-padded
  ','	-	-	y2	y5	-	The result will include locale-specific grouping separators
  '('	-	-	y4	y5	-	The result will enclose negative numbers in parentheses

  ¹ Depends on the definition of Formattable.

  ² For 'd' conversion only.

  ³ For 'o', 'x', and 'X' conversions only.

  ⁴ For 'd', 'o', 'x', and 'X' conversions applied to BigInteger or 'd' applied to byte, Byte, short, Short, int and Integer, long, and Long.

  ⁵ For 'e', 'E', 'f', 'g', and'G' conversions only.

  Any characters not explicitly defined as flags are illegal and are reserved for future extensions. 

• Width

  The width is the minimum number of characters to be written to the output. For the line separator conversion, width is not applicable; if it is provided, an exception will be thrown.

  Precision

  For general argument types, the precision is the maximum number of characters to be written to the output.

  For the floating-point conversions 'a', 'A', 'e','E', and 'f' the precision is the number of digits after the radix point. If the conversion is'g' or 'G', then the precision is the total number of digits in the resulting magnitude after rounding.

  For character, integral, and date/time argument types and the percent and line separator conversions, the precision is not applicable; if a precision is provided, an exception will be thrown.

  Argument Index

  The 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.

  Another way to reference arguments by position is to use the '<' ('\u003c') flag, which causes the argument for the previous format specifier to be re-used. For example, the following two statements would produce identical strings:

     Calendar c = ...;
     String s1 = String.format("Duke's Birthday: %1$tm %1$te,%1$tY", c);
  
     String s2 = String.format("Duke's Birthday: %1$tm %<te,%<tY", c);

最重要的是conversion：

Conversion	Argument Category	Description
'b', 'B'	general	If the argument arg is null, then the result is "false". Ifarg is a boolean or Boolean, then the result is the string returned by String.valueOf(arg). Otherwise, the result is "true".
'h', 'H'	general	If the argument arg is null, then the result is "null". Otherwise, the result is obtained by invokingInteger.toHexString(arg.hashCode()).
's', 'S'	general	If the argument arg is null, then the result is "null". Ifarg implements Formattable, then arg.formatTo is invoked. Otherwise, the result is obtained by invokingarg.toString().
'c', 'C'	character	The result is a Unicode character
'd'	integral	The result is formatted as a decimal integer
'o'	integral	The result is formatted as an octal integer
'x', 'X'	integral	The result is formatted as a hexadecimal integer
'e', 'E'	floating point	The result is formatted as a decimal number in computerized scientific notation
'f'	floating point	The result is formatted as a decimal number
'g', 'G'	floating point	The result is formatted using computerized scientific notation or decimal format, depending on the precision and the value after rounding.
'a', 'A'	floating point	The result is formatted as a hexadecimal floating-point number with a significand and an exponent. This conversion isnot supported for the BigDecimal type despite the latter's being in thefloating point argument category.
't', 'T'	date/time	Prefix for date and time conversion characters. See Date/Time Conversions.
'%'	percent	The result is a literal '%' ('\u0025')
'n'	line separator	The result is the platform-specific line separator

示例代码：

package main;

public class StringFormat {

	public static void main(String[] args) {
		// TODO Auto-generated method stub

		String format = "Haha.%1$(g%2$s%3$s";
		Object obj = 100f, object1 = "fdsafds", object2 = new StringFormat();
		String format2 = String.format(format, obj, object1, object2);
		System.out.println(format2);

		// String Format總結
		// %[argument_index$][flags][width][.precision]conversion 标准表达式

	}

}

运行结果：

Haha.100.000fdsafdsmain.StringFormat@6d6f6e28