String.format其實是調用的Formatter.format：

public static String format(String format, Object... args) {return new Formatter().format(format, args).toString();}
第一個參數是格式化字符串，第二個參數是可變的被格式化的參數。

我們主要看的是格式化參數的說明：

• The format specifiers for general, character, and numeric types have the following syntax: %[argument_index$][flags][width][.precision]conversion

這個參數是用來格式化通用的、字符串、數字類型，主要說一下每個參數的作用：
% 用于表示這后面將是一個格式化數據的模板，
argument_index$ 表示要作用在第幾個參數上，注意一定要加上'$'，
flags 以下是flag的對照表，對不同的類型有不同的作用：

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

  FlagGeneralCharacterIntegralFloating PointDate/TimeDescription

  '-'	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： ConversionArgument CategoryDescription

'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?

示例代碼：

package main;public class StringFormat {public static void main(String[] args) {// TODO Auto-generated method stubString 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 標準表達式}}

運行結果：

Haha.100.000fdsafdsmain.StringFormat@6d6f6e28

總結

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