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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$][flags][width][.precision]conversion
-
The following table summarizes the supported flags. y means the flag is supported for the indicated argument types.
Flag General Character Integral Floating Point Date/Time Description '-' y y y y y The result will be left-justified. '#' y1 - y3 y - The result should use a conversion-dependent alternate form '+' - - y4 y - The result will always include a sign ' ' - - y4 y - The result will include a leading space for positive values '0' - - y y - The result will be zero-padded ',' - - y2 y5 - The result will include locale-specific grouping separators '(' - - y4 y5 - The result will enclose negative numbers in parentheses 1 Depends on the definition of
Formattable.2 For
'd'conversion only.3 For
'o','x', and'X'conversions only.4 For
'd','o','x', and'X'conversions applied toBigIntegeror'd'applied tobyte,Byte,short,Short,intandInteger,long, andLong.5 For
'e','E','f','g', and'G'conversions only.Any characters not explicitly defined as flags are illegal and are reserved for future extensions.
-
Width
The width is the minimum number of characters to be written to the output. For the line separator conversion, width is not applicable; if it is provided, an exception will be thrown.
Precision
For general argument types, the precision is the maximum number of characters to be written to the output.
For the floating-point conversions
'a','A','e','E', and'f'the precision is the number of digits after the radix point. If the conversion is'g'or'G', then the precision is the total number of digits in the resulting magnitude after rounding.For character, integral, and date/time argument types and the percent and line separator conversions, the precision is not applicable; if a precision is provided, an exception will be thrown.
Argument Index
The argument index is a decimal integer indicating the position of the argument in the argument list. The first argument is referenced by "
1$", 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$tm %<te,%<tY", c);
| Conversion | Argument Category | Description |
|---|---|---|
'b', 'B' | general | If the argument arg is null, then the result is "false". Ifarg is a boolean or Boolean, then the result is the string returned by String.valueOf(arg). Otherwise, the result is "true". |
'h', 'H' | general | If the argument arg is null, then the result is "null". Otherwise, the result is obtained by invokingInteger.toHexString(arg.hashCode()). |
's', 'S' | general | If the argument arg is null, then the result is "null". Ifarg implements Formattable, then arg.formatTo is invoked. Otherwise, the result is obtained by invokingarg.toString(). |
'c', 'C' | character | The result is a Unicode character |
'd' | integral | The result is formatted as a decimal integer |
'o' | integral | The result is formatted as an octal integer |
'x', 'X' | integral | The result is formatted as a hexadecimal integer |
'e', 'E' | floating point | The result is formatted as a decimal number in computerized scientific notation |
'f' | floating point | The result is formatted as a decimal number |
'g', 'G' | floating point | The result is formatted using computerized scientific notation or decimal format, depending on the precision and the value after rounding. |
'a', 'A' | floating point | The result is formatted as a hexadecimal floating-point number with a significand and an exponent. This conversion isnot supported for the BigDecimal type despite the latter's being in thefloating point argument category. |
't', 'T' | date/time | Prefix for date and time conversion characters. See Date/Time Conversions. |
'%' | percent | The result is a literal '%' ('\u0025') |
'n' | line separator | The result is the platform-specific line separator |
示例代码：
package main;public class StringFormat {public static void main(String[] args) {// TODO Auto-generated method stubString format = "Haha.%1$(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 标准表达式}}
运行结果：
Haha.100.000fdsafdsmain.StringFormat@6d6f6e28