How to convert 123,456 this form 123456

As the title belongs, put a float type data into a string 12,345,678.90 this form?<-! Main posts under Banner (D4) -><-! Posts under the main text (D5) ->
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 String.format ("% 1 $, .2 f", 12345678.9) 

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 String formatFloat (float num) {
StringBuffer buf = new StringBuffer (Float.toString (num)); 
buf = buf.reverse (); 
int dotPos = buf.indexOf ("."); 
final int start = dotPos == -1 0: dotPos; 
?StringBuffer sb = new StringBuffer (); 
for (int i = 0; i if ((i - start)% 3 == 0 && (i -! start) = 0) {
sb.append (','); 
} 
sb.append (buf.charAt (i)); 
} 
return sb.reverse () toString ();. 
} 

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float f = 12345678.9f; 
String str = String.format ("% .2 f", f) replaceAll. (, "," "(= (\ \ D {3}) + \ \)?."); 

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NumberFormat numberFormat = new DecimalFormat ("# # # #, # # #"); 
System.out.println (numberFormat.format (Integer.parseInt ("1234564589"))); 

Note that the length of type int
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DecimalFormat good
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cited a floor tantahe reply:

 String.format ("% 1 $, .2 f", 12345678.9) 

I ask: "% 1 $, .2 f" What does this mean?
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reference to the third floor hjywyj reply:

 
float f = 12345678.9f; 
String str = String.format ("% .2 f", f) replaceAll. (, "," "(= (\ \ D {3}) + \ \)?."); 

658 returns yes, 658.00
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reference to the 6th floor knxw0001 reply:

Quote: references to a floor tantahe reply:

 String.format ("% 1 $, .2 f", 12345678.9) 

I ask: "% 1 $, .2 f" What does this mean?

1 $ represents the first argument behind that 12345678.9
Before, which means that every three decimal places separated by commas
.2 F represents two digits after the decimal point
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mark about the study.