Default Locale = en_GB value = 123,456,789
The language is english and the country is Great Britain. Locales specify both language and country.
 DecimalFormat  Objects 
The program creates a DecimalFormat object.
The format() method of this object converts a number
into a StringBuffer.
A StringBuffer is almost the same as a String
except it can be modified.
No modification of a StringBuffer will be done in this chapter,
so it will be treated exactly as if it were a String.
The default locale determines the format of this StringBuffer.
The number may be a primitive (like int or double)
or a wrapper object (like  Integer or Double).
import java.util.Locale;
import java.text.*;
class IODemo2
{
  public static void main ( String[] args )
  {
    int value = 123456789 ;
    
    System.out.println( "Default Locale = " + Locale.getDefault() );
    DecimalFormat decform = new DecimalFormat();   
    System.out.println( "value = " + decform.format(value) );
  }
}
The program uses the default behavior of DecimalFormat.format()format().
(Review :) What three characters are used to separate the digits of an integer into groups of three?