The %g format specifier causes Formatter to use either %f or %e, whichever is shorter : Formatter Specifiers « Development « Java Tutorial

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2. Development
3. Formatter Specifiers

import java.util.Formatter;

public class MainClass {
  public static void main(String args[]) {
    Formatter fmt = new Formatter();

    for (double i = 1000; i < 1.0e+10; i *= 100) {
      fmt.format("%g ", i);
      System.out.println(fmt);
    }
  }
}

1000.00 
1000.00 100000 
1000.00 100000 1.00000e+07 
1000.00 100000 1.00000e+07 1.00000e+09