In the Java class DecimalFormat, what do the hashtags at the left side of the decimal do?

Question

My title of this question I have isn't so great, but hopefully I can explain it more in this post.

import java.io.*;
import java.text.*;
public class Output {
public static void main(String[] args) {

    /*double number = 438.978;
    /*UpperCase <- naming convention for classes.DecimalFormat x = new DecimalFormat("#.#");
    System.out.println(x.format(number));*/

    double number = 43.97;
    DecimalFormat x = new DecimalFormat(".###");
    System.out.println(x.format(number));

  }
}

Don't mind the comments. During my Gr 11 Comp Sci class I asked my teacher if the hashtags on the left of the decimal point(11th line) did anything to the double number, we tried it as a class and we found that it did not change the output of the System.out.println statement.

~/workspace/Java/ $ java Output
43.97

Can someone explain to me the purpose of the parameters to the left of the decimal? Someone programmed it to do something so I was just curious.

Answer 1

Like others have said # formats a digit but drops zeros.

In your example where you apply .### to a double like 12.34 it would format to 12.340 but since it drops zeros it does not.

The same happens when you put a # before the decimal, for example ###.### to format 12.34 would display 012.340 but since it drops zeros it displays as 12.34.

So a # before the decimal will really do nothing.

An example of something useful before the decimal, so you can see formats before the decimal can work is 0 which formats a digit but does not drop zeros, and can also be used in the DecimalFormat. A pattern like 000.000 applied to 12.34 results in 012.340:

double d = 12.34;
DecimalFormat df = new DecimalFormat("000.000");
System.out.print(df.format(d));

Patterns like 0, # and more are defined in DecimalFormat.