How do I add left-padded zeros to a number in Java?

Question

I have an integer 100, how do I format it to look like 00000100 (always 8 digits long)?

Answer 1

Try this:

String formattedNumber = String.format("%08d", number);

Answer 2

You can also use the class DecimalFormat, like so:

NumberFormat formatter = new DecimalFormat("00000000");
System.out.println(formatter.format(100)); // 00000100

Answer 3

Yet another way. ;)

int x = ...
String text = (""+(500000000 + x)).substring(1);

-1 => 99999999 (nines complement)

import java.util.concurrent.Callable;
/* Prints.
String.format("%08d"): Time per call 3822
(""+(500000000+x)).substring(1): Time per call 593
Space holder: Time per call 730
 */
public class StringTimer {
    public static void time(String description, Callable<String> test) {
        try {
            // warmup
            for(int i=0;i<10*1000;i++)
                test.call();
            long start = System.nanoTime();
            for(int i=0;i<100*1000;i++)
                test.call();
            long time = System.nanoTime() - start;
            System.out.printf("%s: Time per call %d%n", description, time/100/1000);
        } catch (Exception e) {
            System.out.println(description+" failed");
            e.printStackTrace();
        }
    }

    public static void main(String... args) {
        time("String.format(\"%08d\")", new Callable<String>() {
            int i =0;
            public String call() throws Exception {
                return String.format("%08d", i++);
            }
        });
        time("(\"\"+(500000000+x)).substring(1)", new Callable<String>() {
            int i =0;
            public String call() throws Exception {
                return (""+(500000000+(i++))).substring(1);
            }
        });
        time("Space holder", new Callable<String>() {
            int i =0;
            public String call() throws Exception {
                String spaceHolder = "00000000";
                String intString = String.valueOf(i++);
                return spaceHolder.substring(intString.length()).concat(intString);
            }
        });
    }
}

Answer 4

If Google Guava is an Option:

String output = Strings.padStart("" + 100, 8, '0');

Alternatively Apache Commons Lang:

String output = StringUtils.leftPad("" + 100, 8, "0");

Answer 5

String.format uses a format string which is described here

Answer 6

If you just need to print it out, this is a shorter version:

System.out.printf("%08d\n", number);

Answer 7

This also works:

int i = 53;
String spaceHolder = "00000000";
String intString = String.valueOf(i);
String string = spaceHolder.substring(intString.lenght()).contract(intString);

But the other examples are much easier.

Answer 8

If you need to parse this string and or support i18n consider extending the

java.text.Format 

object. Use the other answers to help you get the format.