How to pad a number with leading zeros

Short answer

The %05d pattern prints the number 5 characters wide, padded with zeros:

void main() {
    IO.println(String.format("%05d", 42));
    IO.println("%03d".formatted(7));
}

Output:

00042
007

An order number

Leading zeros are often used in document numbers:

void main() {
    for (int id : new int[] {1, 25, 310}) {
        IO.println("INV-%04d".formatted(id));
    }
}

Output:

INV-0001
INV-0025
INV-0310

Time

Hours and minutes are usually printed with two digits:

void main() {
    int totalSeconds = 3725;
    int hours = totalSeconds / 3600;
    int minutes = totalSeconds % 3600 / 60;
    int seconds = totalSeconds % 60;
 
    IO.println("%02d:%02d:%02d".formatted(hours, minutes, seconds));
}

Output:

01:02:05

A string of digits

If the value is already a string, pad it with repeat():

void main() {
    String code = "815";
    IO.println("0".repeat(Math.max(0, 6 - code.length())) + code);
}

Output:

Common mistake

Storing a number with leading zeros in an int. A number has no use for zeros, so they disappear when parsed. Keep postal codes, phone numbers and codes as strings:

void main() {
    int zip = Integer.parseInt("01067");
    IO.println(zip);
}

Output:

Thanks for reading!

Short answer

An order number

Time

A string of digits

Common mistake

Functions used in this recipe

Thanks for reading!