public class LeadingZerosExample {
    public static void main(String[] args) {
       int number = 1500;

       // String format below will add leading zeros (the %0 syntax) 
       // to the number above. 
       // The length of the formatted string will be 7 characters.

       String formatted = String.format("%07d", number);

       System.out.println("Number with leading zeros: " + formatted);
    }
}

Take a look at String.format. Note, however, that it takes format specifiers similar to those of C's printf family of functions -- for example:

String.format("Hello %s, %d", "world", 42);

…would return "Hello world, 42". The "format string" link points to the complete official spec, but for simple cases, this much shorter documentation may be helpful for an introduction to format specifiers even though it's outdated and about Lava. The most commonly used ones are:

• %s - insert a string
• %d - insert a signed integer (decimal)
• %f - insert a real number, standard notation

This is radically different from C#, which uses positional references with an optional format specifier. That means that you can't do things like:

String.format("The {0} is repeated again: {0}", "word");

... without actually repeating the parameter passed to printf/format. (see The Scrum Meister's comment below)

If you just want to print the result directly, you may find System.out.printf (PrintStream.printf) to your liking.

%d means number. %0nd means zero-padded number with a length.

You build n by subtraction in your example.

%s is a string.

Your format string ends up being this:

"%03d%s", 0, "Apple"

So you print a zero-padded 0 that's three digits long, and the string "Apple".

String.format (https://docs.oracle.com/javase/1.5.0/docs/api/java/util/Formatter.html#syntax)

In your case it will be:

String formatted = String.format("%03d", num);

• 0 - to pad with zeros
• 3 - to set width to 3