Padding strings

A padded string has leading or trailing spaces up to a certain number of characters. But no built-in "padding" method is available on the String class.

Instead, we implement padding with format strings and the String.format method. We can specify left and right padding. We can pad strings and numbers.

First example

This program left-justifies color names and right-justifies a parallel array of ints. We do this with just one format string.

Detail The code -10s adds right-padding to a string. We must specify the minus sign here.

Detail The code 10d adds left padding to an integer ("s" would handle a string).

Result The format string applies padding to two values at once. This is efficient and correct.

public class Program {
    public static void main(String[] args) {

        String[] colors = { "red", "orange", "blue", "green" };
        int[] numbers = { 42, 100, 200, 90 };

        // Loop over both arrays.
        for (int i = 0; i < colors.length; i++) {
            // Left-justify the color string.
            // ... Right-justify the number.
            String line = String
                    .format("%1$-10s %2$10d", colors[i], numbers[i]);
            System.out.println(line);
        }
    }
}red                42
orange            100
blue              200
green              90

Right-justify

This program shows how the String.format method can be used to right-justify a string. It uses a total width of 10 chars.

Result The string "Ulysses" is 7 chars, so 3 space chars are added to the start to get to 10 chars.

public class Program {
    public static void main(String[] args) {

        String title = "Ulysses";
        // Pad this string with spaces on the left to 10 characters.
        String padded = String.format("%1$10s", title);
        // Display result.
        System.out.print("[");
        System.out.print(padded);
        System.out.println("]");
    }
}
[   Ulysses]

Left-justify

Here we left-justify a string. We expand the string to 20 chars, adding spaces on the right as needed. We must specify the minus sign.

public class Program {
    public static void main(String[] args) {

        String title = "Finnegans Wake";
        // Left-justify this title to 20 characters.
        String padded = String.format("%1$-20s", title);
        // Our result.
        System.out.print("|");
        System.out.print(padded);
        System.out.println("|");
    }
}
|Finnegans Wake      |

Custom methods

The String.format method is an effective way of aligning text with padding. But we could wrap these String.format calls in methods.

And Custom methods (like padLeft and padRight) would be easier to call in a program.

However With methods that only pad one value, an entire line cannot be composed in one statement. This would likely be less efficient.

A summary

With String.format we apply padding to strings and numbers. We can pad many values at once with the format string syntax. This is powerful but can be complex.