Format

Often we need to compose strings from variables (like integers) and other text. String.format is ideal here. It provides a format language to which we add variables.

To insert numbers, we can use the "d" format. We use ordinals after the initial "%" symbol, starting at 1 and increasing. This indicates which argument to insert.

Integer format

This is a good first example. We add the values 10, 20 and 30 to a format string. We specify the format string as the first argument to format.

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

        // Call String.format with three integer codes.
        String result = String.format("One: %1$d Two: %2$d Three: %3$d",
                10, 20, 30);
        System.out.println(result);
    }
}
One: 10 Two: 20 Three: 30

Pad with zeros

Many format codes can be used with String.format. Here we pad a number with zeros on the left side. The first 0 means "pad with zeros" and the 5 means "use five digits."

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

        for (int i = 0; i < 5; i++) {
            // Pad with zeros and a width of 5 chars.
            String result = String.format("%1$05d %2$05d", i, i + 10);
            System.out.println(result);
        }
    }
}
00000 00010
00001 00011
00002 00012
00003 00013
00004 00014

Precision, double

We can specify the precision of a double or float. We use the "f" to indicate a floating-point. Before this we specify ".3" to mean "three numbers after the decimal."

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

        double number = 1.23456;
        // Specify precision with format.
        String value = String.format("Three numbers after decimal: %1$.3f",
                number);
        System.out.println(value);
    }
}
Three numbers after decimal: 1.235

Insert strings

We can insert a string with the "s" format. This example shows three different syntax forms. We can specify just "%s," use the percentage sign "%," or use a "$" in between.

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

        String first = "Marcus";
        String last = "Aurelius";

        // Use simple string format.
        String value = String.format("%s %s", first, last);
        System.out.println(value);

        // Use indexes before simple string format.
        value = String.format("%1s %2s", first, last);
        System.out.println(value);

        // Use $ symbol before string character.
        value = String.format("%1$s %2$s", first, last);
        System.out.println(value);
    }
}
Marcus Aurelius
Marcus Aurelius
Marcus Aurelius

Argument order

This example uses both a string and an integer in one format string. It also inserts the second argument first—it uses the "%2" syntax for this.

public class Program {
    public static void main(String[] args) {
        String id = "ART1";
        int number = 100;

        // Place the last argument first.
        // ... Use integer and string in same format string.
        String value = String.format("%2$d %1$s", id, number);
        System.out.println(value);
    }
}
100 ART1

Padding, right and left

Here we use right-padding and left-padding in a format string. We can apply padding to strings, numbers or other values.

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

        String[] lefts = { "cat", "dog", "bird", "elephant" };
        String[] rights = { "orange", "black", "blue", "grey" };

        // Loop over both arrays and display paired strings.
        for (int i = 0; i < lefts.length; i++) {
            String left = lefts[i];
            String right = rights[i];

            // Add padding to the right.
            // ... Add padding to the left.
            String value = String.format("%1$-10s %2$10s", left, right);
            System.out.println(value);
        }
    }
}
cat            orange
dog             black
bird             blue
elephant         grey

Calendar

This example uses String.format with a Calendar date. We use "t" and then "B" to insert the long month string. With "t" and then "e" or "Y" we insert the day or year numbers.

import java.util.Calendar;

public class Program {
    public static void main(String[] args) {
        // Create a calendar with a specific date.
        Calendar cal = Calendar.getInstance();
        cal.set(2015, 0, 15);

        // Format the month, day and year into a string.
        String result = String.format("Month: %1$tB Day: %1$te Year: %1$tY",
                cal);
        System.out.println(result);
    }
}
Month: January Day: 15 Year: 2015

Months, days, years

This example shows various ways of formatting months, days, and years with String.format. We specify formatting with uppercase or lowercase letters after the "t."

import java.util.Calendar;

public class Program {
    public static void main(String[] args) {
        Calendar cal = Calendar.getInstance();
        cal.set(2015, 1, 18);

        // Format the month.
        String result = String.format("Month: %1$tB %1$tb %1$tm", cal);
        System.out.println(result);

        // Format the day.
        result = String.format("Day: %1$tA %1$ta %1$td", cal);
        System.out.println(result);

        // Format the year.
        result = String.format("Year: %1$tY %1$ty %1$tj", cal);
        System.out.println(result);
    }
}
Month: February Feb 02
Day: Wednesday Wed 18
Year: 2015 15 049

Hour, minute and second

This is starting to become confusing. Here we format hours, minutes and seconds with String.format. Hours can be formatted in many ways.

import java.time.Instant;
import java.util.Calendar;
import java.util.Date;

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

        // Set time to current time.
        Calendar cal = Calendar.getInstance();
        cal.setTime(Date.from(Instant.now()));

        // Format the hour.
        String result = String.format("Hour: %1$tH %1$tI %1$tk %1$tl", cal);
        System.out.println(result);

        // Format minute.
        result = String.format("Minute: %1$tM", cal);
        System.out.println(result);

        // Format the second.
        result = String.format("Second: %1$tS", cal);
        System.out.println(result);

        // Format the am and pm part.
        result = String.format("AM/PM: %1$tp", cal);
        System.out.println(result);
    }
}
Hour: 18 06 18 6
Minute: 53
Second: 23
AM/PM: pm

File names

Sometimes we have a program that generates files on a regular basis (on a schedule). We can generate file names with String.format that contain the current date and time.

With String.format, a format string is used to insert values (as from variables). We add punctuation and text data in between insertions.