1. Introduction
In this tutorial, we’ll demonstrate different examples of formatting with the printf() method.
The method is part of the java.io.PrintStream class and provides String formatting similar to the printf() function in C.
2. Syntax
We can use one of the following PrintStream methods to format the output:
System.out.printf(format, arguments); System.out.printf(locale, format, arguments);
We specify the formatting rules using the format parameter. Rules start with the ‘%’ character.
Let’s look at a quick example before we dive into the details of the various formatting rules:
System.out.printf("Hello %s!%n", "World");
This produces the following output:
Hello World!
As shown above, the format string contains plain text and two formatting rules. The first rule is used to format the string argument. The second rule adds a newline character to the end of the string.
2.1. Format Rules
Let’s have a look at format string more closely. It consists of literals and format specifiers. Format specifiers include flags, width, precision, and conversion characters in the following sequence:
%[flags][width][.precision]conversion-character
Specifiers in the brackets are optional.
Internally, printf() uses the java.util.Formatter class to parse the format string and generate the output. Additional format string options can be found in the Formatter Javadoc.
2.2. Conversion Characters
The conversion-character is required and determines how the argument is formatted. Conversion characters are only valid for certain data types. Some common ones are:
- s – formats strings
- d – formats decimal integers
- f – formats the floating-point numbers
- t– formats date/time values
We’ll explore these and a few others later in the article.
2.3. Optional Modifiers
The [flags] define standard ways to modify the output and are most common for formatting integers and floating point numbers.
The [width] specifies the field width for outputting the argument. It represents the minimum number of characters written to the output.
The [.precision] specifies the number of digits of precision when outputting floating-point values. Additionally, we can use it to define the length of a substring to extract from a String.
3. Line Separator
To break the string into separate lines, we have a %n specifier:
System.out.printf("baeldung%nline%nterminator");
The code snippet above will produce the following output:
baeldung line terminator
The %n separator printf() will automatically insert the host system’s native line separator.
4. Boolean Formatting
To format boolean values, we use the %b format. It works the following way: If the input value is true, the output is true. Otherwise, the output is false.
So, if we do:
System.out.printf("%b%n", null);
System.out.printf("%B%n", false);
System.out.printf("%B%n", 5.3);
System.out.printf("%b%n", "random text");
Then we’ll see:
false FALSE TRUE true
Notice that we can use %B for uppercase formatting.
5. String Formatting
To format a simple string, we’ll use the %s combination. Additionally, we can make the string uppercase:
printf("'%s' %n", "baeldung");
printf("'%S' %n", "baeldung");
And the output is:
'baeldung' 'BAELDUNG'
Also, to specify a minimum length, we can specify a width:
printf("'%15s' %n", "baeldung");
Which gives us:
' baeldung'
If we need to left-justify our string, then we can use the ‘-‘ flag:
printf("'%-10s' %n", "baeldung");
And the output is:
'baeldung '
Even more, we can limit the number of characters in our output by specifying a precision:
System.out.printf("%2.2s", "Hi there!");
The first ‘x’ number in %x.ys syntax is the padding. ‘y’ is the number of chars.
For our example here, the output is Hi.
6. Char Formatting
The result of %c is a Unicode character:
System.out.printf("%c%n", 's');
System.out.printf("%C%n", 's');
The capital letter C will uppercase the result:
s S
But, if we give it an invalid argument, then Formatter will throw IllegalFormatConversionException.
7. Number Formatting
7.1. Integer Formatting
The printf() method accepts all the integers available in the language; byte, short, int, long and BigInteger if we use %d:
System.out.printf("simple integer: %d%n", 10000L);
With the help of the ‘d’ character, we’ll have:
simple integer: 10000
In case we need to format our number with the thousands separator, we can use the ‘,’ flag. And we can also format our results for different locales:
System.out.printf(Locale.US, "%,d %n", 10000); System.out.printf(Locale.ITALY, "%,d %n", 10000);
As we see, the formatting in the US is different than in Italy:
10,000 10.000
7.2. Float and Double Formatting
To format a float number, we’ll need the ‘f’ format:
System.out.printf("%f%n", 5.1473);
Which will output:
5.147300
Of course, the first thing that comes to mind is to control the precision:
System.out.printf("'%5.2f'%n", 5.1473);
Here we define the width of our number as 5, and the length of the decimal part is 2:
' 5.15'
Here we have one space padding from the beginning of the number to support the predefined width.
To have our output in scientific notation, we just use the ‘e’ conversion character:
System.out.printf("'%5.2e'%n", 5.1473);
And the result is the following:
'5.15e+00'
8. Date and Time Formatting
For date and time formatting, the conversion string is a sequence of two characters: the ‘t’ or ‘T’ character and the conversion suffix. Let’s explore the most common time and date formatting suffix characters with the examples.
Definitely, for more advanced formatting we can use DateTimeFormatter which has been available since Java 8.
8.1. Time Formatting
First, let’s see the list of some useful suffix characters for Time Formatting:
- ‘H’, ‘M’, ‘S’ – characters are responsible for extracting the hours, minutes and second from the input Date
- ‘L’, ‘N’ – to represent the time in milliseconds and nanoseconds accordingly
- ‘p’ – adds am/pm formatting
- ‘z’ – prints out the timezone offset
Now, let’s say we wanted to print out the time part of a Date:
Date date = new Date();
System.out.printf("%tT%n", date);
The code above along with ‘%tT’ combination produces the following output:
13:51:15
In case we need more detailed formatting, we can call for different time segments:
System.out.printf("hours %tH: minutes %tM: seconds %tS%n", date, date, date);
Having used ‘H’, ‘M’, and ‘S’ we get:
hours 13: minutes 51: seconds 15
Though, listing date multiple times is a pain. Alternatively, to get rid of multiple arguments, we can use the index reference of our input parameter which is 1$ in our case:
System.out.printf("%1$tH:%1$tM:%1$tS %1$tp %1$tL %1$tN %1$tz %n", date);
Here we want as an output the current time, am/pm, time in milliseconds, nanoseconds and the timezone offset:
13:51:15 pm 061 061000000 +0400
8.2. Date Formatting
Like time formatting, we have special formatting characters for date formatting:
- ‘A’ – prints out the full day of the week
- ‘d’ – formats a two-digit day of the month
- ‘B’ – is for the full month name
- ‘m’ – formats a two-digit month
- ‘Y’ – outputs a year in four digits
- ‘y’ – outputs the last two digits of the year
So, if we wanted to show the day of the week, followed by the month:
System.out.printf("%1$tA, %1$tB %1$tY %n", date);
Then using ‘A’, ‘B’, and ‘Y’, we’d get:
Thursday, November 2018
To have our results all in numeric format, we can replace the ‘A’, ‘B’, ‘Y ‘ letters with ‘d’, ‘m’, ‘y’:
System.out.printf("%1$td.%1$tm.%1$ty %n", date);
Which will result in:
22.11.18
9. Summary
In this article, we discussed how to use the PrintStream#printf method to format output. We looked at the different format patterns used to control the output for common data types.
Finally, as always, the code used during the discussion can be found over on GitHub.