Java Printf - How to format console output

You will learn

• How to format text output for the console in Java?
• How to print formatted a string in Java?
• How to format text using System.out.printf()?
• Why use System.out.printf() along with System.out.println()?

We will discuss:

• Why converting any data into a string format is necessary
• How to use the built-in utility, System.out.printf(), to format such data
• Why System.out.println() also plays a part in neat console output

We assume you are aware of:

• What the term console means, in a programming context
• The Java built-in System.out.println() method

Tools you will need

• JDK Environment Installed
• A REPL tool like JShell, or an IDE such as Eclipse

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Console Output With System.out.println()

You would be aware that Java has a built-in utility called System.out.println(), that displays data on the console. When we typed in System.out.println(3*4) in our program, we formed an expression 3*4, and passed it to System.out.println().

Now suppose you want to print the exact information 5 * 2 = 10, on the console. Clearly, System.out.println() does not accept 5 * 2 = 10 as a valid parameter. Earlier though, it had accepted 5 * 2, so something is missing here.

Note that 5 * 2 = 10 is no longer an expression, so was rejected. What you want to do is print a string literal, which is a sequence of characters, enclosed within double quotes: ‘"’ and ‘"’. Such a literal can be directly passed to System.out.println(), and will be printed as-is. A literal such as "5 * 2 = 10" will work here.

By now, you have figured out that System.out.println() only accepts a character string as argument. As a programmer, you would need a way to print both numbers and strings, side-by-side on the console. It turns out that Java has a way to do this, and is called formatted output. The built-in System.out.printf() utility comes into play here.

Introducing System.out.printf()

The System.out.printf() method accepts a variable number of arguments: * The first argument is the format in which to print data. This is a string literal, having zero or more format specifiers. A format specifier is predefined (such as %d), that formats data of a particular type. For example, %d formats data of type int. * The trailing arguments are the expressions, whose content you want to display. * To get the display you want, make sure that: * The number of format specifiers matches the number of trailing arguments * The format specifiers are in the same order as the trailing arguments.

jshell> System.out.println("5 * 2 = 10")

5 * 2 = 10

jshell> System.out.printf("5 * 2 = 10")

5 * 2 = 10$1 ==> java.io.PrintStream@4e1d422d

jshell>

• When System.out.printf() is called with a single string argument, it behaves just like System.out.println(). Well, almost! In addition, it also prints some illegible information (the $1 ==> java.io.PrintStream@4e1d422d_ you see above). For now, just take this as information about java.io.PrintStream (a the built-in type).

Beautifying The Output of System.out.printf()

jshell> System.out.printf("5 * 2 = %d", 5*2).println()

5 * 2 = 10

jshell>

• The good news is, that if we call System.out,printf(), and the call System.out.println() on the java.io.Printstream object, the illegible stuff disappears. In addition, a newline is inserted for neater printing!

Passing Arguments to System.out.printf()

As we just mentioned, System.out.printf() accepts a variable number of arguments:

• The first one is a format specifier string
• The rest of the arguments are expressions that need to be displayed, and are formatted by these specifiers.

Let’s try and play around with them a little, shall we? Look at the code examples below.

jshell> System.out.printf("%d + %d + %d", 5, 6, 7).println()

5 + 6 + 7

jshell>

When the number and format of arguments match exactly, there is no problem at all. We get what we want.

jshell> System.out.printf("%d + %d + %d = %d", 5, 6, 7).println()

jshell>

• The code System.out.printf("%d + %d + %d = %d", 5, 6, 7).println() is food for thought. There are four format specifiers (all ‘%d’ for int). However, just three arguments follow it. This is not acceptable in Java, and JShell immediately throws an exception.

jshell>

System.out.printf("%d + %d + %d", 5, 6, 7, 8).println()

5 + 6 + 7

jshell>

• The statement System.out.printf("%d + %d + %d", 5, 6, 7, 8).println() seems to work just fine! That’s because if System.out.printf() is called with excess parameters, they are simply ignored. The program continues running as if nothing has happened.

Other Format Specifiers

So far, we’ve just stuck to formatting integers, and their expressions. Let’s try to format other basic Java types, shall we?

jshell> System.out.printf("Print %s", "Testing").println()

Print Testing

jshell> System.out.printf("%d + %d + %d", 5.5, 6.5, 7.5).println()

jshell> System.out.printf("%f + %f + %f", 5.5, 6.5, 7.5).println()

5.500000 + 6.500000 + 7.500000

jshell>

As you can see with string data, %s is the right format specifier to use. However, even though both integers and floating point data are numbers, their format specifiers are different. Only use %f for floating-point data.

Formatting for a combination of data

jshell> System.out.printf("I saw %f exactly %d time", 7.5, 1).println()

I saw 7.500000 exactly 1 time

jshell>

Did you notice that in the code above, we have combined data of three types (string, floating-point and integer), in a single format string? That is the beauty of System.out.printf(), short and sweet!

Let’s now summarize what we’ve learned in this article.

Summary

We have just explored the following areas:

• Even though System.out.println() is useful, it cannot format all kinds of data
• System.out.printf() can format a combination of data, by using format specifiers.
• System.ut.println() can be used on top of System.out.printf(), for neat output.

We hope this article leaves you better placed to format your data, even when they are a combination of different types. We’ll see you again soon. Until then, bye-bye!

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