Java String formatting with the String.format method (like ‘sprintf’)

Java String formatting FAQ: How can I format Java String output?

Java String formatting solution

One way to format Java string output is with the format method of the String class, which works like a “Java sprintf” method. Here are some examples:

// output a string
log.debug( String.format("The rename status is (%d)", RENAME_SUCCEEDED) );

// format some text and assign it to a string
String status = String.format("The rename status is (%d)", RENAME_SUCCEEDED);

// format a string with multiple variables (interpolation)
log.debug( String.format("%s is %d years old, er, young", "Al", 45) );

For more information on those examples, see the sections below.

A String.format example

Here’s a quick example of how to use the String.format method to format a string before I pass it to a Log4J log.debug method:

log.debug( String.format("The rename status is (%d)", RENAME_SUCCEEDED) );

If you’re familiar with the sprintf function from other languages like C, Perl, or Ruby, this syntax will look familiar. But, if you’re not familiar with this syntax, what happens in the line of code above is that the %d in my Java String is replaced by the value of the variable RENAME_SUCCEEDED, which in this case happens to be a constant in my class. The %d symbol is a placeholder that indicates that a decimal value (something like an int or long in Java) should be printed in place of this symbol. The value of that decimal comes from the variable that follows my string, in this case the RENAME_SUCCEEDED variable.

Java ‘sprintf’: assigning a formatted String to another String

If it helps to see the format method used without the additional log.debug method, here’s an example where I use the String.format method to assign a similarly formatted String to another String:

String status = String.format("The rename status is (%d)", RENAME_SUCCEEDED);

Using String.format with multiple variables

Finally, here is an example of how to use multiple variables with the String format method:

log.debug( String.format("%s is %d years old, er, young", "Al", 45) );

Note that I keep using the log.debug method in these examples for a reason, specifically that I want to show this String format method. However, if you’re interested in just printing formatted strings to system output or system error, there are System.out.format and System.err.format methods that are very similar to the String format method. Rather than make this tutorial any longer, here’s a link to a Java System.out.format (Java printf) example.

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More “printf” style formatting examples

As a final note, if you’re not familiar with the printf function in other languages, including all the different printf print formatting options you can use, I’ve put together this printf format examples page, with printf examples shown in several different programming languages.