6.2. The printf Method

In Java, System.out refers to an object that implements PrintStream methods, including print(Object), println(Object), and printf(String, Object…), for standard output. The printf method is particularly interesting because it includes a varargs parameter in its parameter list:

public PrintStream printf(String format, Object... args)

6.2.1. Format Strings and Specifiers

According to its documentation, printf lets us print a formatted string to output using the format string (format) and other arguments (args) that are supplied when printf is called. Here is a quick example that uses printf and the Person class included in this chapter’s starter code:

A method that used printf to format its output.

public static void printSalary(Person person) {
    String name = person.getName();
    double salary = person.getSalary();
    System.out.printf("The salary for %s is $%.2f.\n", name, salary);
} // printSalary

The argument supplied for format is called a format string, which is just a regular string that also satisfies the Format String Syntax described in Formatter. Each % character in a format string denotes a format specifier for an argument in the array referred to by args. In the example above, %s is the format specifier for name, and %.2f is the format specifier for salary.

To see how the format specifiers impact the output produced by the calls to printf in the printSalary method shown earlier, create and run a program similar to the one below:

An example program that uses the printSalary method shown in an earlier example to display formatted output via printf.

public static void main(String[] args) {
    Person susan = new Person("Susan", 120000.0);
    Person bob = new Person("Bob", 115300.5);
    printSalary(susan); // The salary for Susan is $120000.00.
    printSalary(bob);   // The salary for Bob is $115300.50.
} // main

6.2.2. Use of Varargs

At runtime, printf replaces each format specifier in its format string with a formatted version of its corresponding argument. Since the args parameter is a varargs parameter, the corresponding arguments do need to be manually placed into an array before calling printf. Although Object... technically denotes that zero or more Object references can be supplied, printf considers it an error when args.length does not equal the number of format specifiers in the format string.

6.2.3. A Few Format Specifier Examples

The table below summarizes a few of the format specifiers that can be used in format strings, including the ones used in the examples:

Format Specifier Examples. In each description, arg denotes the corresponding argument.

Format Specifier	Description
%s	arg is formatted using arg.toString().
%d	arg is a int and formatted as an integer.
%f	arg is a float or double and formatted to include one or more places after the decimal point.
%.2f	arg is a float or double and formatted as include exactly two places after the decimal point.

For a complete list of possible format specifiers, refer to the Format String Syntax section in Formatter.