Printf

In addition to println(), System.out provides the printf() method to print formatted strings.

int i = 17;
System.out.printf("%d in hexadecimal is %x.%n", i, i);
// 17 in hexadecimal is 11.

printf() takes a format string and a variable list of arguments (an example of a varargs method.)

The format string contains literal text, as well as format specifiers that start with %.

• Each parameter after the format string is substituted into the corresponding format specifier for output.

• The letter in the format specifier tells printf() how to convert the value for printing:

• d - as a decimal integer

• o - as an octal integer
• x - as a hexadecimal integer
• f - as a floating-point number
• a - as a floating-point number in hexadecimal
• s - as a string

Other special format specifiers include:

• %n - a newline: unlike println(), printf() doesn't output a newline unless you tell it to.
• %% - a literal %.

Between the % and the letter you can specify column width, padding, precision, and other niceties.

System.out.printf("%8d in hexadecimal is 0x%04x.%n", i, i);
//      17 in hexadecimal is 0x0011.

• - in the specifier means left-justify and pad with spaces out to the column width.
• 0 in the specifier means pad with leading zeros out to the column width.
• + in the specifier means indicate the number's sign with + or -.

• ( in the specifier means put negative numbers in parentheses.

• You can find full details of format strings in the Java Platform API documentation: Format String Syntax

What about binary?

printf() doesn't have a format specifier for outputting a number in binary.

However, there is an Integer class with the method toBinaryString(int n).

String bitsAsAString = Integer.toBinaryString(i);
System.out.printf("%+8d in hexadecimal is 0x%04x and in binary is %s.%n", i, i, bitsAsAString);
//     +17 in hexadecimal is 0x0011 and in binary is 10001.

Argument Indexes

printf expects an argument for each format specifier in the format string, but you can re-use arguments by providing an argument index inside a specifier.

• Arguments after the format string are numbered starting at 1 and specified with a $.

System.out.printf("%1$+8d in hexadecimal is 0x%1$04x and in binary is %2$s.%n", i, bitsAsAString);
//     +17 in hexadecimal is 0x0011 and in binary is 10001.

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