تنسيق المخرجات

Written Explanation

The shape of printf

System.out.printf("%s costs %.2f%n", "Pen", 1.5);

The first argument is the format string. Everything in it is printed as written, except format specifiers, which start with %. Each specifier is a hole that takes the next value from the argument list, in order. Output:

Pen costs 1.50

printf does not add a line break by itself, so we end with %n.

The four specifiers you need

%n is the right way to end a line in a format string. It produces the line separator of the current platform. The Javadoc for Formatter (the class behind printf) says it gives the value of System.lineSeparator().

By default %f prints six digits after the point, which is rarely what you want for money. That is where precision comes in.

Precision: %.2f

Write a dot and a number between % and f, and that is how many digits come after the decimal point:

System.out.printf("%.2f%n", 3.14159);   // 3.14
System.out.printf("%.2f%n", 3.0);       // 3.00

Two decimals always print, even 3.00. The number is rounded, not cut off. The Javadoc says rounding uses the HALF_UP mode: a value halfway between two candidates goes up. In compiled tests, %.2f of 0.125 prints 0.13, %.1f of 2.25 prints 2.3, and %.0f of 2.5 prints 3.

One warning from the same tests: %.2f of 1.005 printed 1.01 and 2.675 printed 2.68. That looks right, but do not read it as "decimal arithmetic is exact". The Javadoc explains that the rounding is applied to the decimal text of the value as Double.toString would show it, not to the full binary value. The underlying double can still be slightly off, which is why money needs better tools later. Formatting is a way to display a value, not a way to fix an inexact one.

Width: %8.2f

A number before the dot is the minimum width: how many characters the field takes. Shorter values are padded with spaces on the left, so numbers line up on the right:

System.out.printf("[%8.2f]%n", 3.14159);   // [    3.14]
System.out.printf("[%5d]%n", 42);          // [   42]

The brackets show the padding. %8.2f reads as: width 8, two decimals. The decimal point and the digits all count towards the width.

If the value is longer than the width, the field simply grows. The width is a minimum, never a limit, and that can break alignment, so choose a width a little bigger than your longest value.

Alignment of text: %-12s and %12s

For text, the default is to pad on the left, so the text sits at the right of its field. A minus sign flag - reverses that:

System.out.printf("[%-8s]%n", "Pen");   // [Pen     ]
System.out.printf("[%8s]%n", "Pen");    // [     Pen]

The convention for tables is text left, numbers right: names with %-12s, numbers with %8.2f. It is how a column of prices lines up on the decimal point. The - flag requires a width, and Java reports an error if you use it without one.

Other handy flags

Two more flags, from the Javadoc, that you will use occasionally:

System.out.printf("%,.2f%n", 1234567.891);   // 1,234,567.89
System.out.printf("%05d%n", 42);             // 00042

The comma flag groups thousands, using the separators of the current locale; 0 pads with zeros instead of spaces. To print an actual percent sign, double it: %%.

System.out.printf("%.1f%%%n", 15.0);          // 15.0%

String.format: the same, but as a String

String.format takes the same arguments as printf, but returns the text instead of printing it:

String row = String.format("%-6s%6.2f", "Tea", 2.0);   // "Tea      2.00"

Use it when you need to store, combine, or print the text later. System.out.printf(...) is effectively the same as printing the result of String.format(...). In the receipt (Lesson 6) you will format each line as you read the item and print everything at the end.

Errors to expect

A format string is checked when the program runs, not when it compiles. The compiler will not warn you about a mismatch:

• %d with a decimal value, such as printf("%d", 3.5), throws IllegalFormatConversionException (real message: d != java.lang.Double).
• A specifier with no matching value throws MissingFormatArgumentException.
• A precision with %d (such as %.2d) throws IllegalFormatPrecisionException.

These stop the program, so format strings need testing with real values. The fix is almost always to match the specifier to the type: %d for whole numbers, %f for decimals, and %s for anything else.

Locale

Formatting uses the computer's locale by default. On a machine set up for French, %.2f may print a comma, 3,14, and the thousands separator of %,.2f changes too. For a receipt that must look the same everywhere, you can pass a locale explicitly (String.format(Locale.US, ...), which needs import java.util.Locale;). For learning, the default is fine; if your decimals look different from this lesson's, that is the reason.