Printing and Formatting Output

New words in this topic, in plain English. Come back here whenever one feels fuzzy.

Standard output

The main place a program writes its text. In a terminal it's the screen; it can also be redirected into a file.

PrintStream

The class of System.out. It turns values into text and writes them out, with methods print, println and printf.

Format string

A template like "%s scored %d%n" with blanks (specifiers) that printf fills with values.

Format specifier

A blank in a format string, starting with % and ending with a letter that says what kind of value goes there, such as %d.

Width

The minimum number of characters a filled blank takes up. Shorter values are padded with spaces.

Precision

For decimals, the number of digits after the point (%.2f). For text, the maximum number of characters shown.

Flag

An extra character after % that changes the look: - left-align, 0 pad with zeros, + show the sign, , group digits.

Locale

A language-and-country setting, such as US English or German, that decides details like the decimal point and digit grouping.

Line separator

The characters that end a line: \n on Linux and macOS, \r\n on Windows.

1. 1

   Widen the table

   In the bill example, add an item called "Mechanical pencil". Run it and watch the columns break: the name is longer than the width 10. Change every %-10s to %-18s, fix the dash line and total widths to match, and run again.

2. 2

   Predict the padding

   Before running, write down exactly what each line prints, counting spaces: printf("[%6d]%n", 42), printf("[%-6d]%n", 42), printf("[%06d]%n", 42), printf("[%6.1f]%n", 3.14159).

   Answers: [ 42], [42 ], [000042], [ 3.1].

3. 3

   Trigger a format error

   In the specifier tour, change %d on the int line to %f and run. The program compiles but stops at that line. Read the exception name: it tells you which conversion didn't match which type.

   terminalcopy cmd

   $ java Main

   ── expected output ──

   default 3.141593

   2 places 3.14

   0 places 3

   science 1.234568e+04

   int Exception in thread "main" java.util.IllegalFormatConversionException: f != java.lang.Integer

Errors are the best teachers. Make each change, read the error, guess what went wrong, then reveal the answer.

public class Main {
    public static void main(String[] args) {
        double price = 49.5;
        System.out.printf("Price: %d%n", price);
    }
}

public class Main {
    public static void main(String[] args) {
        String name = "Asha";
        System.out.printf("%s scored %d%n", name);
    }
}

public class Main {
    public static void main(String[] args) {
        int done = 45;
        System.out.printf("Progress: %d%", done);
    }
}

1. 1

   System.out is an object of type PrintStream connected to the program's standard output (usually the terminal). print(x) writes x as text and stays on the same line; println(x) writes x and then a line separator; println() with nothing writes just the line break. Both accept every primitive type, String, char[] and any object (they call its toString()).

2. 2

   printf(format, args...) takes a format string with format specifiers, each starting with %. %d takes a whole number, %f a decimal, %s anything (as text), %c a char, %b a boolean, %x hexadecimal, %e scientific notation, %n a line break and %% a literal percent sign. The values after the format fill the specifiers left to right.

3. 3

   Between % and the letter you can add flags, width and precision: %8d pads to 8 characters on the left (right-aligned), %-8s pads on the right (left-aligned), %08d pads with zeros, %+d always shows the sign, %,d adds grouping separators, and %.2f rounds to 2 decimal places. %10.3f means 'at least 10 wide, 3 decimals'. This is how you print tidy tables.

4. 4

   printf does not add a line break. End the format with %n, which writes the platform's line separator (\r\n on Windows, \n elsewhere), rather than \n. Getting a specifier wrong is a run-time error, not a compile error: %d with a double throws IllegalFormatConversionException, and too few values throws MissingFormatArgumentException.

5. 5

   String.format(format, args...) returns the formatted String instead of printing it; since Java 15 you can write the same thing as "Order #%05d".formatted(42). Numbers are formatted for a locale (a country and language setting): %,.2f prints 1,234,567.89 in the US and 1.234.567,89 in Germany. Pass a Locale as the first argument when the output must not depend on the machine.

6. 6

   %.2f rounds half up (0.125 becomes 0.13) but only changes how the number is shown, never the value stored in the variable. For money, keep exact values in long cents or BigDecimal (Topic 1.4) and format only at the end. Phase 6.6 covers formatting in depth, including DecimalFormat, and Topic 6.5 covers multi-line text blocks.

• ↔

  printf keeps layout and data separate and handles alignment and rounding, but its mistakes surface only at run time; + concatenation is checked by the compiler and faster, but turns messy for tables.

• ↔

  Formatting with the default locale suits messages for the person at the keyboard; a fixed locale suits files, logs and APIs that other programs parse.

• ↔

  System.out.println is fine for learning and small tools; services should log through a logging library, which adds timestamps, levels and output routing.

• I know when to use print, println and printf, and that printf needs %n.

• I can use %d, %f, %s, %c, %b, %x, %e and %% correctly.

• I can control width, alignment, zero padding, sign and decimal places to print an aligned table.

• I can build formatted text with String.format and formatted.

• I can explain the three common format exceptions and fix each.

• I know when to pass an explicit Locale.