Java String format
In this article we show how to format strings in Java.
In Java, we have methods for string formatting. Another way to dynamically create strings is string building.
The System.out.printf , System.out.format , and formatted methods can be used to format strings in Java. They work the same. These three methods write a formatted string to the output stream using the specified format string and arguments. If there are more arguments than format specifiers, the extra arguments are ignored.
%[argument_index$][flags][width][.precision]conversion
The format specifiers for general, character, and numeric types have this syntax.
%[argument_index$][flags][width]conversion
This is the syntax for types which are used to represents dates and times.
The format specifiers begin with the % character and end with a 1 or 2 character conversion that specifies the kind of formatted output being generated. The optional items are placed between the square brackets.
The argument_index is a decimal integer indicating the position of the argument in the argument list. The flags is a set of characters that modify the output format. The set of valid flags depends on the conversion. The width is a non-negative decimal integer indicating the minimum number of characters to be written to the output.
The precision is a non-negative decimal integer usually used to restrict the number of characters. The specific behavior depends on the conversion. The required conversion is a character indicating how the argument should be formatted.
Java String format methods
We use the three methods to format a simple message.
package com.zetcode; public class FormatMethods < public static void main(String[] args) < String name = "John Doe"; String occupation = "gardener"; String txt = "%s is a %s"; String msg = txt.formatted(name, occupation); System.out.println(msg); System.out.format("%s is a %s\n", name, occupation); System.out.printf("%s is a %s%n", name, occupation); >> We build the same string three times.
String name = "John Doe"; String occupation = "gardener"; String txt = "%s is a %s"; String msg = txt.formatted(name, occupation);
The formatted method is an instance method.
System.out.format("%s is a %s\n", name, occupation); System.out.printf("%s is a %s%n", name, occupation); The format and printf methods are static.
$ java FormatMethods.java John Doe is a gardener John Doe is a gardener John Doe is a gardener
Java String format specifiers
Next we use two basic format specifiers.
package com.zetcode; public class Conversions < public static void main(String[] args) < System.out.format("There are %d %s.%n", 5, "pencils"); System.out.printf("The rock weighs %f kilograms.%n", 5.345); >> In this program, we format two simple sentences.
System.out.format("There are %d %s.%n", 5, "pencils"); In this code line, we have three format specifiers. Each specifier starts with the % character. The d specifier formats integer values. The s specifier expects string values. The %n outputs a platform-specific line terminator; it does not require an argument.
System.out.printf("The rock weighs %f kilograms.%n", 5.345); The f formats a floating point value as a decimal value. The System.out.printf works the same as the System.out.format .
$ java Conversions.java There are 5 pencils. The rock weighs 5.345000 kilograms.
Java String format argument index
In the next example, we work with argument indexes.
package com.zetcode; import java.time.LocalDateTime; public class IndexPosition < public static void main(String[] args) < int x = 12; int y = 32; int z = 43; LocalDateTime dt = LocalDateTime.now(); System.out.format("There are %d apples, %d oranges and " + "%d pears%n", x, y, z); System.out.format("There are %2$d apples, %3$d oranges and " + "%1$d pears%n", x, y, z); System.out.format("Year: %tY, Month: %> The example uses argument index to refer to variables included the list of arguments.
System.out.format("There are %d apples, %d oranges and " + "%d pears%n", x, y, z); If we do not specify the index, the variables automatically match the specifiers. The d specifier formats an integer value as a decimal value.
System.out.format("There are %2$d apples, %3$d oranges and " + "%1$d pears%n", x, y, z); The 1$ referes to the x variable, the 2$ referes to the y variable and the 3$ refers to the z variable.
System.out.format("Year: %tY, Month: %
$ java IndexPosition.java There are 12 apples, 32 oranges and 43 pears There are 32 apples, 43 oranges and 12 pears Year: 2022, Month: 10, Day: 17
Java String format flag
The flag modifies the format in a specific way. There are several flags available. For instance, the + flag requires the output to include a positive sign for all positive numbers.
package com.zetcode; public class Flags < public static void main(String[] args) < System.out.format("%+d%n", 553); System.out.format("%010d%n", 553); System.out.format("%10d%n", 553); System.out.format("%-10d%n", 553); System.out.format("%d%n", -553); System.out.format("%(d%n", -553); >> The example presents a few flags of the string format specifier.
The 0 flag will cause the output to be padded with leading zeros to the minimum field width. Our number has three digits. The minimum width is 10. Therefore, we have 7 leading zeros in the output.
Without the 0 flag, the number is right aligned.
The - flag will cause the number to be left aligned.
System.out.format("%d%n", -553); System.out.format("%(d%n", -553); By default, negative numbers have a minus sign. If we use the ( flag, the negative values will be put inside round brackets. (This is used in accounting.)
$ java Flags.java +553 0000000553 553 553 -553 (553)
Java String format width
The width field is the minimum number of characters to be written to the output. It cannot be used together with the line separator.
package com.zetcode; public class WidthSpecifier < public static void main(String[] args) < System.out.println(1); System.out.println(16); System.out.println(1655); System.out.println(16567); System.out.println(166701); System.out.format("%10d%n", 1); System.out.format("%10d%n", 16); System.out.format("%10d%n", 1655); System.out.format("%10d%n", 16567); System.out.format("%10d%n", 166701); >> First, we print five numbers without specifying the field width. The width of the output is equal to the number of the characters being displayed. In the second case, we have a field width of 10. Each of the 5 outputs has a minimum length of 10 characters. The numbers are right aligned.
Number 10 states that the string output must have at least ten characters.
$ java WidthSpecifier.java 1 16 1655 16567 166701 1 16 1655 16567 166701
We can see that in the second case the numbers are right aligned.
Java String format precision
The precision field has different meaning for different conversions. For general argument types, the precision is the maximum number of characters to be written to the output.
package com.zetcode; public class PrecisionSpecifier < public static void main(String[] args) < System.out.format("%.3g%n", 0.0000006); System.out.format("%.3f%n", 54.34263); System.out.format("%.3s%n", "ZetCode"); >> The precision specifier is demonstrated on three different outputs.
System.out.format("%.3g%n", 0.0000006); If the g conversion is used, then the precision is the total number of digits in the resulting magnitude after rounding.
System.out.format("%.3f%n", 54.34263); For floating point values, the precision is the number of digits after the decimal separator.
System.out.format("%.3s%n", "ZetCode"); For strings, it is the maximum number of printed characters. Only three characters out of seven are printed to the console.
$ java PrecisionSpecifier.java 6.00e-07 54.343 Zet
Java String format numbers
The next example formats numeric data.
package com.zetcode; public class FormatNumbers < public static void main(String[] args) < System.out.format("%d%n", 12263); System.out.format("%o%n", 12263); System.out.format("%x%n", 12263); System.out.format("%e%n", 0.03452342263); System.out.format("%d%%%n", 45); >> The example demonstrates the standard formatting specifiers for numbers.
The d conversion specifier will turn an integer value into a decimal value.
The o conversion specifier will format the number into the octal base.
With the x specifier, the result is formatted as a hexadecimal integer.
System.out.format("%e%n", 0.03452342263); Using the e specifier, the number is printed in a scientific notation.
The %% characters are used to print a percent sign.
$ java FormatNumbers.java 12263 27747 2fe7 3.452342e-02 45%
Java String format date and time
Finally, we format date and time data.
package com.zetcode; import java.time.LocalDateTime; public class FormatDateTime < public static void main(String[] args) < LocalDateTime ldt = LocalDateTime.now(); System.out.format("%tF%n", ldt); System.out.format("%tD%n", ldt); System.out.format("%tT%n", ldt); System.out.format("%1$tA, %1$tb %1$tY%n", ldt); System.out.format("%1$td.%1$tm.%1$tY%n", ldt); >> The example demonstrates the standard formatting specifiers for dates. The conversion part of the date and time format string starts with the t character.
This line prints a date in a complete ISO 8601 format, as a result of the tF conversion.
System.out.format("%1$td.%1$tm.%1$tY%n", c); Using these format specifiers, we print a date in the form that is used in Slovakia. The parts are separated by the dot character and the day precedes the month and the month precedes the year. All three format specifiers refer to the c variable.
$ java FormatDateTime.java 2022-10-17 10/17/22 11:30:18 Monday, Oct 2022 17.10.2022
Java localized String format
We can pass the locale to the formatting methods.
package com.zetcode; import java.time.LocalDate; import java.util.Locale; public class Localized < public static void main(String[] args) < double val = 12_568_120.214; LocalDate now = LocalDate.now(); System.out.printf("%f%n", val); System.out.printf(Locale.FRENCH, "%f%n", val); System.out.printf("%tA%n", now); System.out.printf(Locale.FRENCH, "%tA%n", now); >> In the example, we print values in English and French locales.
$ java Localized.java 12568120.214000 12568120,214000 Monday lundi
In this article we have formatted strings in Java.
Author
My name is Jan Bodnar and I am a passionate programmer with many years of programming experience. I have been writing programming articles since 2007. So far, I have written over 1400 articles and 8 e-books. I have over eight years of experience in teaching programming.
String.format в Java
— Я еще хотела тебе рассказать о методе String.format в Java.
Это статический метод класса String , но очень полезный. Но начну издалека.
Когда тебе нужно вывести в одной строке текст, содержащий значения нескольких переменных, как ты будешь это делать?
String name = "Bender"; int age = 12; String friend = "Fry"; int weight = 200;
String name = "Bender"; int age = 12; String friend = "Fry"; int weight = 200; System.out.println("User = ");
— Не очень читаемо – не так ли?
— Но вот если имена переменных у тебя будут длинные или нужно будет вызывать методы для получения данных, будет уже не очень читаемо:
— Ну, если так, то да – не очень читаемо.
— Дело в том, что в реальных программах такое встречается очень часто, поэтому я хочу показать тебе, как можно упростить себе жизнь с помощью метода String.format.
— Да говори же скорее, что это за волшебный метод такой?
— Вот как можно записать приведенный выше код:
String text = String.format("User = ", user.getName(), user.getAge(), user.getFriends().get(0), user.getExtraInformation().getWeight()) System.out.println(text);
В метод String.format первым параметром передается строка-шаблон, в которой, на местах, в которые мы хотим подставить значения, стоят специальные символы: %s, %d.
После строки-шаблона передаются параметры, значения которых и будут подставлены на место символов %s, %d.
Если нужно подставить строку мы пишем %s, если число — %d.
String s = String.format("a = %d, b = %d, c = %d", 1, 4, 3);
s будет равна «a = 1, b = 4, c = 3» — А ведь еще можно делать и так:
int a = -1, b = 4, c = 3; String template; if (a < 0) template = "Warning! a = %d, b = %d, c = %d"; else template = "a = %d, b = %d, c = %d"; System.out.println(String.format(template, a, b, c) );
— Гм. Действительно полезный метод, спасибо, Элли.
— Если ты хочешь использовать в методе format другие типы данных, то вот тебе таблица:
| Обозначение | Тип |
|---|---|
| %s | String |
| %d | целое число: int, long, … |
| %f | вещественное число: float, double |
| %b | boolean |
| %c | char |
| %t | Date |
| %% | Символ % |
На самом деле у этих параметров еще есть свои настройки, а у них свои.
Но для начала тебе этого хватит. Если же нет – вот тебе ссылка на официальную документацию: