Java String format() method
Syntax and Examples

The String.format() method in Java is a powerful tool for creating formatted strings. Think of it as a way to insert variables into a string template, just like you might use placeholders in a word processor or a mail merge application.

Syntax

public static String format(Locale l, String format, Object... args)

public static String format(String format, Object... args)

The format() method has two main overloads. The first allows you to specify a locale for formatting (useful for things like currency symbols or date formats). The second is more commonly used and just requires the format string and the arguments.

Parameters

Parameter	Description
l (optional)	The locale to use for formatting. If omitted, the default locale is used.
format	A format string that specifies how the arguments should be formatted within the resulting string. This string contains plain text and format specifiers (e.g., %d for integers, %f for floating-point numbers).
args	An array of objects to be inserted into the format string. The types of these objects must be compatible with the format specifiers in the format string.

Return Value

The format() method returns a new string that is formatted according to the specified pattern and arguments.

Examples

Example 1: Basic Integer Formatting

This example demonstrates how to format an integer using the %d specifier. The %d specifier tells Java to insert a decimal (integer) value into the string.

int age = 30;
String formattedAge = String.format("You are %d years old.", age);
System.out.println(formattedAge);

You are 30 years old.

Explanation: The value of the age variable (30) is inserted into the format string at the location of the %d placeholder.

Example 2: Formatting Floating-Point Numbers

This example shows how to format a floating-point number using the %f specifier. The %f specifier is used for inserting floating point numbers, and you can control the precision (number of digits after the decimal point).

double price = 19.99;
String formattedPrice = String.format("The price is $%.2f.", price);
System.out.println(formattedPrice);

The price is $19.99.

Explanation: The %.2f format specifier tells Java to insert the value of price as a floating-point number with two digits after the decimal point.

Example 3: Formatting Strings

This example illustrates how to incorporate strings into formatted output using the %s specifier. The %s is used for inserting String values.

String name = "Alice"
String greeting = String.format("Hello, %s!", name);
System.out.println(greeting);

Hello, Alice!

Explanation: The value of the name variable ("Alice") is inserted into the format string at the location indicated by %s.

Example 4: Using Multiple Arguments

This example demonstrates formatting with multiple arguments. You can use several specifiers in a single format string.

String firstName = "John"
String lastName = "Doe"
int id = 12345;
String formattedInfo = String.format("Name: %s %s, ID: %d.", firstName, lastName, id);
System.out.println(formattedInfo);

Name: John Doe, ID: 12345.

Explanation: The format string contains three placeholders (%s, %s, and %d), and their corresponding values are taken from the firstName, lastName, and id variables respectively.

Example 5: Using Locale for Currency Formatting

This example shows how to format a currency value using different locales. This is particularly useful when you need to display amounts in various currencies with the correct symbols and formats (e.g., US dollars vs. Euros).

double amount = 1000.00;
Locale usLocale = Locale.US; // For US Dollars
Locale germanLocale = new Locale("de", "DE"); // For German Euros

String usdFormatted = String.format(usLocale, "$%.2f", amount);
String eurFormatted = String.format(germanLocale, "%.2f €", amount);

System.out.println("US Dollar: " + usdFormatted);
System.out.println("German Euro: " + eurFormatted);

US Dollar: $1000.00
German Euro: 1000,00 €

Explanation: We use the Locale to specify the currency format. The US locale displays a dollar sign ($) before the amount, while the German locale includes the euro symbol (€) after the amount and uses a comma as the decimal separator.