Java .format()

In Java, the .format() method returns a formatted string using the specified format string and arguments. This method is part of the String class and helps improve code readability, localization support, and consistency.

Syntax

string.format(str, ...args);

Parameters:

• str: A format string containing fixed text and format specifiers (like %s, %d, %f, etc.).
• args (Optional): The values that will replace the format specifiers. It can be of any data type.

Return value:

The .format() method returns the formatted version of the given string.

Common Format Specifiers

Notes:

• A NullPointerException will be thrown if the format is null.
• See Java’s similar .printf() method for more information on format specifiers.

Example 1: Basic Usage of .format()

This example demonstrates the usage of the .format() method:

class FormatExample1 {
  public static void main(String[] args) {
    String name = "Katara";
    String job = "bender";

    System.out.println(String.format("%s is a water %s.", name, job));
  }
}

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Here is the output:

Katara is a water bender.

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Example 2: Formatting Numbers Using .format()

This example uses the %.2f format specifier to format the given floating-point number to two decimal places:

public class FormatExample2 {
  public static void main(String[] args) {
    double price = 1234.56789;

    String formatted = String.format("Price: $%.2f", price);

    System.out.println(formatted);
  }
}

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Here is the output:

Price: $1234.57

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Example 3: Formatting Dates Using .format()

This example first uses the Calendar.getInstance() method to create a Calendar object containing the current date. Then, it uses the %1$td, %1$tm, and %1$tY format specifiers to format the date from the object in dd/mm/yyyy format (%1$ refers to the first argument):

import java.util.Calendar;

public class FormatExample3 {
  public static void main(String[] args) {
    Calendar cal = Calendar.getInstance();

    String formatted = String.format("Date: %1$td/%1$tm/%1$tY", cal);

    System.out.println(formatted);
  }
}

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Here is the output:

Date: 21/05/2025

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Note: Since the Calendar object contains the current date, the output may vary each time the code is run.

Frequently Asked Questions

1. What is the difference between .printf() and .format()?

Both use the same formatting syntax. .printf() prints the formatted string directly to the console, while .format() returns the formatted string for further use.

2. Is .format() locale-sensitive?

Yes, there’s an overloaded version for .format() that accepts a Locale, which is especially useful for formatting currency, dates, or numbers:

String.format(Locale.FRANCE, "%.2f", 1234.56); // Uses comma as decimal separator

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3. What happens if the arguments don’t match the format specifiers in .format()?

If the arguments don’t match the format specifiers in .format(), Java throws a java.util.IllegalFormatException, such as MissingFormatArgumentException or IllegalFormatConversionException.