Basics

13. Modern String Formatting

Java offers String.format(), String.formatted() (Java 15+), printf, and MessageFormat for string formatting; use %n for platform-neutral newlines and know the common format specifiers.

Java 21 (core language)

Explanation

String.format(fmt, args...) is the classic static approach. Java 15 added String::formatted as an instance method for more fluent chaining: "Hello %s".formatted("World").

Common format specifiers: %s (toString), %d (decimal integer), %f (floating point, default 6 decimal places), %.2f (2 decimal places), %n (platform newline — prefer over \n for portability in cross-platform output), %x (hex), %05d (zero-padded width 5), %-10s (left-aligned in width 10).

MessageFormat (from java.text) uses {0}, {1} indexed placeholders and supports locale-aware number/date formatting. For complex localisation use MessageFormat; for simple string building prefer String.formatted() or text blocks with formatted().

Example

import java.text.MessageFormat;

public class StringFormattingDemo {
    public static void main(String[] args) {
        String name  = "Alice";
        int    age   = 30;
        double score = 98.567;

        // String.format — classic static
        String s1 = String.format("Name: %-10s Age: %3d Score: %.2f", name, age, score);
        System.out.println(s1); // Name: Alice      Age:  30 Score: 98.57

        // String::formatted — fluent instance method (Java 15+)
        String s2 = "Hello, %s! You scored %.1f%%".formatted(name, score);
        System.out.println(s2); // Hello, Alice! You scored 98.6%

        // %n vs \n: %n is platform-neutral newline
        System.out.printf("Line 1%nLine 2%n");

        // Hex and zero-padding
        int value = 255;
        System.out.printf("Dec: %d  Hex: %#x  Padded: %08d%n", value, value, value);
        // Dec: 255  Hex: 0xff  Padded: 00000255

        // MessageFormat for localisation/indexed args
        String pattern = "User {0} has {1} messages.";
        String msg = MessageFormat.format(pattern, name, 5);
        System.out.println(msg); // User Alice has 5 messages.

        // Text block + formatted (clean for multi-line templates)
        String report = """
                Name:  %s
                Age:   %d
                Score: %.2f
                """.formatted(name, age, score);
        System.out.print(report);
    }
}

Sources