String.format() in Java

Direct Answer

String.format(format, args...) returns text using Formatter syntax such as %s, %d, %.2f, and %n. The no-locale overload uses the default formatting locale, so use String.format(Locale.ROOT, ...) or a chosen locale when output must be stable. In logging frameworks, prefer their placeholder API instead of eagerly formatting a message.

Code Example

public class FormatDemo {
    public static void main(String[] args) {

        String name = "Alice";
        int age = 28;
        double score = 95.678;

        // Basic usage — %s for string, %d for int, %f for double
        String msg = String.format("Name: %s, Age: %d, Score: %f", name, age, score);
        System.out.println(msg); // Name: Alice, Age: 28, Score: 95.678000

        // Control decimal places with %.Nf
        String scoreMsg = String.format("Score: %.2f", score);
        System.out.println(scoreMsg); // Score: 95.68

        // Padding and alignment
        System.out.printf("%-15s %5d%n", "Alice", 95);  // Alice              95
        System.out.printf("%-15s %5d%n", "Bob", 72);    // Bob                72

        // printf() = String.format() + print — no assignment needed
        System.out.printf("Hello, %s! You scored %d points.%n", name, (int) score);
        // Hello, Alice! You scored 95 points.

        // Using formatted() — instance method in Java 15+
        String result = "User %s has %d tokens".formatted(name, 10);
        System.out.println(result); // User Alice has 10 tokens
    }
}

Common Mistakes & Pitfalls

• Passing a non-integer to %d — String.format("%d", 3.14) throws IllegalFormatConversionException at runtime, not compile time, so it sails through your build and fails in production. Use %f for decimals, or %s when the type is uncertain.
• Mismatching the argument count — too few arguments for the placeholders throws MissingFormatArgumentException; the compiler can't catch it because the template is just a String.
• Using \n instead of %n in a template — %n emits the platform line separator, while a hardcoded \n produces the wrong line ending for Windows-style consumers and log parsers.
• Expecting %f to print clean decimals — bare %f always prints 6 digits (3.140000); use %.2f to control precision for prices and scores.
• Reaching for + on multi-value messages — concatenation is harder to read, easy to misorder, and painful to localize compared to a single template.