String Formatting

Format numbers, text, and dates precisely using String.format(), printf(), and formatted() with format specifiers.

Overview

String formatting lets you control exactly how values appear in output - decimal places, padding, alignment, and more. Java provides System.out.printf() for direct formatted printing, String.format() to build a formatted String, and the newer String.formatted() instance method. Format specifiers start with % and describe the type and width of the value to insert. %s for String, %d for integer, %f for floating-point, %c for char, %n for newline.

Key Points

•%s - String argument
•%d - integer (int, long)
•%f - floating-point; %.2f means 2 decimal places
•%c - char
•%n - platform-correct newline (prefer over \n in printf)
•%10s - right-align in 10 characters wide; %-10s - left-align
•%05d - pad with leading zeros: 007 → %03d gives "007"
•String.format(pattern, args) returns a String; printf prints directly

Examples

Basic format specifiers

public class Main {
    public static void main(String[] args) {
        String name = "Alice";
        int age = 28;
        double gpa = 3.756;

        System.out.printf("Name: %s%n", name);
        System.out.printf("Age: %d%n", age);
        System.out.printf("GPA: %.2f%n", gpa);  // 3.76 (rounded)

        // String.format builds and returns a String
        String report = String.format("%-10s | %3d | %.1f", name, age, gpa);
        System.out.println(report); // Alice      |  28 | 3.8
    }
}

%.2f rounds to 2 decimal places. %-10s left-aligns in a 10-character field. %3d right-aligns in a 3-character field.

Formatting a table

public class Main {
    public static void main(String[] args) {
        System.out.printf("%-15s %8s %8s%n", "Product", "Price", "Qty");
        System.out.println("-".repeat(33));
        System.out.printf("%-15s %8.2f %8d%n", "Apple",   0.99, 150);
        System.out.printf("%-15s %8.2f %8d%n", "Banana",  0.49, 300);
        System.out.printf("%-15s %8.2f %8d%n", "Cherry", 2.99,  75);
    }
}

Using consistent field widths creates aligned table columns.

String.formatted() - modern Java

public class Main {
    public static void main(String[] args) {
        // Java 15+ - instance method on String
        String msg = "Hello, %s! You have %d messages.".formatted("Bob", 5);
        System.out.println(msg);

        // Padding and zero-fill
        System.out.printf("Code: %05d%n", 42);    // Code: 00042
        System.out.printf("Hex: %X%n", 255);       // Hex: FF
        System.out.printf("Sci: %e%n", 12345.678); // Sci: 1.234568e+04
    }
}

String.formatted() (Java 15+) is the object-oriented version of String.format().

Common Mistakes

Mismatching format specifier and argument type: `printf("%d", 3.14)` throws IllegalFormatConversionException
Using \n instead of %n in printf on Windows - %n produces the platform-correct line separator
Too few arguments for the format string: `printf("%s %s", "only one")` throws MissingFormatArgumentException

Note

Use String.format() (or .formatted()) when building a String to store or return. Use printf() when printing directly. Both use the same format syntax.

String Formatting

Overview

Key Points

Examples

Common Mistakes

Note

Exercises

Knowledge Check

What does `%.2f` format mean?