String Formatting in Java

printf prints formatted text straight to the screen, but often you need the formatted text as a string: to save in a file, send in an email, or show in an app. String.format and formatted build that string using the same % specifiers you met in "Format Specifiers". This lesson focuses on building strings, and on formatting numbers the way people in a particular country expect to read them.

Building a Formatted String

public class Receipt {
    public static void main(String[] args) {
        String header = String.format("%-10s %5s %8s", "Item", "Qty", "Amount");
        String row1 = String.format("%-10s %5d %8.2f", "Rice", 3, 54.5);
        String row2 = "%-10s %5d %8.2f".formatted("Ghee", 1, 249.0);
 
        System.out.println(header);
        System.out.println(row1);
        System.out.println(row2);
        System.out.println("Each row is " + row1.length() + " characters");
 
        System.out.println(String.format("%2$s scored %1$d", 91, "Navika"));
    }
}

Item         Qty   Amount
Rice           3    54.50
Ghee           1   249.00
Each row is 25 characters
Navika scored 91

How it works

1. String.format(template, values...) returns the filled-in template as a String; nothing is printed until you print it.
2. "template".formatted(values...) (Java 15) does exactly the same, written on the template.
3. Widths line up the columns: %-10s is a left-aligned 10-character column, %5d and %8.2f are right-aligned.
4. %2$s and %1$d pick arguments by position, so the template can use them in any order. That's useful when the same template is translated into another language with a different word order.

Formatting for a Country

People in different countries write numbers differently. Java's Locale describes a country's conventions, and the java.text.NumberFormat class uses it:

import java.text.NumberFormat;
import java.util.Locale;
 
public class Locales {
    public static void main(String[] args) {
        Locale india = Locale.of("en", "IN");
        double amount = 1234567.5;
 
        NumberFormat rupees = NumberFormat.getCurrencyInstance(india);
        NumberFormat dollars = NumberFormat.getCurrencyInstance(Locale.US);
        System.out.println(rupees.format(amount));
        System.out.println(dollars.format(amount));
        System.out.println(String.format(Locale.GERMANY, "%,.2f", amount));
 
        NumberFormat compact = NumberFormat.getCompactNumberInstance(
            india, NumberFormat.Style.SHORT);
        System.out.println(compact.format(1_500_000));
 
        System.out.println(NumberFormat.getPercentInstance().format(0.256));
    }
}

₹1,234,567.50
$1,234,567.50
1.234.567,50
15L
26%

How it works

1. getCurrencyInstance(india) adds the rupee symbol and two decimal places. Java still groups the digits in threes here, not as 12,34,567.50.
2. The US format uses $; the German format swaps the roles of . and ,.
3. getCompactNumberInstance (Java 12) writes short forms, and for India it uses lakhs: 15,00,000 becomes 15L.
4. getPercentInstance multiplies by 100 and rounds: 0.256 shows as 26%.

Common Mistakes

Formatting money with double and %f for real invoices. Calculate with BigDecimal, then format.

Expecting %,d to give Indian grouping. Java groups by thousands even for India; use compact format or your own helper if you need 12,34,567.

Passing the wrong type for a specifier. %d with a double throws IllegalFormatConversionException (see "Format Specifiers").

Practice

1. Easy: Build a string "Advitha | 95.50% | Rank 1" with String.format and print it.
2. Harder: Format the amounts 99.5, 125000 and 2500000 in rupees, in US dollars and in compact Indian form, as a three-column table.

Key Takeaways