Formatting Values

Formatting Values

Java provides several APIs for converting numbers, dates, and other values into human-readable strings — String.format, NumberFormat, and DateTimeFormatter. Each has its own syntax and use cases that the OCP exam tests directly.

String.format and formatted()

String.format(String format, Object... args) returns a formatted string using format specifiers as placeholders. In Java 15+, the instance method formatted() on String does the same thing:

java
String s1 = String.format("Hello, %s! You are %d years old.", "Alice", 30);
String s2 = "Hello, %s! You are %d years old.".formatted("Alice", 30);
// Both produce: "Hello, Alice! You are 30 years old."

System.out.printf uses the same format string syntax but writes directly to stdout instead of returning a String.

Format Specifiers

Specifier Type Example Output
%s String (or any object via toString) "%s".formatted("hello") hello
%d Integer (int, long) "%d".formatted(42) 42
%f Floating-point "%f".formatted(3.14) 3.140000
%.2f Float with 2 decimal places "%.2f".formatted(3.14159) 3.14
%n Platform line separator "line1%nline2" line1\nline2
%b Boolean "%b".formatted(null) false
%c Character "%c".formatted('A') A 
java
double price = 19.99;
int qty = 3;
String line = String.format("%-20s %5.2f x %d = %7.2f",
        "Widget", price, qty, price * qty);
// "Widget               19.99 x 3 =   59.97"

Width and flags:

  • %10d — right-align in field of width 10.
  • %-10d — left-align in field of width 10.
  • %05d — zero-pad to width 5.

NumberFormat

java.text.NumberFormat provides locale-aware formatting for numbers, currencies, and percentages. You obtain instances through factory methods — the constructor is not public.

java
import java.text.NumberFormat;
import java.util.Locale;

double value = 1234567.89;

NumberFormat nf  = NumberFormat.getNumberInstance();
NumberFormat curr = NumberFormat.getCurrencyInstance(Locale.US);
NumberFormat pct  = NumberFormat.getPercentInstance();

System.out.println(nf.format(value));      // 1,234,567.89  (default locale)
System.out.println(curr.format(value));    // $1,234,567.89
System.out.println(pct.format(0.856));     // 86%

Parsing a formatted number back to a Number:

java
NumberFormat nf = NumberFormat.getNumberInstance(Locale.US);
Number parsed = nf.parse("1,234.56");
double d = parsed.doubleValue();   // 1234.56

parse() throws the checked ParseException if the string cannot be parsed.

DateTimeFormatter

java.time.format.DateTimeFormatter formats and parses LocalDate, LocalTime, LocalDateTime, and ZonedDateTime. It is immutable and thread-safe.

Predefined Formatters

java
import java.time.*;
import java.time.format.*;

LocalDate date = LocalDate.of(2024, 3, 15);
System.out.println(date.format(DateTimeFormatter.ISO_LOCAL_DATE));   // 2024-03-15
System.out.println(date.format(DateTimeFormatter.BASIC_ISO_DATE));   // 20240315

Custom Patterns

java
DateTimeFormatter fmt = DateTimeFormatter.ofPattern("MM/dd/yyyy");
LocalDate d = LocalDate.of(2024, 3, 15);
System.out.println(d.format(fmt));   // 03/15/2024

LocalDate parsed = LocalDate.parse("03/15/2024", fmt);
System.out.println(parsed);          // 2024-03-15

Common pattern symbols:

Symbol Meaning Example
yyyy 4-digit year 2024
MM 2-digit month 03
dd 2-digit day 15
HH Hour (00–23) 14
mm Minutes 05
ss Seconds 30
a AM/PM PM

Locale-Sensitive Formatting

java
DateTimeFormatter fmt = DateTimeFormatter
        .ofLocalizedDate(FormatStyle.LONG)
        .withLocale(Locale.FRANCE);

LocalDate date = LocalDate.of(2024, 3, 15);
System.out.println(date.format(fmt));   // 15 mars 2024

FormatStyle options: FULL, LONG, MEDIUM, SHORT.

Key Rules Summary

  • %s works for any object; %d is for integers; %f for floats; %n for a newline; %b returns false for null.
  • String.format and formatted() produce the same result — formatted() is the more modern instance-method form.
  • NumberFormat instances are created via factory methods (getNumberInstance, getCurrencyInstance, getPercentInstance); pass a Locale for locale-aware output.
  • DateTimeFormatter is immutable and thread-safe; use ofPattern() for custom patterns and ofLocalizedDate/Time/DateTime() for locale-aware patterns.
  • Parsing with NumberFormat.parse() throws checked ParseException; parsing with DateTimeFormatter throws unchecked DateTimeParseException.

References