Displaying Formatted Output with String.format()

Displaying Formatted Output with String.format()

1. What It Does (beginner)

String.format() builds a formatted String (doesn't print — returns!). Twin of printf (which prints). Use when you need to store/send formatted text (bill, report, log).

2. Same Specifiers, Two Uses

double pi = 3.14159;
String s = String.format("pi=%.2f", pi); // s = "pi=3.14" (stored!)
System.out.println(s);                    // print stored
System.out.printf("pi=%.2f%n", pi);       // direct print (same output!)
// Rule: printf(format, args) = print(format(format, args))

Spec	Example → String
%d	String.format("%d", 42) → "42"
%.2f	String.format("%.2f", pi) → "3.14"
%-10s	left-align name in 10 chars
%05d	zero-pad: String.format("%05d", 7) → "00007"
%,d	commas: String.format("%,d", 1000000) → "1,000,000"

3. Beginner Project — Bill Line

String item = "Tea";
int qty = 2;
double price = 25.5;
String bill = String.format("%-10s %2d x %6.2f = %7.2f", item, qty, price, qty*price);
System.out.println(bill); // Tea         2 x  25.50 =   51.00

4. Traps

• String.format RETURNS (must assign/print — calling alone does nothing visible!).
• Mismatch %d ← double → IllegalFormatConversionException.
• %% prints %: String.format("%.1f%%", 92.5) → "92.5%".

5. Exam Points

• printf vs String.format (print vs return) — 5 marks, very frequent.
• Bill/table program + zero-pad/commas — 5 marks, frequent.