Formatting

Building output by gluing strings with + works, but it gets noisy fast and gives you no control over decimals, padding, or alignment. Java's answer is a pair of near-twin tools, both using C-style format specifiers:

String s = String.format("%s is %d years old", "Alice", 30);  // returns the String
System.out.printf("%s is %d years old%n", "Alice", 30);       // prints it directly

Same mini-language, different destination: String.format hands you the result to store or pass around; System.out.printf writes straight to stdout. Learn one and you've learned both.

The specifier mini-language

A format specifier starts with % and says "a value goes here, formatted like this":

Width and precision compose: %8.2f means "8 columns wide, 2 decimal places" — the backbone of every aligned table you'll ever print.

System.out.printf("%-8s | %6.2f%n", "total", 12.5);
// total    |  12.50

The traps

Type mismatches blow up at runtime, not compile time. The format string is just text — the compiler doesn't look inside it:

System.out.printf("%d%n", "Alice");   // compiles fine...
// runtime: IllegalFormatConversionException: d != java.lang.String

printf does not add a newline. println always appends one; printf prints exactly what the format string says — nothing more. If your format string doesn't end with %n (or \n), the line is left unterminated and whatever prints next continues on the same line — which silently corrupts every line after it. (At the very END of a program it slips past this platform's comparator, which ignores trailing whitespace, but do not rely on that: terminate your lines.)

System.out.printf("done");     // no newline — output line never ends
System.out.printf("done%n");   // complete line

Which to use when

• One value, no special formatting → println with simple + concatenation is fine.
• Decimals, alignment, or several values woven into a sentence → printf / String.format.
• Need the string for later (store it, return it) → String.format.

Java 15+ also has text blocks (""" triple-quoted multi-line strings) for big literals — good to recognize in modern code, rarely needed for line-based output like these exercises.

Your exercise

Read a name and an age (the starter parses them for you), then print exactly:

Hi, {name}! You are {age} years old.

Two equally correct paths: a single printf with a %s and a %d woven into the sentence, or plain + concatenation inside a println.

For Alice / 25 the grader expects Hi, Alice! You are 25 years old. — character for character. The mistakes it will catch: dropping the comma after Hi, the ! after the name, or the final period. If you go the printf route, remember that it prints exactly what the format string says and nothing more.