How can you format a string of characters?

There are actually several solutions given to you by the JDK, but the simplest one is probably the format factory method of the String class. It takes a format as its first argument, followed by the objects you want to pass to that format so they get rendered into the final string.

java
String message = String.format("%s scored %d points", "Ana", 92);
// message = "Ana scored 92 points"

This format syntax is inspired by the (in)famous format used by C's printf function — the one everybody knows, or has at least run into. It is fully described in the Javadoc of the Formatter class, which is the engine behind String.format, PrintStream.printf, and PrintWriter.printf.

There are, however, differences between C's printf and Java's Formatter:

• Errors are not handled the same way. C's printf trusts the format string and reads memory according to it, regardless of what was actually passed — a mismatch can silently corrupt output or crash. Java's Formatter type-checks each argument against its specifier and throws IllegalFormatConversionException on a mismatch instead.
• Some customization has been made on top of the original C syntax — for instance the n$ argument index and the %n platform-independent line separator have no equivalent in C's printf.

java
System.out.printf("%-10s | %5.2f%n", "Total", 42.5);   // writes straight to System.out
String s = String.format("%-10s | %5.2f", "Total", 42.5); // same rendering, returned as a String

Formatting strings might tempt you to share a Formatter instance to save some resources — but be careful: a Formatter carries mutable internal state (its output buffer), so sharing one across threads is not safe. String.format and printf never run into this because each call creates its own private Formatter internally.