A format string contains directives that control how subsequent arguments are interpreted and rendered. Java’s String.format() and PrintStream.printf() use format specifiers to substitute values into the string. section 6.4
Treat mismatched format arguments as a practical correctness and reliability hazard:
- Update the arguments whenever the format string changes; unused arguments may be silently ignored, while incompatible arguments can produce incorrectly formatted output or runtime exceptions. section 6.4
- Do not assume every specifier enforces the intended type: %s accepts any value, and %b converts non-Boolean values to true when non-null and false when null. section 6.4
- Use static analysis when the format string is constant; IntelliJ IDEA and SonarLint can detect several mismatches, but may not flag semantically wrong uses such as %b with a non-Boolean. section 6.11 section 6.4
Argument order changes the meaning
With int x = 10 and int y = 20, System.out.printf("Point is: %d\n", x, y); prints only Point is: 10 because the extra argument is ignored. For typed values, align each argument with its directive: System.out.printf("Day: %s; isWeekend = %b\n", dw, isWeekend);. section 6.4
int x = 10;
int y = 20;
System.out.printf("Point is: %d\n", x, y);
Inspect the minimal example where an unused argument produces plausible but incomplete output. section 6.4
