print, println and printf: Java's Three Ways to Write a Line

The three methods#

System.out.print("A");
System.out.print("B");
System.out.println("C");
System.out.println("next line");

ABC
next line

print leaves the cursor where it is. println writes its argument and then ends the line. That is the whole difference, and it is why the first three calls above land on one line.

System.out.println() with nothing in the brackets is legal and useful — it just ends the current line, which is how you finish output that was built with print.

The third method is printf, which takes a format string containing placeholders, followed by the values to drop into them:

System.out.printf("%s is %d years old%n", "Ada", 36);

Ada is 36 years old

The plus sign is two operators#

This is the part worth slowing down for, because it is the source of more wrong output than anything else in this topic.

System.out.println("1" + 2 + 3);
System.out.println(1 + 2 + "3");

123
33

Same three values, same operator, completely different answers. + means numeric addition when both sides are numbers, and string concatenation when either side is a String. Evaluation runs strictly left to right:

Once a String enters the expression, everything after it is concatenation. Here is the version that shows up in real code:

System.out.println(10 + 5 + " total");

15 total

That one happens to be what you wanted. Swap the operands to " total " + 10 + 5 and you get total 105. When in doubt, bracket the arithmetic: " total " + (10 + 5).

Escape sequences#

Some characters cannot be typed directly inside a string, because they would end it or be invisible. A backslash escapes them:

The backslash one catches people writing Windows paths: to print one backslash you type two.

printf format specifiers#

You will use four of these constantly and the rest rarely:

The width specifiers are what make columns line up in a table of output, which is usually why an assignment asks for printf in the first place.

printf does not end the line for you#

System.out.printf("no newline here");
System.out.println();
System.out.println("done");

no newline here
done

Unlike println, printf writes exactly what you asked for and nothing more. If you want the line to end, put %n at the end of the format string.

Use %n rather than \n#

Both end a line. \n is always one line-feed character; %n is whatever the platform you are running on considers a line separator, which on Windows is two characters. Inside a format string, prefer %n.

A rounding result worth knowing#

This behavior surprises people who have used other languages, so it is worth seeing rather than being told:

System.out.printf("%.2f%n", 3.14159);
System.out.printf("%.2f%n", 2.345);
System.out.printf("%.2f%n", 2.675);

3.14
2.35
2.68

That last one is the interesting case. The value 2.675 cannot be stored exactly as a double — what is actually in memory is 2.674999999999999822..., which is just below the halfway point. Rounding the stored value would give 2.67, and that is exactly what Python and C do.

Java gives 2.68, because printf rounds the shortest decimal text that would round-trip back to the same double — the string "2.675" — rather than the exact stored value.

Too few arguments fails at run time#

System.out.printf("%d and %d%n", 1);

1 and
Exception in thread "main" java.util.MissingFormatArgumentException: Format specifier '%d'
	at java.base/java.util.Formatter.format(Formatter.java:2790)
	...
	at Fmt.main(Fmt.java:3)

Two things to notice. It compiled — the compiler does not count your placeholders. And 1 and was printed before it failed, which is why partial output above a stack trace is a strong hint that a format string is the culprit.

Where these go wrong#

• Concatenating before adding. "Total: " + a + b glues the two numbers together. Bracket the sum.
• Expecting printf to end the line. It never does. Add %n.
• Mismatched specifiers. %d with a double throws IllegalFormatConversionException at run time; the compiler will not catch it.
• Single quotes for strings. 'A' is a char and "A" is a String. 'Hi' is a compile error — a char holds exactly one character.
• One backslash in a Windows path. "C:\Users" will not compile the way you meant; \U is not a valid escape.
• Rounding money with printf. Fine for showing a number to a human, wrong as a step in a calculation.

Frequently asked questions#