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☕ Java Foundations · Beginner

Printing output in Java

System.out.println vs print vs printf, and System.err.

🧩 The mysteryThree print commands, one or two letters apart, and picking the wrong one scrambles your output. Let's master them.

println vs print

System.out.println prints its text and then moves to a new line. System.out.print prints and stays on the same line, so the next output continues right after it.

System.out.print("A");
System.out.println("B");
System.out.print("C");
// Output:
// AB
// C
🔮 Predict it

Your turn

What does this print?

System.out.print("Ja");
System.out.println("va");
System.out.print("!");
  1. Ja va !
  2. Java !
  3. Java!
Show the answer

print("Ja") stays on the line, println("va") finishes the line, so ! starts a new one: Java then !.

printf fills in blanks

printf takes a format string with placeholders: %d for a whole number, %s for text, %.2f for a decimal with exactly two digits after the point, and %n for a new line.

System.out.printf("%s has %d coins%n",
        "Ada", 3);
// Ada has 3 coins
🔮 Predict it

Your turn

What does this print?

System.out.printf("Total: %.2f%n", 7.5);
  1. Total: 7.5
  2. Total: 7.50
  3. Total: %.2f
Show the answer

%.2f means "exactly two digits after the point", so 7.5 is shown as 7.50. %n ends the line.

Width and alignment

A number after % sets a minimum width. %5d pads with spaces on the left (right-aligned). %-5d pads on the right (left-aligned). %05d pads with zeros. Great for neat tables.

System.out.printf("[%5d]%n", 7);  // [    7]
System.out.printf("[%-5d]%n", 7); // [7    ]
System.out.printf("[%05d]%n", 7); // [00007]

Where do errors go?

✗ Mixed in
System.out.println("Error: no file");

Goes into the same stream as normal output. Redirect output to a file and the error vanishes into it.

✓ Separate stream
System.err.println("Error: no file");

System.err is a separate stream, so errors still reach the screen when normal output is redirected.

💼 In the real world

In real projects

Servers and containers capture standard output and standard error separately, and monitoring tools often watch stderr for trouble. printf-style formatting shows up everywhere: String.format, log messages, invoices and reports.

Key takeaways

  1. println adds a newline, print doesn't
  2. printf("%d %s %.2f%n", …) formats values; %n is a newline
  3. System.err is for errors, so they can be redirected separately
🤯 Did you know?

%n prints the platform's own line separator (\r\n on Windows, \n on macOS and Linux), while \n is always a single line-feed character.

Practice questions

What does this print?

System.out.print("A");
System.out.println("B");
System.out.print("C");
  1. A B C
  2. AB C
  3. ABC
  4. A BC
Check your answer

AB C. print("A") stays on the line, println("B") finishes the line with a newline, then C starts a new line.

What does this print?

System.out.printf("%d items: %.2f%n", 3, 4.5);
  1. 3 items: 4.5
  2. %d items: %.2f
  3. 3 items: 4.500000
  4. 3 items: 4.50
Check your answer

3 items: 4.50. %d is replaced by the int 3 and %.2f formats 4.5 with exactly two digits after the decimal point. %n ends the line.

Next: notes in your code that the compiler completely ignores, and the comment that refuses to nest.