Basic Output and Formatted Output

When a Java program prints text and the result looks wrong, the value being displayed is often not the problem. The real issue is frequently the position of the next piece of text. Every output operation does two jobs: it displays text, and it decides where later text will begin. This article separates those two jobs so you can trace them one statement at a time.

The difference between print and println is easiest to see by following the cursor. Think of the cursor as the position where the next character will appear. A print call writes its text at the cursor and leaves the cursor just after that text. A println call writes its text the same way, and then adds a line feed, which moves the cursor to the start of the next line.

In the first case, both pieces of text sit on the same line because neither call ends with a line feed. In the second case, the line feed added by println("A") pushes the B onto a new line. The text displayed is identical in both cases; only the position of the cursor after the first call has changed.

The two methods share the same display behavior. The only difference is what happens after the text is written. This means you can decide the layout of output by choosing between them: print when the next piece of text should continue on the same line, and println when the next piece of text should begin a new line.

Trace the example line by line. The first two print calls place Hello and , world on the same line without any line feed. The println call writes ! and then adds a line feed, so the cursor moves down. The last print call writes Done at the start of the new line, and nothing follows it, so no line feed is added after it.

print does not only accept text. It accepts values and expressions too. When the argument is not already a string, print converts it to a string before the characters reach the console. The source describes this as converting x to a string when necessary, so a number such as 7 is shown as the characters 7, and an expression such as 3 + 4 is evaluated first and then its result is displayed.

System.out.println() with no argument outputs no visible text. Its only purpose is to add a line feed. This makes it a tool for controlling layout: it ends the current line without adding any characters to it, so the next output starts on a fresh line.

Without the empty println call, the word Second would appear directly after First on the same line. The empty call separates the text produced before it from the text produced afterward, and it does so without introducing any extra characters.

When a program has several output statements, trace them in order. For each statement, ask two questions: what text does it display, and does it end with a line feed? The diagram below follows four statements and shows where each piece of text lands and where the cursor is left for the next call.

In the programs covered here, standard output is generally the place where a user sees program text. Standard output is not permanently fixed to one visible screen, however. It can be redirected to another destination, and in a GUI program it may appear somewhere a typical user does not see.