Text Input and Output Fundamentals

System.out.print(x) outputs x after converting it to a string when necessary.

Programming

When the Text Is Right but the Layout Is Wrong

Suppose a Java program prints the correct words, yet the console shows them on the wrong lines or jammed together. It is tempting to look only at the values being displayed. In many cases, though, the value is not the problem. The real issue is 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 at a time.

What do you think happens?

Before running it, predict what the console shows for these two statements: System.out.print("A"); followed by System.out.print("B");

A on one line and B on the next line
AB on a single line
B only, because the second statement replaces the first

Reveal answer

Answer:

AB on a single line

Both calls use print. Print displays its text and does not add a line feed, so the text from the second call continues directly after the first.

Tracing Two Print Calls

Start with the simplest case. The statement System.out.print(x) outputs x. If x is not already text, the value is converted to a string when necessary before it is output. Each print call sends its text to standard output, which is generally where a user sees program text in the programs covered here. After a print call finishes, the output position stays where the printed text ended, so the next output continues on the same line.

java

System.out.print("A");
System.out.print("B");

System.out.print("A");
System.out.print("B");

Output

AB

Notes on the code

The first call displays A and leaves the position just after the A.
The second call displays B beside the A, because print adds no line feed.
The console therefore shows the single line AB.

After System.out.print("A") and then System.out.print("B"), where does B appear relative to A? Print keeps the position on the same line.

The second statement is the trace point that matters most. It both displays B and changes the position for future output. Because print leaves the position on the same line, anything printed later will also begin right after the B unless some other operation moves to a new line.

Print Versus Println

Definition

System.out.println(x) outputs the same text that System.out.print(x) would output, and then adds a line feed. A line feed moves the output position to the start of the next line.

The two methods share the same display behavior. The difference is what happens after the text is shown. Print ends by leaving the position where the text stopped. Println ends by moving the position down to a new line. Once you know this, you can predict the layout of any sequence of output calls by asking one question for each statement: does this operation end by moving to a new line?

java

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

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

Output

A
BC

Notes on the code

The println call displays A and then adds a line feed.
The print call displays B on the new line, and the position stays after B.
The next print call displays C directly after B, so B and C share a line.

Statement	Text displayed	Line feed added	Where later output begins
System.out.print(x)	Yes, the value of x as text	No	Right after the displayed text, on the same line
System.out.println(x)	Yes, the value of x as text	Yes	At the start of the next line
System.out.println()	No visible text	Yes	At the start of the next line

How the three output operations differ in what they display and where the next output begins

Following a Sequence of Output Calls

When a program contains several output statements, the console shows the result of each statement in order. Trace the sequence from top to bottom, and mark each operation that adds a line feed. Those marks tell you exactly where each new line begins. The method below follows that habit on a four-statement program.

Worked example

Tracing print, print, println, and print

What does the console show after each statement of the following program runs in order? System.out.print("A"); System.out.print("B"); System.out.println(); System.out.print("C");

1
Statement 1: print A
A is displayed. Print adds no line feed, so the position stays right after A. The console shows A.
2
Statement 2: print B
B is displayed right after A. The position stays after B. The console shows AB on one line.
3
Statement 3: println with no argument
No visible text is displayed. The call adds only a line feed, so the position moves to the start of the next line. The text AB is now complete on its line.
4
Statement 4: print C
C is displayed at the start of the new line, because the previous line feed moved the position there. The console shows AB on the first line and C on the second line.

Result

The final console shows AB on the first line and C on the second line.

After each statement runs in order, what does the console show, and at which point does a new line begin?

Notice that statement 3 displays nothing of its own. Its entire contribution is the line feed. Without it, C would have appeared right after B on the same line, which is exactly the kind of unexpected placement this trace is designed to catch.

Using Println With No Argument

The no-argument form System.out.println() outputs no visible text. Its only purpose is to add a line feed. This makes it useful for separating text produced before it from text produced afterward, without displaying anything new. The next visual shows the same starting point before and after one such call.

What does the console look like when println() is called with no argument? Only the position changes, not the text.

Mistakes When Tracing Output

Common mistake
Assuming print adds a line break after its text
What it looks like
```
System.out.print("A"); System.out.print("B"); displays AB, not A and B on separate lines.
```
Why it fails
Print displays its text and stops there. The output position stays after the displayed text.
Do this instead
Use println when the next output should begin on a new line, or add a println() call between the two statements.
Common mistake
Ignoring statements that display no visible text
What it looks like
In the sequence print A, print B, println(), print C, a reader may expect C to appear beside B.
Why it fails
The no-argument println() adds a line feed even though it displays nothing, so C starts on a new line.
Do this instead
Mark every operation that adds a line feed as you trace, including println() with no argument.
Common mistake
Looking only at the displayed values when the layout is wrong
What it looks like
A program shows the right words, but they appear on the wrong lines.
Why it fails
Each output operation both displays text and decides where later text begins, so the layout problem may come from a line feed rather than from the values.
Do this instead
Trace the output statements in order, and identify where println or an empty println changes the position.

Predicting the Console Lines

MEDIUM

Without running the code, predict the console output of this program, then check your answer by tracing each statement in order: System.out.print("Hi "); System.out.println("there"); System.out.print("friend"); System.out.println();

Hints

Start with the first statement. Print displays Hi and a space, and the position stays after the space.
The second statement displays there and then adds a line feed. Where does the line end?
The third statement starts on a new line. Does the final println() add any visible text?

Your attempt (not saved)

What do you think happens?

Which lines does the console show for the program in the challenge above?

Hi there on line one, and friend on line two
Hi there friend on a single line
Hi on line one, there on line two, and friend on line three