PublishedBEG-008-L03

Printing output

Lesson 3 of 6
11 min read
Beginner

On This Lesson 11 topics

Lesson Overview
Learning Objectives
Written Explanation
Examples
Backend Insight
Common Mistakes
Exercises
Mini Challenge
Summary & Next Lesson
References
Practice

Lesson Overview

So far every program you wrote printed something, but we treated System.out.println(...) as a magic phrase. This lesson explains it. You will learn the difference between print and println, how to print text and numbers, how to join several pieces into one line with +, and how to include a new line, a tab, or a quotation mark inside text.

In the recipe-card picture, printing is the card's output: the finished dish you put on the table. Text in quotes is a label you write by hand. A number is a measured amount. The + is the cook combining a label and an amount on one line of the card.

Why this matters: output is how a program shows what it did. In backend work, the same skill produces log lines and console messages that you read when something breaks. A program you cannot see inside is a program you cannot debug.

This lesson builds on Lessons 1 and 2: you know where statements go and that each ends with a semicolon.

Learning Objectives

By the end of this lesson you will be able to:

Explain the difference between System.out.print and System.out.println.
Print text, whole numbers, and decimal numbers.
Explain why text needs double quotes and numbers do not.
Join text and values with +, and predict the result.
Use \n, \t, \", and \\ inside text.
Write a short program that prints a multi-line, aligned layout.

Written Explanation

The statement, word by word

java

System.out.println("Hello");

Read it from left to right. System is a built-in class that comes with Java. out is a thing inside it that stands for the standard output, usually your terminal or the IDE's output panel. println is a method, a named action, that writes to it. The parentheses hold what to write. The dots connect the names, and the semicolon ends the statement.

You will not need to build anything like System.out yourself in this module. Use it as a fixed phrase, like main. Remember that it is case sensitive: a capital S in System and lowercase out.

`println` and `print`

The official documentation for PrintStream, the type of System.out, says println behaves as if it first calls print with the value and then ends the line. So the only difference is the line ending:

System.out.println(x) writes x and moves to a new line.
System.out.print(x) writes x and stays on the same line.

java

System.out.print("Name: ");
System.out.println("Ana");
System.out.println("Age: " + 30);

Output:

text

Name: Ana
Age: 30

The first statement leaves the cursor right after Name: . The second continues on the same line and then ends it. The third starts a new line.

Calling println() with nothing in the parentheses ends the current line and prints nothing else. Use it to make an empty line:

java

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

prints AB on the first line and C on the second.

One detail: the line ending that println writes comes from the system's line separator, which is not the same on every operating system. You will not notice it on screen, but a file written on Windows may differ in invisible characters from one written on Linux.

Text and numbers

Text, which Java calls a string, is written in double quotes. Everything between the quotes is printed exactly, including spaces:

java

System.out.println("Hello, world");   // prints: Hello, world
System.out.println("a   b");          // prints: a   b  (three spaces)

Numbers are written without quotes:

java

System.out.println(42);     // prints: 42
System.out.println(3.5);    // prints: 3.5

The quotes matter. "42" is text that happens to look like a number, and 42 is a number. Both print the same here, but they behave differently once we start calculating in later modules. System.out.println(true) also works and prints true. Which kinds of values Java has, and what they can do, is the subject of the next module.

What about text without quotes? System.out.println(Hello); makes the compiler look for something named Hello, not find it, and report cannot find symbol. Quotes are how you tell Java "this is words, not a name."

Joining with `+`

When one side of + is text, Java turns the other side into text and joins them:

java

System.out.println("Age: " + 30);    // Age: 30

You can chain several pieces:

java

System.out.println("Total: " + 3 + " items");   // Total: 3 items

Be careful with spaces: Java joins exactly what you give it. The space after Total: and the space before items are inside the quotes, and they are what separates the pieces on screen.

Here is a trap. When both sides of a + are numbers, Java adds them. The expression is evaluated from left to right:

java

System.out.println("Total: " + 3 + 4);      // Total: 34
System.out.println("Total: " + (3 + 4));    // Total: 7
System.out.println(3 + 4 + " items");       // 7 items

The first prints Total: 34: after "Total: " + 3 becomes text, the + 4 joins again. The second adds the numbers first because of the parentheses. The third adds 3 + 4 before the text appears, because it is evaluated left to right. If you want to be sure, use parentheses around arithmetic.

Special characters inside text

Some characters are hard to type in the middle of a string, so Java uses a backslash \ followed by a letter or symbol. This pair is called an escape sequence. The Java Language Specification lists the allowed ones. These four cover most of what you need now:

Write this	You get	Example
`\n`	A new line	`"Line one\nLine two"` prints on two lines
`\t`	A tab (a jump to the next column)	`"Tea\t5"`
`\"`	A double quote that does not end the text	`"She said \"hi\""`
`\\`	One backslash	`"C:\\temp"` prints `C:\temp`

Any other letter after a backslash is a compile-time error. A string literal also may not contain a real line break between its quotes, so "Line one followed by a line break and Line two" does not compile. Use \n instead.

\n lets you do with one print what println does for you. These two statements print the same thing:

java

System.out.println("Done");
System.out.print("Done\n");

(The first one uses your system's line separator, which can differ from \n on some systems, so prefer println for ordinary lines.)

Examples

Example 1: print versus println (simple)

java

public class Pr {
    public static void main(String[] args) {
        System.out.print("Name: ");
        System.out.println("Ana");
        System.out.println("Age: " + 30);
        System.out.println(3 + 4);
        System.out.println("3 + 4");
    }
}

Output:

text

Name: Ana
Age: 30
7
3 + 4

The last two lines show the difference between numbers and text. 3 + 4 without quotes is a calculation, and "3 + 4" with quotes is just characters.

Example 2: a small receipt (practical)

java

public class Rcpt {
    public static void main(String[] args) {
        System.out.println("=== Receipt ===");
        System.out.print("Tea");
        System.out.print("\t");
        System.out.println(5);
        System.out.print("Cake");
        System.out.print("\t");
        System.out.println(12);
        System.out.println("Total:\t" + (5 + 12));
        System.out.println("Thank you!");
    }
}

Output:

text

=== Receipt ===
Tea    5
Cake    12
Total:    17
Thank you!

Each tab jumps to the next tab stop, so the numbers line up in a column in most terminals. The exact width of a tab depends on the terminal or editor, so alignment can differ slightly from what you see here.

Example 3: escapes in a realistic message (realistic)

Programs often print messages that include quotation marks or file paths. Here is one that does both:

java

public class Msg {
    public static void main(String[] args) {
        System.out.println("Config file: C:\\app\\config.txt");
        System.out.println("Status: \"OK\"");
        System.out.println("Line one\nLine two");
    }
}

Output:

text

Config file: C:\app\config.txt
Status: "OK"
Line one
Line two

Without the doubled backslashes, \a in \app would be an invalid escape and the file would not compile. Without \" the quotes around OK would end the text early.

Backend Insight

When a backend service runs on a server, nobody is watching a window. The way you learn what it did is through log lines, which are text messages written to the console or to a file, usually with a timestamp and a short description. Real services use a logging library, which you will meet later, rather than System.out. But the habit is the one you are building now: print the right fact, in a readable form, with the value next to its label, like "Order id: " + 42.

Two lessons from this one carry over directly. First, a message must be easy to read at 3 a.m., so labels and consistent layout matter. Second, text that joins values can be built wrongly, as in the "Total: " + 3 + 4 trap, and logs that print the wrong number mislead everyone reading them.

Common Mistakes

Mistake 1: Forgetting the quotes

Incorrect: System.out.println(Hello); (compiler: cannot find symbol)
Correct: System.out.println("Hello");

Mistake 2: Expecting `print` to start a new line

If you use print three times, all the text appears on one line. Use println, or add \n, when you want separate lines.

Mistake 3: Joining a number after text when you wanted to add

Incorrect: "Total: " + 3 + 4 prints Total: 34
Correct: "Total: " + (3 + 4) prints Total: 7

Mistake 4: Single quotes for text

Incorrect: System.out.println('Hello');
Correct: System.out.println("Hello");

Single quotes are for a single character, which is a separate idea. Text of several characters needs double quotes, and the compiler will report an unclosed character literal.

Mistake 5: A single backslash in a path

Incorrect: "C:\temp" (the \t becomes a tab)
Correct: "C:\\temp"

Exercises

Try each exercise yourself before opening the solutions. Hands-on items must be done on your machine.

Level A: Foundation

A1. What is the single difference between print and println?

Show solution A1

A1. println ends the line after printing and print does not.

A2. What does each line print? (a) System.out.println("Hi " + "there"); (b) System.out.println(2 + 3); (c) System.out.println("2 + 3");

Show solution A2

A2. (a) Hi there. (b) 5. (c) 2 + 3.

A3. Which escape sequence gives you (a) a new line, (b) a tab, (c) a double quote inside text?

Show solution A3

A3. (a) \n. (b) \t. (c) \".

Level B: Application

B1. Write a program, Intro, that prints exactly this, using only print for the first line and println for the second:

text

City: Cairo
Done

Show solution B1

B1.

java

public class Intro {
    public static void main(String[] args) {
        System.out.print("City: Cairo\n");
        System.out.println("Done");
    }
}

This prints the two lines. The first statement must use print, so the line break comes from \n inside the text. (Printing "City: " and "Cairo\n" as two print calls is also fine.)

B2. Predict the output of each statement, then run to check: (a) System.out.println("Count: " + 1 + 2); (b) System.out.println("Count: " + (1 + 2)); (c) System.out.println(1 + 2 + " items");

Show solution B2

B2. (a) Count: 12. (b) Count: 3. (c) 3 items.

B3. Write a program that prints one println statement producing this two-line output, using \n once:

text

Line one
Line two

Show solution B3

B3.

java

public class Lines {
    public static void main(String[] args) {
        System.out.println("Line one\nLine two");
    }
}

Level C: Challenge

C1. Write a program, Menu, that prints this price list so the second column lines up using tabs, with the title in its own line. Items and prices: Tea 5, Cake 12, Juice 8.

Show solution C1

C1. For example:

java

public class Menu {
    public static void main(String[] args) {
        System.out.println("Price list");
        System.out.println("Tea\t5");
        System.out.println("Cake\t12");
        System.out.println("Juice\t8");
    }
}

Output (tab widths vary by terminal): Price list, then Tea, Cake, Juice with the prices lined up in a column. With names of different lengths, such as Cake and Juice, tabs may not align perfectly in every terminal, which is a limit of tabs.

C2. The program below fails to compile. Without running it, list every problem and fix them all:

java

public class Broken {
    public static void main(String[] args) {
        System.out.println('Path: ' + "C:\data");
        System.out.println(Total: + 5 + 6);
    }
}

Show solution C2

C2. Problems: (1) 'Path: ' uses single quotes, so the compiler reports an unclosed character literal; use double quotes. (2) "C:\data" has \d, an invalid escape; write "C:\\data". (3) Total: is not in quotes, so it is invalid; write "Total: ". Also, + 5 + 6 after text joins the numbers one by one and would print Total: 56; if the sum 11 is wanted, use (5 + 6). A corrected version:

java

public class Broken {
    public static void main(String[] args) {
        System.out.println("Path: " + "C:\\data");
        System.out.println("Total: " + (5 + 6));
    }
}

It prints Path: C:\data and then Total: 11.

Mini Challenge

Create a program named Card that prints a business card of your own with four lines: a name, a role, an email address, and a phone number. Put a label before each value using print for the label and println for the value on every line, and add a top and bottom border line made of repeated characters, for example ==========. Do not use real private data; invent a phone number and email.

Check your work: it compiles, there are exactly six lines of output, each label and value sit on the same line, and your own borders use println. This is the core of the module's lab.

Summary & Next Lesson

Key takeaways

println prints and moves to a new line. print prints and stays on the same line.
Text goes in double quotes. Numbers are written without quotes.
+ joins when one side is text, and adds when both sides are numbers. Evaluation goes left to right, so use parentheses for sums inside text.
Escape sequences \n, \t, \", and \\ put special characters inside text.
A real line break inside a string literal is a compile-time error.

Next lesson

You can now make the program show things. Next, "Comments" teaches how to leave notes in the code for people: single-line, multi-line, and Javadoc comments, and how to write ones that help.

Self-check

What does System.out.print("A"); System.out.print("B"); print?
What is the output of "x" + 1 + 2?
Why does System.out.println(Hello); fail?
How do you print a double quote inside text?
How do you print C:\logs?

Show answers

AB on a single line.
x12.
Without quotes, Java looks for something named Hello and cannot find it. It needs "Hello".
Put a backslash before it: \".
System.out.println("C:\\logs");

References

Oracle, PrintStream API, Java SE 21. It says println(String) behaves like print(String) followed by println(), that println() writes the line separator, and that the separator comes from the line.separator system property.
Oracle, The Java Language Specification, Java SE 21, section 3.10.5 (String Literals). It lists the escape sequences, says other characters after a backslash are errors, and says a raw line break inside a string literal is an error.
The outputs shown, including Total: 34 for "Total: " + 3 + 4, were produced by compiling and running the examples with JDK 21. The left-to-right rule for + is described here from that run and not read from the specification's section on the additive operators.

Practice

Apply this lesson with the module's lab, exercises and quiz.

Lab and exercises Quiz

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Lesson Overview

Learning Objectives

Written Explanation

The statement, word by word

println and print

Text and numbers

Joining with +

Special characters inside text

Examples

Example 1: print versus println (simple)

Example 2: a small receipt (practical)

Example 3: escapes in a realistic message (realistic)

Backend Insight

Common Mistakes

Mistake 1: Forgetting the quotes

Mistake 2: Expecting print to start a new line

Mistake 3: Joining a number after text when you wanted to add

Mistake 4: Single quotes for text

Mistake 5: A single backslash in a path

Exercises

Level A: Foundation

Level B: Application

Level C: Challenge

Mini Challenge

Summary & Next Lesson

Key takeaways

Next lesson

Self-check

References

Practice

`println` and `print`

Joining with `+`

Mistake 2: Expecting `print` to start a new line