Expressions and Output

A program that calculates a grade or a ticket price is useless if nobody can see the answer. Programs need a way to show information to the person using them. In Topic 1.1 you saw System.out.println display a line of text. This subsection explains exactly what it does, because the details decide where every piece of output lands.

When a Java program displays text, it goes to the console: the output window of your IDE, sometimes called the terminal. Picture the console as a page that the program writes on from top to bottom. A blinking marker called the cursor shows where the next character will be written. Every time the program displays something, it starts writing at the cursor, and the cursor moves along behind the text.

System.out.println(...) does two things, always in this order:

1. It displays the value inside the parentheses, starting at the cursor.
2. It moves the cursor to the beginning of the next line.

The name is short for “print line”: print this, then finish the line. Displaying information like this is called output.

public class MenuBoard
{
    public static void main(String[] args)
    {
        System.out.println("Today's lunch:");
        System.out.println("Veggie tacos");
        System.out.println();
        System.out.println("Enjoy!");
    }
}

Today's lunch:
Veggie tacos

Enjoy!

The empty println() is the usual way to put a blank line in your output. Each println statement, with or without a value, ends exactly one line.

public class PrintlnTypo
{
    public static void main(String[] args)
    {
        System.out.printIn("Practice starts at 4.");
    }
}

PrintlnTypo.java:5: error: cannot find symbol

public class BusSchedule
{
    public static void main(String[] args)
    {
        System.out.println("Bus 12");
        System.out.println();
        System.out.println("Leaves at 7:45");
        System.out.println("Arrives at 8:10");
    }
}

Bus 12

Leaves at 7:45
Arrives at 8:10

Sometimes you do not want a new line after every piece of output. A scoreboard should show Score: 95 on one line, even if the label and the number come from two separate statements. You need a way to display something and leave the cursor right where it stopped.

That is what System.out.print(...) does. It displays its value starting at the cursor and does not move the cursor to a new line. The cursor stays just after the last character displayed, so the next output continues on the same line.

System.out.print("Ready ");
System.out.print("set ");
System.out.println("go!");
System.out.println("Race over.");

Ready set go!
Race over.

The most common use of print is a label followed by a value. Change one thing: leave out the space at the end of a label.

System.out.print("Score: ");
System.out.println(95);
System.out.print("Level:");
System.out.println(3);

Score: 95
Level:3

"Score: " ends with a space, so 95 appears one space after the colon. "Level:" has no space, so 3 lands right against the colon. Both pairs end with println, so each label-and-value pair gets its own line. (Later, in Topic 1.15, you will learn a way to join text and numbers in one statement. For now, a print followed by a println does the job.)

System.out.print("Line one");
System.out.print("Line two");

Line oneLine two

System.out.println("Line one");
System.out.println("Line two");

Line one
Line two

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

AB
CD
E

System.out.print("Name: ");
System.out.println("Ada");
System.out.print("Grade: ");
System.out.print(11);
System.out.println();

Name: Ada
Grade: 11

You have now displayed text, and in Question 4 a number, 11. The two were written differently: the text had double quotes and the number did not. Why? Because Java needs to know what kind of value you mean, and the way you write a value in code tells it.

A literal is a fixed value written directly in the code. When Java sees 42, it does not look anything up; the value is exactly forty-two, every time the program runs. Each type from Topic 1.2 has its own way of writing literals:

A string literal is text written between a pair of double quotes: any characters, one after another. “Characters” means anything you can type: letters, digits, spaces, punctuation. The double quotes are not part of the text. They are markers that tell Java where the text starts and stops, so they are not displayed. Everything between them is kept exactly, including spaces. A pair of quotes with nothing between, "", is called the empty string.

System.out.println(42);
System.out.println(3.75);
System.out.println(true);
System.out.println("42");
System.out.println("Room 101, 2nd floor!");

42
3.75
true
42
Room 101, 2nd floor!

Change one thing: look at how double literals and spacing come out.

System.out.println(2.50);
System.out.println(7.0);
System.out.println("  spaced   out");
System.out.println("");
System.out.println("end");

2.5
7.0
  spaced   out

end

A double is displayed without extra zeros at the end, so 2.50 shows as 2.5. But it always keeps at least one digit after the decimal point, so 7.0 shows as 7.0, not 7. That .0 is how you can tell a double result from an int on the console. In the string literal, every space between the quotes is kept. The empty string "" displays nothing, and println then ends the line, so you see an empty line.

public class MissingQuotes
{
    public static void main(String[] args)
    {
        System.out.println(Welcome);
    }
}

MissingQuotes.java:5: error: cannot find symbol

System.out.println("3.0");
System.out.println(3.0);
System.out.println(3.50);
System.out.println("3.50");

3.0
3.0
3.5
3.50

A double quote marks where a string literal begins and ends. So what if the text itself needs a double quote, as in She said "hi" to me.? Try the obvious thing:

public class QuoteInside
{
    public static void main(String[] args)
    {
        System.out.println("She said "hi" to me.");
    }
}

Java reads "She said " as a complete string literal, because the second quote closes it. Then it finds hi, which is not text and not in a place that makes sense, and the compiler reports errors such as ')' or ',' expected and ';' expected (the exact wording varies between Java versions). Nothing runs.

Java solves this with a code. Inside a string literal, a backslash \ means “the next character is special; do not treat it normally.” A backslash followed by a character is an escape sequence: a two-character code inside a string that stands for one character that would be hard or impossible to type there directly. This course uses three:

Note the direction: escape sequences use the backslash \, which leans left, not the forward slash /.

System.out.println("She said \"hi\" to me.");
System.out.println("A backslash looks like this: \\");
System.out.println("\"\\\"");

She said "hi" to me.
A backslash looks like this: \
"\"

Change one thing: use the third escape sequence, \n, which moves the cursor to the next line in the middle of a string.

System.out.println("Roses are red,\nViolets are blue.");
System.out.print("one\ntwo\n");
System.out.println("three");

Roses are red,
Violets are blue.
one
two
three

Line 1 displays two lines from one statement: \n acts like pressing Enter between red, and Violets, and then println ends the second line as usual. Line 2 uses print, which never ends a line on its own. But its text ends with \n, so the cursor still ends up at the start of a new line, and three appears on its own line.

public class PathIllegal
{
    public static void main(String[] args)
    {
        System.out.println("Saved in C:\data");
    }
}

PathIllegal.java:5: error: illegal escape character

System.out.println("Saved in C:\new");

Saved in C:
ew

System.out.println("Saved in C:\\new");

Saved in C:\new

System.out.print("Title: \"Hopper\"\n");
System.out.println("Use \\n for a new line");
System.out.println("Done\n");
System.out.println("!");

Title: "Hopper"
Use \n for a new line
Done

!

System.out.println("Line 1/nLine 2");

System.out.println("Line 1\nLine 2");

Line 1
Line 2

Displaying fixed text only goes so far. A ticket booth needs to show the cost of 3 tickets at $12 each, and the answer depends on the numbers. You could work out 36 yourself and type it in, but then the program is no help when the numbers change. You want Java to do the calculation.

Java calculates with arithmetic expressions. An expression is a piece of code that Java evaluates (works out) to produce a single value. An arithmetic expression is made of numeric values (literals such as 12 or 2.5, and numeric variables), joined by arithmetic operators, the symbols that say which calculation to do. The values an operator works on are its operands. In 3 * 12, the operator is * and the operands are 3 and 12.

Multiplication uses * because there is no × key on a keyboard. Division and remainder have surprises of their own, so each gets its own subsection (1.3.6 and 1.3.7). When you put an expression inside println, Java first evaluates it, then displays the value.

System.out.println(3 * 12);
System.out.println("3 * 12");
System.out.println(20 - 7 + 1);
System.out.println(6.5 + 2);

36
3 * 12
14
8.5

The type of the result

Every value in Java has a type, so the result of an arithmetic operation has a type too. Java decides it from the two operands:

• If both operands are int values, the result is an int.
• If at least one operand is a double, the result is a double.

The reason: an int can hold only whole numbers, so whenever a double is involved the answer might need a decimal part, and only a double can hold one. Java does not look at whether the answer happens to be a whole number. It decides from the types alone.

System.out.println(6 * 2);
System.out.println(6 * 2.0);
System.out.println(1.5 + 2.5);
System.out.println(10 - 0.25);

12
12.0
4.0
9.75

6 * 2 uses two int values, so the result is the int 12. Change one operand to 2.0 and the result is the double 12.0: the same amount, but a different type, which is why it displays with .0. 1.5 + 2.5 is 4.0 and not 4, because both operands are double values, even though the sum is a whole number.

Variables in expressions

An operand can also be a variable. When Java evaluates the expression, it uses the value the variable holds at that moment. Here the variables are declared with starting values, as in Topic 1.2. (The = sign that gives a variable its value is studied fully in Topic 1.4.)

int tickets = 3;
double price = 12.5;
int fee = 2;
System.out.println(tickets * price);
System.out.println(tickets * fee);
System.out.println(price - fee);

37.5
6
10.5

int laps = 4;
System.out.println("laps * 400");
System.out.println(laps * 400);

laps * 400
1600

int boxes = 5;
double weight = 2.5;
System.out.println(boxes + 10);
System.out.println(boxes * weight);
System.out.println(weight * 4);
System.out.println("boxes + 1");

15
12.5
10.0
boxes + 1

Seventeen cookies are shared equally among 5 friends, and cookies cannot be broken. Each friend gets 3, with some left over. Java’s division of two int values works exactly like this, and it surprises nearly every new programmer.

You just learned that an operation on two int values must give an int. But 17 divided by 5 is 3.4, and an int cannot hold 3.4. Java’s answer: keep only the whole-number part of the quotient and throw the decimal part away. This is called integer division. It does not round: 19 / 5 is 3.8 in math, and Java gives 3, not 4. When at least one operand is a double, the result is a double, so Java keeps the full quotient, decimal part included.

System.out.println(17 / 5);
System.out.println(19 / 5);
System.out.println(2 / 3);
System.out.println(17.0 / 5);
System.out.println(17 / 5.0);

3
3
0
3.4
3.4

Change one thing: average two test scores whose total is odd. The total of 85 and 90 is 175.

int total = 175;
System.out.println(total / 2);
System.out.println(total / 2.0);

87
87.5

total / 2 divides two int values, so the .5 is lost and the average shows as 87. Writing the divisor as 2.0 makes one operand a double, so Java keeps the full answer, 87.5. Same numbers, different types, different results.

System.out.println(1 / 2);

0

System.out.println(1.0 / 2);

0.5

System.out.println(29 / 10);
System.out.println(29 / 10.0);
System.out.println(9 / 10);
System.out.println(30 / 10);

2
2.9
0
3

Back to the 17 cookies and 5 friends. Integer division told you each friend gets 3. But how many cookies are left over? Integer division throws that part away, and sometimes the leftover is exactly what you need: the minutes left after counting whole hours, or the people left after forming full teams.

The remainder operator % gives the amount left over when one whole number is divided by another. In a % b, Java divides a by b as many whole times as possible and reports what is left. For 17 and 5: 5 goes into 17 three whole times, which uses up 15, and 2 are left, so 17 % 5 is 2. The two operators fit together like this:

17 = 5 × (17 / 5) + (17 % 5)
17 = 5 ×    3     +    2

/ tells you how many whole groups, % tells you how many are left over. The % sign has nothing to do with percent here; it is just the symbol Java uses. In this course, a is never negative and b is always positive.

System.out.println(17 / 5);
System.out.println(17 % 5);

3
2

Change things: try different first and second numbers to see what the remainder depends on.

System.out.println(20 % 5);
System.out.println(3 % 5);
System.out.println(1234 % 10);
System.out.println(125 / 60);
System.out.println(125 % 60);
System.out.println(14 % 2);
System.out.println(15 % 2);

0
3
4
2
5
0
1

Notice the second row. When the first number is smaller than the second, there is not even one whole group, so the whole first number is left over. So 3 % 5 is 3, not 0.

System.out.println(50 % 10);

0

int days = 365;
System.out.println(days / 7);
System.out.println(days % 7);

52
1

What is 2 + 3 * 4? If you work left to right you get 20, but if you multiply first you get 14. A program cannot be ambiguous, so Java needs a fixed rule for which operation happens first whenever an expression has more than one operator.

An expression with more than one operator is a compound expression. Java follows the same order you learned in math class. Operator precedence is the rule that decides which operators are grouped with their values first:

1. Anything in parentheses is evaluated first.
2. Next, *, /, and % (they share the same, higher precedence).
3. Last, + and - (they share the same, lower precedence).
4. When operators have the same precedence, they are evaluated from left to right.

The grouping is decided when the program is compiled. The compiler reads the expression and works out which operator belongs with which values, as if it were adding invisible parentheses: 2 + 3 * 4 is understood as 2 + (3 * 4). The actual calculation happens later, when the program runs.

System.out.println(2 + 3 * 4);
System.out.println((2 + 3) * 4);
System.out.println(20 - 6 - 4);
System.out.println(24 / 4 * 2);
System.out.println(17 % 5 * 3);

14
20
10
12
6

Change one thing: use precedence with integer division, in the classic average-of-two-quizzes problem.

int quiz1 = 85;
int quiz2 = 90;
System.out.println(quiz1 + quiz2 / 2);
System.out.println((quiz1 + quiz2) / 2);
System.out.println((quiz1 + quiz2) / 2.0);

130
87
87.5

Without parentheses, only quiz2 is divided, and the “average” is 130, which is higher than either quiz. Parentheses make the sum happen first. Dividing by 2.0 also keeps the .5.

The type is decided one operation at a time

Precedence also matters for the int/double rule. Java decides the type of each operation separately, using only that operation’s two operands.

System.out.println(1 / 2 * 4.0);
System.out.println(1.0 / 2 * 4);
System.out.println(4.0 * 1 / 2);

0.0
2.0
2.0

In the first row, the 4.0 cannot rescue the answer. By the time Java reaches it, 1 / 2 has already been done as integer division and has given 0.

int pizzaCost = 30;
int tip = 6;
int people = 4;
System.out.println(pizzaCost + tip / people);

31

System.out.println(10 - 2 * 3);
System.out.println((10 - 2) * 3);
System.out.println(36 / 6 / 2);
System.out.println(7 + 10 % 4);

4
24
3
9

int fahrenheit = 212;
System.out.println(fahrenheit - 32 * 5 / 9);

System.out.println((fahrenheit - 32) * 5 / 9);

In Topic 1.1 you saw 10 / 0 stop a program with an exception. Now that you know how / works, you can see exactly why, and why the compiler lets it through.

Dividing by zero has no answer: no number of zeros adds up to 10. Integer division must produce an int, and there is no int it could honestly give. Rather than make up a number and let the program continue with a wrong value, Java stops the program. When an int is divided by the int zero, Java throws an ArithmeticException, the exception for an impossible arithmetic operation.

This is a run-time error, not a syntax error. snacks / teams follows every rule of Java, so the compiler accepts it. The compiler only checks how the code is written and does not work out the values. The divisor is only known to be zero when the program actually runs and evaluates the expression. (This course only covers dividing by zero when both values are int. Dividing by zero when a double is involved behaves differently and is not part of this course.)

public class TeamSplit
{
    public static void main(String[] args)
    {
        int snacks = 24;
        int teams = 0;
        System.out.println("Splitting snacks...");
        System.out.println(snacks / teams);
        System.out.println("Done!");
    }
}

Splitting snacks...
Exception in thread "main" java.lang.ArithmeticException: / by zero
	at TeamSplit.main(TeamSplit.java:8)

Change one thing: put the zero on the other side, and build a zero divisor from an expression.

System.out.println(0 / 24);
System.out.println(24 / (3 - 3));
System.out.println("Finished");

0
Exception in thread "main" java.lang.ArithmeticException: / by zero

Line 1, 0 / 24, is fine: 24 goes into 0 zero times, so the result is 0. Zero is only a problem as the second operand, the divisor. Line 2 divides by (3 - 3). Parentheses go first, the divisor works out to 0, and the exception is thrown. The compiler did not catch this either, because the divisor is not known until the expression is evaluated. Finished never appears.

public class ScoreBoard
{
    public static void main(String[] args)
    {
        int points = 90;
        int games = 6;
        int forfeits = 6;
        System.out.print("Average: ");
        System.out.println(points / games);
        System.out.println("Per played game:");
        System.out.println(points / (games - forfeits));
        System.out.println("Season over");
    }
}

Average: 15
Per played game:

Maya is planning a pizza party: 3 pizzas, 8 slices each, 5 friends, and $12.50 per pizza. Her program uses every idea from this chapter. Read it, predict the output, then check the trace.

public class PizzaParty
{
    public static void main(String[] args)
    {
        int pizzas = 3;
        int slicesPerPizza = 8;
        int friends = 5;
        double pricePerPizza = 12.5;

        System.out.println("*** \"Pizza Party\" Planner ***");
        System.out.print("Total slices: ");
        System.out.println(pizzas * slicesPerPizza);
        System.out.print("Slices each: ");
        System.out.println(pizzas * slicesPerPizza / friends);
        System.out.print("Left over: ");
        System.out.println(pizzas * slicesPerPizza % friends);
        System.out.print("Total cost: $");
        System.out.println(pizzas * pricePerPizza);
        System.out.print("Cost each: $");
        System.out.println(pizzas * pricePerPizza / friends);
        System.out.println("\nThanks for planning!");
    }
}

*** "Pizza Party" Planner ***
Total slices: 24
Slices each: 4
Left over: 4
Total cost: $37.5
Cost each: $7.5

Thanks for planning!

Connect each line to its subsection:

• Output (1.3.1, 1.3.2): each label uses print, so the value lands on the same line; the value uses println, so the next label starts a new line.
• Literals and escape sequences (1.3.3, 1.3.4): \" puts quotes inside the title. The \n at the start of the last string creates the empty line. The $ inside "Total cost: $" is just a character.
• Types (1.3.5): slices are all int, so they stay whole. The cost involves the double pricePerPizza, so it shows a decimal point.
• Integer division and remainder (1.3.6, 1.3.7): 24 slices among 5 friends is 4 each with 4 left over: 5 × 4 + 4 = 24.
• Precedence (1.3.8): *, /, and % share a level, so pizzas * slicesPerPizza / friends multiplies first (left to right), then divides.
• Dividing by zero (1.3.9): every division here has friends (5) after the /. Before trusting a line like 14, ask whether that divisor could ever be zero.

A lookup table for the messages and symptoms in this chapter. “Compiler” means nothing runs; the other rows run.

• System.out.println displays a value and then moves the cursor to the next line; System.out.print displays a value and leaves the cursor where it stopped, so the next output continues on the same line.
• A literal is a fixed value in code. A string literal is characters in double quotes; the quotes are not displayed. Inside one, the escape sequences \", \\, and \n stand for a double quote, a backslash, and a new line.
• Arithmetic expressions combine numeric values and variables with +, -, *, /, and %. Two int operands give an int; at least one double gives a double.
• int divided by int keeps only the whole-number part, with no rounding. % gives the remainder, what is left after taking out as many whole groups as possible.
• Operator precedence: parentheses first, then *, /, %, then +, -, with equal-precedence operators left to right. The type of each operation depends only on its own two operands.
• Dividing an int by the int zero compiles, but throws an ArithmeticException when the line runs, stopping the program.

Go "Team"!
\o/

int pages = 7;
int days = 2;
double daysExact = 2.0;
System.out.println(pages / days);
System.out.println(pages / daysExact);
System.out.println(pages % days);
System.out.println(pages + days * 3);
System.out.println((pages + days) / 4);

3
3.5
1
13
2

public class QuizAverage
{
    public static void main(String[] args)
    {
        int quiz1 = 80;
        int quiz2 = 85;
        int quiz3 = 87;
        System.out.print("Average: ");
        System.out.println(quiz1 + quiz2 + quiz3 / 3);
    }
}

System.out.println((quiz1 + quiz2 + quiz3) / 3.0);

int totalMinutes = 200;
System.out.print(totalMinutes / 60);
System.out.print(" h ");
System.out.print(totalMinutes % 60);
System.out.println(" min");
System.out.println(totalMinutes / 60 * 60 + totalMinutes % 60);

3 h 20 min
200

Tickets: 4
Total: $50.0

public class TicketReceipt
{
    public static void main(String[] args)
    {
        int tickets = 4;
        double price = 12.5;
        System.out.print("Tickets: ");
        System.out.print(tickets);
        System.out.print("Total: $");
        System.out.println("tickets * price");
    }
}

System.out.println(tickets);

System.out.println(tickets * price);

System.out.print(3 / 4 * 8.0);
System.out.print(" ");
System.out.println(8.0 * 3 / 4);
System.out.println(2 + 12 % 5 * 3 - 1);
System.out.print("Done\n\"Bye\"");

0.0 6.0
7
Done
"Bye"

Check each statement. If the answer is “not yet,” reread the linked subsection.

• I can explain what println does after it displays its value. Reread
• I can predict where output lands when print and println are mixed. Reread
• I can tell a string literal apart from a number literal and predict how each is displayed. Reread
• I can use escape sequences to put a double quote, a backslash, or a new line inside a string. Reread
• I can evaluate an arithmetic expression and say whether the result is an int or a double. Reread
• I can predict the result of dividing two int values. Reread
• I can use % to find what is left over after division. Reread
• I can evaluate a compound expression using precedence, left-to-right order, and parentheses. Reread
• I can explain why dividing an int by zero compiles but throws an ArithmeticException. Reread