Operators

An operator is a symbol that performs an action on values: adds them, compares them, combines conditions. The values it works on are called operands: in 2 + 3 the operator is + and the operands are 2 and 3.

Arithmetic

Addition, subtraction and multiplication work as usual. The % operator gives the remainder of a division:

void main() {
    IO.println(7 + 2);
    IO.println(7 - 2);
    IO.println(7 * 2);
    IO.println(7 % 2);
}

Output:

9
5
14
1

The remainder is handy for checking parity: a number is even when n % 2 equals zero.

Division

If both operands are whole numbers, the result is a whole number too — the fractional part is simply dropped. To get a fractional answer, at least one number must be a double:

void main() {
    IO.println(7 / 2);
    IO.println(7.0 / 2);
}

Output:

3
3.5

Compound assignment

x += 5 is a short form of x = x + 5. -=, *=, /= and %= work the same way. To increase or decrease by one there are the ++ and -- operators:

void main() {
    int count = 10;
    count += 5;
    count *= 2;
    count++;
    IO.println(count);
}

Output:

31

Comparison

Comparison operators return a boolean: == (equal), != (not equal), <, >, <=, >=. Note that equality uses two = signs; a single one is assignment:

void main() {
    int age = 20;
    IO.println(age >= 18);
    IO.println(age == 21);
    IO.println(age != 21);
}

Output:

true
false
true

Logical operators

Logical operators combine conditions: && (and) is true when both conditions are true; || (or) when at least one is; ! (not) flips a value:

void main() {
    int age = 25;
    boolean hasTicket = false;
    IO.println(age >= 18 && hasTicket);
    IO.println(age >= 18 || hasTicket);
    IO.println(!hasTicket);
}

Output:

false
true
true

There are also operators that work with the individual bits of a number — see the bitwise operators lesson.

Order of evaluation

As in math, multiplication and division happen before addition and subtraction. Parentheses change the order:

void main() {
    IO.println(2 + 3 * 4);
    IO.println((2 + 3) * 4);
}

Output:

14
20

When in doubt about the order, add parentheses — the code becomes clearer.

Common mistake

Integer division silently loses the fractional part. Here the average of two grades should be 4.5, but we get 4:

void main() {
    int a = 4;
    int b = 5;
    double average = (a + b) / 2;
    IO.println(average);
}

Output:

4.0

9 / 2 was computed as integer division before the value went into the double. Divide by 2.0 and the result is 4.5.