Now that we know how to declare and initialize variables in Java,
we can do some mathematics with those variables.

What You Should See

The plus sign (+) will add two integers or two doubles together, or one
integer and one floating-point value (in either order). With two Strings
(like on line 32) it will concatenate^{5} the two Strings together.

The minus sign (-) will subtract one number from another. Just like
addition, it works with two integers, two floating-point values, or one
integer and one double (in either order).

An asterisk (*) is used to represent multiplication. You can also see
on line 15 that Java knows about the correct order of operations. b
is multiplied by 3 giving 81 and then a is added.

A slash (/) is used for division. Notice that when an integer
is divided by another integer (like on line 17) the result is also
an integer and not floating-point.

The percent sign (%) is used to mean ‘modulus’, which is essentially
the remainder left over after dividing. On line 19, b is divided by 10
and the remainder (7) is stored into the variable g.

Modular arithmetic is a fairly simple mathematical operation that just isn’t
often taught in public school or even introductory university math curriculum.
Wikipedia’s example is good enough: we do modular arithmetic every time
we add times on a typical 12-hour clock. If it is 7 o’clock now, what
time will it be in eight hours? Well, once we hit 12:00 we “wrap around”,
so it will be 3 o’clock. (8+7 = 15, 15-12 = 3)

Put another way, 15 divided by 12 is 1 with a remainder of 3.

Modular arithmetic is used more than you would think in programming, but I
won’t be using it too much in the book.