For the first few years of his life, Peter did not have a clue
what birthdays were, let alone his own birth date. He was
incapable of responding to your inquiry on his birthday. It was
his parents who planned for his elaborate birthday parties
months in advance. We can think of Peter then as a rather "dumb"
person with very little intelligence and capability. Now Peter
is a college student. There is a piece of memory in his brain
that stores his birth date: it's September 12, 1985! Peter is
now a rather smart person. He can figure out how many more
months till his next birthday and e-mail his wish list two
months before his birth day. How do we model a "smart" person
like Peter? Modeling such a person entails modeling
- a birth date and
- the computation of the number of months till the next
birth day given the current month.
A birth date consists of a month, a day and a year. Each of
these data can be represented by an integer, which in Java is
called a number of type
int. As
in the computation of the area of a rectangle, the computation
of the number of months till the next birth day given the
current month can be represented as a method of some class. What
we will do in this case that is different from the area
calculator is we will lump both the data (i.e. the birth date)
and the computation involving the birth date into one class. The
grouping of data and computations on the data into one class is
called encapsulation. Below is the Java code modeling an
intelligent person who knows how to calculate the number of
months before his/her next birth day. The line numbers shown are
there for easy referencing and are not part of the code.
1 class Person {
2 /**
3 * Data fields:
4 */
5 int _bDay; // birth day
6 int _bMonth; // birth month; for example, 3 means March.
7 int _bYear; // birth year
8 /**
9 * Uses "modulo" arithmetic to compute the number of months till the next
10 * birth day given the current month.
11 * @param currentMonth an int representing the current month.
12 */
13 int nMonthTillBD(int currentMonth) {
14 return (_bMonth - currentMonth + 12) % 12;
15 }
16 }
We now explain what the above Java code
means.
- line 1 defines a class called Person. The opening curly
brace at the end of the line and the matching closing brace
on line 16 delimit the contents of
class Person.
- lines 2-54 are comments. Everything between /* and */ are
ignored by the compiler.
- lines 5-7 define three integer variables. These
variables are called fields
of the class. Only code inside of the class can access them. Each
field is followed by a comment delimited by // and the
end-of-line. So there two ways to comment code in Java:
start with /* and end with */ or start with // and end with
the end-of-line.
- lines 8-12 are comments.
- line 11 is a special format for documenting the
parameters of a method. This format is called the javadoc
format. We will learn more about javadoc in another module.
- lines 13-15 constitute the definition of a method in
class Person.
- line 14 is the formula for computing the number of
months before the next birthday using the remainder operator
"%".
x % y gives the remainder of the integer division between
the dividend x and the divisor
y.
