| << Chapter < Page | Chapter >> Page > |
Here is an example of how we use
AreaCalc to compute area of a rectanglee of width 4.5 and height 7.2. In the Interactions pane of DrJava, enter the following lines of code.
AreaCalc calc = new AreaCalc();
calc.rectangle(4.5, 7.2) The first line of code defines
calc as a variable of type
AreaCalc and assign to it an
instance of the class
AreaCalc .
new is a keyword in Java. It is an example of what is called a class operator. It operates on a class and creates an instance (also called
object ) of the given class. The second line of code is a call to the object
calc to perform the
rectangle task where
width is assigned the value 4.5 and
height is assigned the value 7.2. To get the area of a 5.6 by 8.4 rectangle, we simply use the same calculator
calc again:
calc.rectangle(5.6, 8.4); So instead of solving just one proble -given a rectangle 4.5 ft wide and 7.2 ft high, compute its area- we havebuilt a "machine" that can compute the area of
any given rectangle. But what about computing the area of a right triangle with height 5 and base 4? We cannot simply use this calculator. We need another specialized calculator, the kind that can compute the area of a circle.
There are at least two different designs for such a calculator.
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
1 public class Person {
2 /**3 * All data fields are private in order to prevent code outside of this
4 * class to access them.5 */
6 private int _bDay; // birth day7 private int _bMonth; // birth month; for example, 3 means March.
8 private int _bYear; // birth year9 /**10 * Constructor: a special code used to initialize the fields of the class.
11 * The only way to instantiate a Person object is to call new on the constructor.12 * For example: new Person(28, 2, 1945) will create a Person object with
13 * birth date February 28, 1945.14 */
15 public Person(int day, int month, int year) {16 _bDay = day;
17 _bMonth = month;18 _bYear = year;
19 }20 /**21 * Uses "modulo" arithmetic to compute the number of months till the next
22 * birth day given the current month.23 * @param currentMonth an int representing the current month.24 */
25 public int nMonthTillBD(int currentMonth) {26 return (_bMonth - currentMonth + 12) % 12;
27 }28 } (
Download the above code ) We now explain what the above Java code means.
new Person(28, 2, 1945) will create an instance of a Person with _bDay = 28, _bMonth = 2 an d_bYear = 1945.
Notification Switch
Would you like to follow the 'Principles of object-oriented programming' conversation and receive update notifications?
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|