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This program computes the cost of a song as the product of its length
in seconds and the price per second. A discount is appliedto “long” songs. A class
Song is defined
to encapsulate the field
seconds and the methods
computePrice and
discount .
song1 and
song2 .computePrice is called
on the object referenced by
song1 . In Jeliot this is visualized by an arrow to the object
placed in the
Expression Evaluation Area followed by a period and
the method name and parameters.this is initialized by the implicit reference and
pricePerSecond is initialized from the actual parameter.price is declared and initialized by the
expression calculated from the formal parameter
pricePerSecond and
the field of the object
seconds that is implicitly accessed through
this .discount ,
declared in the same class, is invoked and returns a boolean value.A new activation is allocated for this method and deallocated when it terminates.
The implicit actual parameter is
this and it is used to initialize the
implicit formal parameter
this of the method
discount .computePrice is deallocated
and the value returned is stored in the variable
price1 .song2 .price1 and
price2 are printed.
Exercise Modify the program so that discount does not use the explicit
parameter
s .
Program: Method06B.java
// Learning Object Method06B
// calling a method on the same objectclass Song {
int seconds;
Song(int s) { seconds = s;
}
static int level(int n) { return n * 100;
}
boolean discount(int s) { return s > level(3);
}
double computePrice(double pricePerSecond) { double price = seconds * pricePerSecond;
if (discount(seconds)) price = price * 0.9;
return price; }
}
public class Method06B { public static void main(/* String[] args */) { Song song1 = new Song(164);
Song song2 = new Song(403); double price1 = song1.computePrice(0.01);
double price2 = song2.computePrice(0.01); System.out.println(price1);
System.out.println(price2); }
}
Given a call to a method
m2 within a method
m1 :
void m1() {
m2();}
it is impossible to tell from the call if m2 is being implicitly called on the same object or if it is a static method defined in the class.
s with 300 in the method
discount , it is compared with
the value returned by the method
level . It is impossible to tell
from the calls alone to
discount and
level that the first is
a call on an object while the second is a call to a static method.
Exercise Modify the calls to
discount and
level so that it is immediately apparent which is definitely a call on an object and which
is definitely a call to a static method.
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