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Exercise Write a recursive method to compute the n'th Fibonacci number:
Exercise Write a more efficient nonrecursive method for the same function.
Concept Nonstatic methods defined in a class must be invoked
on an object of the class. A reference to the object becomes
an
implicit actual parameter that initializes a formal variable called
this in the method. The variable
this need not be
explicitly mentioned when accessing fields of the object unless there is an ambiguity.
Program: Method05.java
// Learning Object Method05
// calling methods on an objectclass Song {
int seconds;
Song(int s) { seconds = s;
}
double computePrice(double pricePerSecond) { return seconds * pricePerSecond;
}}
public class Method05 {
public static void main(/*String[] args*/) {
Song song1 = new Song(164); Song song2 = new Song(103);
double price1 = song1.computePrice(0.01); double price2 = song2.computePrice(0.02);
System.out.println(price1); System.out.println(price2);
}}
This program computes the cost of a song as the product of its length
in seconds and the price per second. A class
Song is defined
to encapsulate the field
seconds and the method
computePrice .
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.this is used to obtain
the value of the field
seconds . An expression is evaluated and
its value returned.price1 .song2 .price1 and
price2 are printed.
Exercise Modify the method so that the formal parameter is
also named
seconds . Yes, it can be done!
(Hint: read the
Concept paragraph above.)
Concept A nonstatic method defined in a class that is invoked
on an object of the class can invoke another such method on the same object.
The object for the second call is the same as the one on the first call,namely, the only referenced by
this .
There is no need to explicitly write
this and the object may be accessed implicitly.
Program: Method06A.java
// Learning Object Method06A
// calling a method on the same objectclass Song {
int seconds;
Song(int s) { seconds = s;
}
boolean discount(int s) { return s > 300;
}
double computePrice(double pricePerSecond) { double price = seconds * pricePerSecond;
if (discount(seconds)) price = price * 0.9;
return price; }
}
public class Method06A { 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); }
}
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