Java Reference
In-Depth Information
Changing the
end
object now alters the line, so we get a different intersection point for the two lines
after we move the point
end
. This is because the
Line
object,
line1
, contains references to the
Point
objects defined in
main()
, not independent
Point
objects.
Recursion
The methods you have seen so far have been called from within other methods, but a method can also call
itself - something referred to as
recursion
. Clearly you must include some logic in a recursive method so that
it will eventually stop calling itself. We can see how this might be done with a simple example.
We can write a method that will calculate integer powers of a variable, in other words, evaluate
xn
, or
x*x...*x
where
x
is multiplied by itself
n
times. We can use the fact that we can obtain
xn
by
multiplying
xn-1
by x. To put this in terms of a specific example, we can calculate 2
4
as 2
3
multiplied
by 2, and we can get 2
3
by multiplying 2
2
by 2, and 2
2
is produced by multiplying 2
1
, which is 2 of
course, by 2.
Try It Out - Calculating Powers
Here is the complete program including the recursive method,
power()
:
public class PowerCalc {
public static void main(String[] args) {
double x = 5.0;
System.out.println(x + " to the power 4 is " + power(x,4));
System.out.println("7.5 to the power 5 is " + power(7.5,5));
System.out.println("7.5 to the power 0 is " + power(7.5,0));
System.out.println("10 to the power -2 is " + power(10,-2));
}
// Raise x to the power n
static double power(double x, int n) {
if(n > 1)
return x*power(x, n-1); // Recursive call
else if(n < 0)
return 1.0/power(x, -n); // Negative power of x
else
return n == 0 ? 1.0 : x; // When n is 0 return 1, otherwise x
}
}
This program will produce the output:
5.0 to the power 4 is 625.0
7.5 to the power 5 is 23730.46875
7.5 to the power 0 is 1.0
10 to the power -2 is 0.01
