5.8. Recursion

The methods you have seen so far have been called from within other methods, but a method can also call itself. A method that calls itself is described as a recursive method, and the process is referred to as recursion. You can also have indirect recursion where a method A calls another method B, which in turn calls the method A. Clearly you must include some logic in a recursive method so that it will eventually stop calling itself if the process is not to continue indefinitely. You can see how this might be done with a simple example.

You 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. You can use the fact that you can obtain xn by multiplying xn-1 by x. To put this in terms of a specific example, you can calculate 24 as 23 multiplied by 2, and you can get 23 by multiplying 22 by 2, and 22 is produced by multiplying 21, 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, ...

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