Java Reference
In-Depth Information
When this suspended computation resumes, it executes an output statement that out-
puts the value
12%10
, which is
2
. That ends that computation, but there is yet another
suspended computation waiting to resume. When this last suspended computation
resumes, the situation is
output the
digit
2
if
(123 < 10)
{
System.out.println(123);
}
else //n is two or more digits long:
{
writeVertical(123/10);
System.out.println(123%10);
}
Computation resumes
here.
When this last suspended computation resumes, it outputs the value
123%10
, which is
3
, and the execution of the original method call ends. And, sure enough, the digits
1
,
2
, and
3
have been written to the screen one per line, in that order.
output the
digit
3
A Closer Look at Recursion
The definition of the method
writeVertical
uses recursion. Yet, we did nothing new
or different in evaluating the method call
writeVertical(123)
. We treated it just like
any of the method calls we saw in previous chapters. We simply substituted the argu-
ment
123
for the parameter
n
and then executed the code in the body of the method
definition. When we reached the recursive call
writeVertical(123/10)
;
we simply repeated this process one more time.
The computer keeps track of recursive calls in the following way. When a method
is called, the computer plugs in the arguments for the parameter(s) and begins to exe-
cute the code. If it should encounter a recursive call, then it temporarily stops its com-
putation, because it must know the result of the recursive call before it can proceed. It
saves all the information it needs to continue the computation later on, and proceeds
to evaluate the recursive call. When the recursive call is completed, the computer
returns to finish the outer computation.
The Java language places no restrictions on how recursive calls are used in method
definitions. However, in order for a recursive method definition to be useful, it must
be designed so that any call of the method must ultimately terminate with some piece
of code that does not depend on recursion. The method may call itself, and that recur-
sive call may call the method again. The process may be repeated any number of times.
how
recursion
works
how
recursion
ends
