Recursion - Absolute Java - page 657

PITFALL: Infinite Recursion

In the example of the method writeVertical discussed in the previous subsections,
the series of recursive calls eventually reached a call of the method that did not
involve recursion (that is, a stopping case was reached). If, on the other hand, every
recursive call produces another recursive call, then a call to the method will, in theory,
run forever. This is called infinite recursion . In practice, such a method will typically
run until the computer runs out of resources and the program terminates abnormally.
Examples of infi nite recursion are not hard to come by. The following is a syntacti-
cally correct Java method defi nition, which might result from an attempt to defi ne an
alternative version of the method writeVertical :

public static void newWriteVertical( int n)
{
newWriteVertical(n / 10);
System.out.println(n % 10);
}

If you embed this definition in a program that calls this method, the program
will compile with no error messages and you can run the program. Moreover,
the definition even has a certain reasonableness to it. It says that to output the
argument to newWriteVertical , first output all but the last digit and then output
the last digit. However, when called, this method will produce an infinite sequence
of recursive calls. If you call newWriteVertical(12) , that execution will stop
to execute the recursive call newWriteVertical(12/10) , which is equivalent to
newWriteVertical(1) . The execution of that recursive call will, in turn, stop to
execute the recursive call

newWriteVertical(1 / 10);

which is equivalent to

newWriteVertical(0);

This, in turn, will stop to execute the recursive call newWriteVertical(0 / 10); which
is also equivalent to

newWriteVertical(0);

This will produce another recursive call to again execute the same recursive
method call newWriteVertical(0); and so on, forever. Because the definition of
newWriteVertical has no stopping case, the process will proceed forever (or until
the computer runs out of resources). ■