Java Reference
In-Depth Information
For simplicity, suppose that we invoke this method with the statement
countDown(3);
from within a
main
method of the class that defines
countDown
. This call behaves like any other call
to a nonrecursive method. The argument 3 is copied into the parameter
integer
and the following
statements are executed:
System.out.println(3);
if
(3
>
1)
countDown(3 - 1);
// first recursive call
A line containing 3 is displayed, and the recursive call
countDown(2)
occurs, as Figure 7-2a shows.
Execution of the method is suspended until the results of
countDown(2)
are known. In this
particular method definition, no statements appear after the recursive call. So although it
appears that nothing will happen when execution resumes, it is here that the method returns to
the client.
FIGURE 7-2
The effect of the method call
countDown(3)
(a)
(b)
(c)
countDown(3)
countDown(2)
countDown(1)
Display 3
Call
countDown(2)
Display 2
Call
countDown(1)
Display 1
7.9
Continuing our trace,
countDown(2)
causes the following statements to execute:
System.out.println(2);
if
(2
>
1)
countDown(2 - 1);
// second recursive call
A line containing 2 is displayed, and the recursive call
countDown(1)
occurs, as shown in
Figure 7-2b. Execution of the method is suspended until the results of
countDown(1)
are known.
The call
countDown(1)
causes the following statements to execute:
System.out.println(1);
if
(1
>
1)
A line containing 1 is displayed, as Figure 7-2c shows, and no other recursive call occurs.
Figure 7-3 illustrates the sequence of events from the time that
countDown
is first called. The
numbered arrows indicate the order of the recursive calls and the returns from the method. After 1
is displayed, the method completes execution and returns to the point (arrow 4) after the call
countDown(2
-
1)
. Execution continues from there and the method returns to the point (arrow 5)
after the call
countDown(3
-
1)
. Ultimately, a return to the point (arrow 6) after the initial recursive
call in
main
occurs.
