Java Reference
In-Depth Information
S
ELF
C
HECK
3.
What are all permutations of the four-letter word beat?
4.
Our recursion for the permutation generator stops at the empty string.
What simple modification would make the recursion stop at strings of
length 0 or 1?
595
596
C
OMMON
E
RROR
13.2: Tracing Through Recursive
Methods
Debugging a recursive method can be somewhat challenging. When you set a
breakpoint in a recursive method, the program stops as soon as that program line is
encountered in any call to the recursive method. Suppose you want to debug the
recursive
getArea
method of the
Triangle
class. Debug the
TriangleTester
program and run until the beginning of the
getArea
method. Inspect the
width
instance variable. It is 10.
Remove the breakpoint and now run until the statement
return
smallerArea + width
; (see
Figure 1
). When you inspect
width
again, its
value is 2! That makes no sense. There was no instruction that changed the value
of
width
. Is that a bug with the debugger?
No. The program stopped in the first recursive call to
getArea
that reached the
return
statement. If you are confused, look at the call stack (top left in the
figure). You will see that nine calls to
getArea
are pending.
You can debug recursive methods with the debugger. You just need to be
particularly careful, and watch the call stack to understand which nested call you
currently are in.
