RECURSION - Introduction to Java Programming - page 750

public static long getSize(File file) {

getSize method

15
16 long size = 0 ; // Store the total size of all files

17
18 if ( ) {
19 // All files and subdirectories
20 for ( int i = 0 ; files != null && i < files.length; i++) {
21 size +=

file.isDirectory() is directory?

all subitems File[] files = file.listFiles();

getSize(files[i])

recursive call

; // Recursive call

22 }
23 }
24 else { // Base case
25 size += file.length();
26 }
27
28

base case

return size;

29 }
30 }

Enter a directory or a file:
48619631 bytes

c:\book

Enter a directory or a file:
172 bytes

c:\book\Welcome.java

c:\book\NonExistentFile

Enter a directory or a file:
0 bytes

If the file object represents a directory (line 18), each subitem (file or subdirectory) in the
directory is recursively invoked to obtain its size (line 21). If the file object represents a file
(line 24), the file size is obtained (line 25).
What happens if an incorrect or a nonexistent directory is entered? The program will detect
that it is not a directory and invoke file.length() (line 25), which returns 0 . Thus, in this
case, the getSize method will return 0 .

Tip
To avoid mistakes, it is a good practice to test all cases. For example, you should test the
program for an input of file, an empty directory, a nonexistent directory, and a nonexis-
tent file.

testing all cases

20.16 What is the base case for the getSize method?
20.17 How does the program get all files and directories under a given directory?
20.18 How many times will the getSize method be invoked for a directory if the directory
has three subdirectories and each subdirectory has four files?

✓ ✓

Check

Point

20.7 Case Study: Towers of Hanoi
The Towers of Hanoi problem is a classic problem that can be solved easily using
recursion, but it is difficult to solve otherwise.

Key
Point

The problem involves moving a specified number of disks of distinct sizes from one tower to
another while observing the following rules:

There are n disks labeled 1, 2, 3, . . . , n and three towers labeled A, B, and C.

■

No disk can be on top of a smaller disk at any time.

■