RECURSION - Introduction to Java Programming: Comprehensive Version - page 718

14
15

public static long getSize(File file) {

getSize method

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

17
18 if (file.isDirectory()) {
19 File[] files = file.listFiles(); // All files and subdirectories
20 for ( int i = 0 ; files != null && i < files.length; i++) {
21 size += getSize(files[i]); // Recursive call
22 }
23 }
24 else { // Base case
25 size += file.length();
26 }
27
28

is directory?
all subitems

recursive call

base case

return size;

29 }
30 }

Enter a directory or a file: c:\book
48619631 bytes

Enter a directory or a file: c:\book\Welcome.java
172 bytes

Enter a directory or a file: c:\book\NonExistentFile
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 and added to the total size (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
nonexistent file.

testing all cases

18.16 What is the base case for the getSize method?
18.17 How does the program get all files and directories under a given directory?
18.18 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

18.19

Will the program work if the directory is empty (i.e., it does not contain any files)?

18.20

Will the program work if line 20 is replaced by the following code?
for ( int i = 0 ; i < files.length; i++)