20.7. Key Terms

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18.5. Case Study: Copying File

This case study develops a program that copies files. The user needs to provide a source file and a target file as command-line arguments using the following command:

java Copy source target

The program copies a source file to a target file and displays the number of bytes in the file. If the source does not exist, tell the user that the file has not been found. If the target file already exists, tell the user that the file exists. A sample run of the program is shown in Figure 18.13.

Figure 18.13. The program copies a file.

To copy the contents from a source to a target file, it is appropriate to use a binary input stream to read bytes from the source file and a binary output stream to send bytes to the target file, regardless of the contents of the file. The source file and the target file are specified from the command line. Create an InputFileStream for the source file and an OutputFileStream for the target file. Use the read() method to read a byte from the input stream, and then use the write(b) method to write the byte to the output stream. Use BufferedInputStream and BufferedOutputStream to improve the performance. Listing 18.3 gives the solution to the problem.

Listing 18.3. Copy.java

(This item is displayed on pages 615 - 616 in the print version)

 1   import   java.io.*; 2 3   public class   Copy { 4  /** Main method  5   @param args[0] for source file   6   @param args[1] for target file   7  */  8   public static void   main(String[] args)   throws   IOException { 9  // Check command line parameter usage  10   if   (args.length !=   2   ) { 

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 11 System.out.println( 12   "Usage: java CopyFile sourceFile targetfile.dat"   ); 13 System.exit(     ); 14 } 15 16  // Check if source file exists  17  File sourceFile =   new   File(args[     ]);  18   if   (!sourceFile.exists()) { 19 System.out.println(   "Source file "   + args[     ] +   " not exist"   ); 20 System.exit(     ); 21 } 22 23  // Check if target file exists  24  File targetFile =   new   File(args[   1   ]);  25   if   (targetFile.exists()) { 26 System.out.println(   "Target file "   + args[   1   ] +   " already exists"   ); 27 System.exit(     ); 28 } 29 30  // Create an input stream  31  BufferedInputStream input =  32    new   BufferedInputStream(   new   FileInputStream(sourceFile));  33 34  // Create an output stream  35  BufferedOutputStream output =  36    new   BufferedOutputStream(   new   FileOutputStream(targetFile));  37 38  // Display the file size  39 System.out.println(   "The file "   + args[     ] +   " has "   + 40  input.available()  +   " bytes"   ); 41 42  // Continuously read a byte from input and write it to output  43   int   r; 44   while   ((  r = input.read()  ) !=   - 1   ) 45  output.write((   byte   )r);  46 47  // Close streams  48 input.close(); 49 output.close(); 50 51 System.out.println(   "Copy done!"   ); 52 } 53 } 

• The program first checks whether the user has passed two required arguments from the command line in lines 10 “14.

• The program uses the File class to check whether the source file and target file exist. If the source file does not exist (lines 18 “21) or if the target file already exists, exit the program.

• An input stream is created using BufferedInputStream wrapped on FileInputStream in lines 31 “32, and an output stream is created using BufferedOutputStream wrapped on FileOutputStream in lines 35 “36.

• The available() method (line 40) defined in the InputStream class returns the number of bytes remaining in the input stream.

• The expression ((r = input.read()) !=-1) (line 44) reads a byte from input.read() , assigns it to r , and checks whether it is -1 . The input value of -1 signifies the end of a file. The program continuously reads bytes from the input stream and sends them to the output stream until all of the bytes have been read.