Java

1. Java Chapter 9 File Input and Output

2. Objectives In this chapter you will: • Learn how bits, characters, records, and files fit into a data hierarchy • Explore the differences between text files and binary files • Use the File class to identify, manage, and manipulate files • Perform data input and output with text files • Perform data input and output with binary files

3. File Input and Output • Most real-world applications use data files • Data stored in files are called persistent data • Reading data from a file is called file input • Writing data to a file is called file output • File processing refers to file input or file output

4. File Input and Output (continued) • Two types of files • Text files contain only human-readable characters • Binary files contain human-readable characters and other characters understood by the software or hardware • Files containing data needed by an application may be either text or binary

5. Inputting Data from a Text File • Many programs are designed to use external data • External data can be numeric • Stock prices, GPS coordinates temperatures • External data can be string • Names, descriptions, passwords • If such data is stored permanently in a data file, and is designed to be human-readable, it is a text file

6. Text File Organization • Recall all data ultimately is a sequence of 0s and 1s, which represent two states of electronic circuits: on and off • A localized group of 8 bits forms a byte and bytes represent characters • Example: ‘A’ is 65 in Unicode, 01000001 in binary: 0X27+ 1X26+ 0X25+ 0X24+ 0X23+ 0X22+ 0X21+ 1X20

7. Text File Organization (continued) • A data field is a group of characters that has a specific meaning • Example: last_name, student_ID, test_score • A data record is a group of related fields • Example: Smith 12345 95.5 • Attributes of the same individual • A data file is a group of related data records stored in a single file

8. Text File Organization (continued)

9. The File Class • The File class establishes the file’s name and location and opens the file for input • If the file is in a directory other than the current, its path must be specified • The relative path is the path of folders that leads to the file relative to the current file • The absolute path is the path from the drive letter to the file • The syntax for declaring a File object: File myFl = new File (“./mydata.txt”);

10. The FileReader and BufferedReader Classes • After the data file is established using a File object, the data in the file can be read • A source file provides data to a program • Analogous to a pipeline • The pipeline has 2 ends connected to the source (the input file) and destination (the program file) • The pipeline has a valve that controls the amount of data allowed into the program

11. The FileReader and BufferedReader Classes (continued)

12. The FileReader and BufferedReader Classes (continued) • Establish a data source File infl = new File (“./source.txt”); • Create a pipeline from source to program FileReader frdr = new FileReader (infl); • Create a valve BufferedReader aBfrd = new BufferedReader(frdr);

13. The FileReader and BufferedReader Classes (continued) • Read one line of data String aRcrd = aBfrd.readLine(); • Input can also be read using the Scanner class File infl = new File (./source.txt); Scanner input = new Scanner (infl); String aFld = input.next();

14. Apply the Concept • Develop an application to read IDs, names, and scores of students from a file and display the average score for each student • A while loop tests whether the program is at the end of the student names file • As IDs are read scores are read and totalScore and scoreCount are updated

15. Working with Directories • The file object has many methods. • The file can be a folder or directory. • Review the API documentation for all of the methods. • http://java.sun.com/j2se/1.4.2/docs/api/index.html • Note the list() and list(filter) methods that can be used to return a list of files (as well as other folders) in a folder.

16. Outputting Data to a Text File • Previously we have accessed data from a source file • Next, we output data to a destination file • A destination file receives data from a program

17. The File Class • File objects for output are created just as for input • Unlike source files, destination files do not have to exist before the program is run • If the destination file already exists, it is overwritten when the program is run • If the destination file does not exist, it is created

18. The File Class (continued)

19. The FileWriter and PrintWriter Classes • Instantiate a File object to create the data destination • Instantiate a FileWriter object to create a pipeline from the program to the output file • FileWriter throws an IOException if it can’t create the output file • Instantiate a PrintWriter object to enable writing to the output file • Use the println method in PrintWriter to write the output

20. The FileWriter and PrintWriter Classes (continued)

21. Apply the Concept • Modify previous example to print each student’s score average to file, as well as to the command window • The application uses the same input files • Import java.io.IOException, java.io.FileWriter, and java.io.PrintWriter • Instantiate a File object for writing output • PrintWriter is declared outside of the try block to be accessible in the finally block

22. Apply the Concept (continued) • A runtime error can occur if the output file location is nonexistent

23. Apply the Concept (continued)

24. Performing Input and Output with Binary Files • The previous sections discussed reading from and writing to text files • Java classes Scanner, FileReader, BufferedReader, FileWriter, and PrintWriter work with pipelines or streams that carry text data • All files not classified as text files are binary files • Binary files can be compiled programs, image files, sound files, compressed files

25. Identifying an Input/Output File • The process of using the File class is the same for text files and binary files • Create a binary file to write to in the current directory File file1 = new File ( “./myFile.dat” );

26. Writing to a Binary File • To write to a text file there are 5 steps: File oFl = new File ( “averages.txt”); FileWriter fwt = new FileWriter (oFl); PrintWriter pwt = new PrintWriter (fwt); pwt.println (aLineOfData); pwt.close(); • To write to a binary file, substitute: • FileOutputStream for FileWriter • DataOutputStream for PrintWriter • writeChar for println

27. Reading from a Binary File • To read from a binary file, identify the input file using a File object • A FileInputStream object connects the input file to the program • The DataInputStream allows different types of data to be read • Methods in DataInputStream read different types of data

28. Reading from a Binary File

29. Summary • Data is arranged in a hierarchy: files, records, fields, characters, bits • The File class identifies a file to the program so that it can be read from or written to • An absolute path is the path from the drive letter to the file • A relative path is the path to the file relative to the current file • The class FileReader reads a continuous stream of characters from a text file

30. Summary (continued) • The class BufferedReader controls the flow of characters through the FileReader object • The class FileWriter establishes a data stream from the program to a text file • The class PrintWriter enables writing formatted text to a text file • The classes FileOutputStream and DataOutputStream write program data to binary files • The classes FileInputStream and DataInputStream read data from binary files