COP 3503 FALL 2012 Shayan Javed Lecture 13

1. COP 3503 FALL 2012ShayanJavedLecture 13 Programming Fundamentals using Java

2. Storage and Retrieval

3. Storage and Retrieval • Looked at how to read and write files.

4. Storage and Retrieval • Looked at how to read and write files. • How can we store and later retrieve the objects we need for our programs?

5. Storage and Retrieval • Method 1: • Create a file format

6. Storage and Retrieval • Method 1: • Create a file format • Store all your info as text

7. Storage and Retrieval • Example: • Let’s look at the UndergradStudent class from Project 1

8. Storage and Retrieval • Example: • Let’s look at the UndergradStudent class from Project 1 • We want to store all the students

9. Storage and Retrieval public class UndergradStudentextends Student { publicUndergradStudent(String name, int UFID, String dob, double gpa) { super(name, UFID, dob, gpa); } }

10. Storage and Retrieval public class UndergradStudentextends Student { publicUndergradStudent(String name, int UFID, String dob, double gpa) { super(name, UFID, dob, gpa); } } Properties: String name intUFID String dob double gpa (Ignore courses for now)

11. Storage • Need to decide on a format to store the info

12. Storage • Need to decide on a format to store the info • Sample format: Undergrad Student: Name: name UFID: UFID D.O.B: dob GPA: gpa

13. Storage • Need to decide on a format to store the info • Sample format: Undergrad Student: Name: name UFID: UFID D.O.B: dob GPA: gpa

14. Storage • Sample method to write to file: publicstaticvoidwriteStudent(PrintWriter pw, UndergradStudent student) throwsIOException pw.println("Undergrad Student: "); pw.println("Name: " + student.getName()); pw.println("UFID: " + student.getUFID()); pw.println("D.O.B: " + student.getDob()); pw.println("GPA: " + student.getGpa()); }

15. Retrieval • Now we have files with student info in them

16. Retrieval • Now we have files with student info in them • Need to be able to read the info back and construct student objects

17. Retrieval • Now we have files with student info in them • Need to be able to read the info back and construct student objects • Have to parse the file

18. Retrieval publicstaticUndergradStudentreadStudent(Scanner in) throwsIOException { // read line by line // name String line1 = in.nextLine(); .... // UFID line1 = in.nextLine(); ..... // DOB line1 = in.nextLine(); ..... // gpa line1 = in.nextLine(); .... // Create the student object UndergradStudent student = newUndergradStudent(name, ufid, dob, gpa); return student; }

19. Storage and Retrieval • So we looked at one method of storing and retrieving objects.

20. Storage and Retrieval • Method 1 • Advantages: • Stored in human-readable format (open in text editor)

21. Storage and Retrieval • Method 1 • Advantages: • Stored in human-readable format (open in text editor) • Can edit data directly

22. Storage and Retrieval • Method 1 • Advantages: • Stored in human-readable format (open in text editor) • Can edit data directly • Can be read in any programming language

23. Storage and Retrieval • Method 1 • Disadvantages: • Need to write read/write methods

24. Storage and Retrieval • Method 1 • Disadvantages: • Need to write read/write methods • Reading/Writing can be slow • (Create Scanner, read next line, then next token, etc. etc...)

25. Storage and Retrieval • There is another way of writing and reading objects.

26. Storage and Retrieval • There is another way of writing and reading objects. • Create persistent objects in memory.

27. Storage and Retrieval • There is another way of writing and reading objects. • Create persistent objects in memory. • Objects stored as streams of bytes.

28. Storage and Retrieval • Objects are serialized (writing)

29. Storage and Retrieval • Objects are serialized (writing)  Serialization is the process of converting an object state into a format that can be stored (for example, in a file or memory buffer) and "resurrected" later in the same or another computer environment.

30. Storage and Retrieval • Objects are serialized (writing)  Serialization is the process of converting an object state into a format that can be stored (for example, in a file or memory buffer) and "resurrected" later in the same or another computer environment. • “Resurrection” = Deserialization (reading)

31. Storage and Retrieval • Objects are serialized (writing)  Serialization is the process of converting an object state into a format that can be stored (for example, in a file or memory buffer) and "resurrected" later in the same or another computer environment. • “Resurrection” = Deserialization (reading)

32. Serialization • In Java, implement the Serializable interface

33. Serialization • In Java, implement the Serializable interface • Indicates that it can be converted into a stream of bytes for writing

34. Serialization • In Java, implement the Serializable interface • Indicates that it can be converted into a stream of bytes for writing • Marker interface importjava.io.Serializable; publicclassNameimplementsSerializable {

35. Serialization // Create an UndergradStudent object UndergradStudent student1 = new UndergradStudent("John Smith”, ...) FileOutputStreamfos = null; ObjectOutputStream out = null; try { // Create FileOutputStream fos = newFileOutputStream(args[0]); // pass in file name out = newObjectOutputStream(fos); // write out the objects out.writeObject(student1); // close the stream fos.close(); } catch (IOExceptionio) {...}

36. DeSerialization // Create an UndergradStudent object UndergradStudent student; FileInputStreamfis = null; ObjectInputStream in = null; try { // Create FileInputStream fis = newFileInputStream(args[0]); // pass in file name in = newObjectInputStream(fis); // read the objects student1 = (UndergradStudent) in.readObject(); // close the stream fis.close(); } catch (IOException/ClassNotFoundExceptionio) {...}

37. Storage and Retrieval • Method 2: Serialization • Advantages: • Very simple to write and read. • Fast

38. Storage and Retrieval • Method 2: Serialization • Disadvantages: • Not human-readable. • Cannot be edited by text-editors • Restricted to one programming language (not exactly…have to implement reading/writing algorithm for other languages).

39. Storage and Retrieval • What if you don’t want a variable in a class to be written out?

40. Storage and Retrieval • What if you don’t want a variable in a class to be written out? publicStudent implementsSerializable{ protected Course[] courses; … }

41. Storage and Retrieval • What if you don’t want a variable in a class to be written out? publicStudent implementsSerializable{ protected Course[] courses; … } We don’t want to store the Courses array when writing out

42. Storage and Retrieval • What if you don’t want a variable in a class to be written out? publicStudent implementsSerializable{ protectedtransient Course[] courses; … } Use the transient keyword

43. Summary • Two ways of storage/retrieval: • Text files • specify a file format, write parsers • Serializable • stream of bytes • write out objects directly, read back directly TODO: Look into “JSON” as a format