Download - COP 3503 FALL 2012 Shayan Javed Lecture 13

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COP 3503 FALL 2012SHAYAN JAVED

LECTURE 13

Programming Fundamentals using Java

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Storage and Retrieval

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Looked at how to read and write files.

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Looked at how to read and write files.

How can we store and later retrieve the objects we need for our programs?

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Method 1:

Create a file format

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Method 1:

Create a file format

Store all your info as text

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Example:

Let’s look at the UndergradStudent class from Project 1

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Example:

Let’s look at the UndergradStudent class from Project 1

We want to store all the students

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public class UndergradStudent extends Student {

public UndergradStudent(String name, int UFID, String dob, double gpa) {

super(name, UFID, dob, gpa);}

}

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public class UndergradStudent extends Student {

public UndergradStudent(String name, int UFID, String dob, double gpa) {

super(name, UFID, dob, gpa);}

}

Properties: String nameint UFIDString dobdouble gpa(Ignore courses for now)

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Storage

Need to decide on a format to store the info

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Storage


Sample format:

Undergrad Student: Name: name UFID: UFID D.O.B: dob GPA: gpa

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Storage


Sample format:

Undergrad Student: Name: name UFID: UFID D.O.B: dob GPA: gpa

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Storage

Sample method to write to file:

public static void writeStudent(PrintWriter pw, UndergradStudent student) throws IOException

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()); }

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Retrieval

Now we have files with student info in them

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Retrieval


Need to be able to read the info back and construct student objects

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Retrieval


Need to be able to read the info back and construct student objects

Have to parse the file

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Retrievalpublic static UndergradStudent readStudent(Scanner in) throws

IOException {// read line by line// nameString line1 = in.nextLine(); ....// UFIDline1 = in.nextLine(); .....// DOBline1 = in.nextLine(); .....// gpaline1 = in.nextLine(); ....

// Create the student objectUndergradStudent student = new UndergradStudent(name, ufid, dob, gpa);return student;

}

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So we looked at one method of storing and retrieving objects.

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Method 1

Advantages: Stored in human-readable format (open in text editor)

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Method 1

Advantages: Stored in human-readable format (open in text editor) Can edit data directly

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Method 1

Advantages: Stored in human-readable format (open in text editor) Can edit data directly Can be read in any programming language

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Method 1

Disadvantages: Need to write read/write methods

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Method 1

Disadvantages: Need to write read/write methods Reading/Writing can be slow

(Create Scanner, read next line, then next token, etc. etc...)

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There is another way of writing and reading objects.

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Create persistent objects in memory.

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Create persistent objects in memory.

Objects stored as streams of bytes.

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Objects are serialized (writing)

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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.

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“Resurrection” = Deserialization (reading)

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“Resurrection” = Deserialization (reading)

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Serialization

In Java, implement the Serializable interface

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Serialization


Indicates that it can be converted into a stream of bytes for writing

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Serialization


Indicates that it can be converted into a stream of bytes for writing

Marker interfaceimport java.io.Serializable;

public className implements Serializable {

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Serialization// Create an UndergradStudent objectUndergradStudent student1 = new UndergradStudent("John Smith”, ...)

FileOutputStream fos = null;ObjectOutputStream out = null;try {

// Create FileOutputStreamfos = new FileOutputStream(args[0]); // pass in file nameout = new ObjectOutputStream(fos);// write out the objectsout.writeObject(student1);// close the streamfos.close();

} catch (IOException io) {...}

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DeSerialization// Create an UndergradStudent objectUndergradStudent student;

FileInputStream fis = null;ObjectInputStream in = null;try {

// Create FileInputStreamfis = new FileInputStream(args[0]); // pass in file namein = new ObjectInputStream(fis);// read the objectsstudent1 = (UndergradStudent) in.readObject();// close the streamfis.close();

} catch (IOException/ClassNotFoundException io) {...}

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Method 2: Serialization

Advantages: Very simple to write and read. Fast

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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).

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What if you don’t want a variable in a class to be written out?

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public Student implements Serializable {protected Course[] courses;…

}

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public Student implements Serializable {protected Course[] courses;…

}

We don’t want to store the Courses array when writing out

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public Student implements Serializable {protected transient Course[] courses;…

}

Use the transient keyword

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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