This week and next

• Tuesday 11am – IO• Friday 10am – IO and handout of mini -assignment• Friday 12 noon – review – probably of CS12230

exam?

• Monday 1pm – IO• Tuesday 11am – IO (if needed)• Friday – over to Chris

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First, the world’s quickest review

What are the 3 main characteristics of OO?– Decomposition of problem by data– Encapsulation and Information hiding– Inheritance and Polymorphism

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Student

-name: String-address: Address

DegreeScheme

-name: String

Module

-name: String

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Person

-name: String-address: Address

DegreeScheme

-name: String

Module

-name: String

StudentStaff

0..* 0..*

0..*

1..1

This leads to links between classes (and objects)

In next week

• We will focus on reuse through inheritance in order to do Input and Output

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CS12420IO and Exceptions

(This was also touched on in CS12230 and CS12130?)

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0. You need to be able to save databetween program executions

• In a Database – next year• In files – not sure how much you did last

semester• We’ll look at a few ways now:

– Human readable (as text files)– Whole objects (Serializable)– XML

CS12420

The java.io.File class• Provides facilities for file manipulation File f = new File("grades.dat");• Can now use

– exists()– canWrite() / canRead()– isFile() / isDirectory()– renameTo(File newName) / delete()– mkdir() / list () (if directory)– lastModified()– length()– createNewFile()

CS12420

Files are based on StreamsStream Fundamentals

filtered flowbuffered flow

data in asource (e .g. a

file )flow of data

InputStream

DataInputS tream

BufferedInputStream

• Streams read from a source and pass data to a sink - stream can be a source and/or a sink

• Streams build on each other – if ‘text’ is a filename:BufferedReader br = new BufferedReader

(new InputStreamReader(new(FileInputStream("text"))));

What do buffered

and filtered mean?

CS12420

No Buffering of Data reading a byte arrayYourClass FileInputStream

read(byte[])

Bytes read from disk

Bytes returned in your byte array (this is the rawest form)

• One problem here is efficiency of disk accesses– Every read can potentially lead to a disk head move

• Buffering improves efficiency…

CS12420

YourClassFileInputStream

Bytes read from disk

BufferedInputStream

read(byte[], …) If buffer empty, read a whole buffer worth of bytes

Bytes returned in yourbyte array

Bytes returned to fillinternal buffer

• This solves the efficiency issue but your classes normally wish to read ints, doubles etc NOT bytes!

• Filtering presents a better interface for your classes to use…

Buffering of Data reading a byte array

If buffer not empty – fill from here

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Filtering Data reading an int

Bytes read from disk

Returns an int 4 bytes returned for the int

YourClass

FileInputStream

BufferedInputStream

DataInputStream

Read 4 bytes for intreadInt()

Read bytes if emptyBytes returned to fillinternal buffer

CS12420

Categories of stream• Defined in java.io• Read and write streams• “node” streams - connected to a basic source

(or sink) e.g. a file• “filter” and “buffer” streams - connected to

another stream as a source (or sink)• Lots of stream classes - they fit together

CS12420

Stream class hierarchy

Object

InputStream

OutputStream

PrintStream

Reader

InputStreamReader

BufferedReader

FileInputStream

FileReader

StringReader

LineNumberInputStream

DataInputStream

FilterInputStream

ObjectInputStream

SequencenputStream

BufferedInputStream

FilterReader

Writer

DataOutputStream

FilterOutputStream BufferedOutputStream

FileOutputStream

ObjectOutputStream

OutputStreamWriter

StringWriter

FilterWriter

FileWriter

BufferedWriter

CS12420

Reading textBufferedReader br = new BufferedReader(new InputStreamReader( new(FileInputStream("text"))));

Two Alternative short cuts:BufferedReader br = new BufferedReader(new FileReader("text"));

String st = br.readLine(); br.close();Scanner infile =new Scanner(new InputStreamReader (new FileInputStream(“text”))); String st = infile.nextLine(); infile.close();

FileInputStream(bytes)

InputStreamReader

BufferedReader

(strings)

Human readable

Writing text

• Text output use a PrintWriter - can essentially use a PrintWriter to write to a file just like you would use System.out

PrintWriter pw = new PrintWriter(new OutputStreamWriter (new FileOutputStream (“text”)));

pw.println("John");pw.close();

• There is also a BufferedWriter which has different methods and can generate more exceptions

What does this mean for you?

public void load(String fn) throws IOException{ Scanner infile =new Scanner(newInputStreamReader (new FileInputStream(fn)));

int num=infile.nextInt(); infile.nextLine(); //why?

for (int i=0;i<num;i++) { String n=infile.nextLine(); String p=infile.nextLine(); Contact c=new Contact(n,p); contacts.add(c); } infile.close(); }

Aberystwyth University - CS12420

For me:I find an incantation that works and stick with it – this is mine

public void save(String fn) throws IOException{

PrintWriter outfile = new PrintWriter (new OutputStreamWriter (new FileOutputStream (fn))); outfile.println(contacts.size());

for (Contact c:contacts) { outfile.println(c.getName() );

outfile.println(c.getPhone() );//so as to match loading!

} outfile.close(); }

Date file looks like2Fred01970622452Sue01654234234

contacts is an ArrayList

Do I need to review ArrayList?

• Or introduce it?• You are welcome to use any other kind of

collection if you like (CS121 what did you do?)

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CS12420

Reading data directly (skip)

DataInputStream dis = new DataInputStream(new BufferedInputStream( new FileInputStream("data")));int i = dis.readInt();dis.close();

FileInputStream(bytes)

DataInputS tream

BufferedInputStream

(int, float, short)

CS12420

Writing data directly (skip) • Writing data requires output streamsDataOutputStream dos = new DataOuputStream(new BufferedOutputStream( new FileOutputStream(“data”)));

Person.java example (skip)• See online code example (or next page)• This saves the object’s data using normal IO

stream class operations – faster than as text• The static read method is used since we run

read on the Person class and after it reads the data it constructs a Person object which it returns

public void write(String pathname) throws IOException{ DataOutputStream dos = new DataOutputStream(new

BufferedOutputStream(new FileOutputStream(pathname))); dos.writeUTF(this.name); dos.writeInt(this.age); dos.close(); } public static Person read(String pathname) throws IOException{ DataInputStream dis = new DataInputStream(new

BufferedInputStream(new FileInputStream(pathname))); String theName = dis.readUTF(); int theAge = dis.readInt(); return new Person(theName, theAge); }

1. Exceptions and Text Files

• What Java does when you have a run-time error.• Examples??

• You can either throw on or catch Exceptions• First, look at them in the context of reading from

a text (editor produced) file

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Trivially – reading a filepublic void load(String fn) throws IOException{ Scanner infile =new Scanner(new InputStreamReader (new FileInputStream(fn))); int num=infile.nextInt();infile.nextLine(); for (int i=0;i<num;i++) { String n=infile.nextLine(); int a=infile.nextInt(); infile.nextLine(); //note this Person c=new Person(n,a); contacts.add(c); } infile.close();}

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Of the form:2Fred21Mary23

And writing ….

public void save(String fn) throws IOException{ PrintWriter outfile = new PrintWriter(new

OutputStreamWriter(new FileOutputStream(fn))); outfile.println(contacts.size()); for (Person p:contacts) { outfile.println(p.getName()); outfile.println(p.getAge()); //as to match loading! } outfile.close();}

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In form:2Fred21Mary23

Better

public void load(String fn) {try {… possibly problem code}catch (IOException e) {… what to do}finally{ // (usually omitted)… no matter what

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What if you have several possible things that can go wrong?

try{….}catch (IOException e) {…}catch (InputMismatchException e){…}catch (Exception e) { //this catches everything – so put last if at all!…..}

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Notice how this all relates to inheritance.Exception is the most

general oneIOException is a special

case of it, FileNotFoundException is a special case of that

See API

Dealing with InputMismatch Exceptions

boolean ok=false; do{ try{ System.out.println("enter number"); Scanner in=new Scanner(System.in); int input=in.nextInt(); //may throw exception in.nextLine(); System.out.println("you entered "+input); ok=true; } catch (InputMismatchException e) { //so ‘ok’ is still false } }while(!ok);

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Beware!• You CANNOT deal with NullPointerExceptions

this way – they are a sign that there is a logic error.

• Usually that you have done something like:Person p;….p.methodcall();

• Without ever saying p=new Person();

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Day 2

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You can define your own exceptions

• To deal with ‘exceptional’ situationspublic class MyException extends RuntimeException {

……}

• Then when one of those situations happens say throw new MyException(“message”);

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do { // The exception is expected to be thrown in the try try { System.out.print("Please enter your age: "); ageOfPerson = in.nextInt(); if (ageOfPerson<0 || ageOfPerson>130) throw new MyInvalidAgeException(); in.nextLine(); successful = true; } // if an exception is thrown, it is caught here catch (InputMismatchException e){ System.out.println("That wasn't an integer " + e); in.nextLine(); } catch (MyInvalidAgeException e) { System.out.println("That was a ridiculous age " + e); in.nextLine(); } finally { System.out.println("I have just executed the finally block"); } } while (!successful); System.out.println("Ok success - you typed in " + ageOfPerson); }

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2. Serializable

• What is the matter with always using text files?• Have to save each attribute AND consider if more than

one kind of thing all linked (Monsters, Treasures, Weapons) would have to save and link!

• It is possible to save an entire object• Even if it contains links to other objects• Trouble is that the resulting file is only readable by a

computer – not a human like a text file is• Let us see what this means ….

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Here is part of the class diagram from lecture 1

• Consider the Facebook system (lots of Users, lots of Posts)

• Now consider some point in time where you have 2 users, each has several posts

• Draw the object (instance) diagram for that situation <<do it>>

• Once you have done that, think about how you could store the data in simple text files

Users file:

2FredMary

Posts file:

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Fred post 1 info

Fred post 2 info

Mary post 1 info

Mary post 2 info

Fred post 3 info

Not very satisfactory is it!

Have to relink all the data in the program

OR

One file:

2

Fred

3

Fred post 1 info

Fred post 2 info

Fred post 3 info

Mary

2

Mary post 1 info

Mary post 2 info

Look at your object diagram

• Wouldn’t it be great if we could just dump the whole system object out when we save and then reload it all at once ..

• .. complete with all the links between objects, no matter how complex (consider if users are tagged in other users posts)

• We can ….

Serializable interface

public class Person implements Serializable{…..}• Serializable is a marker interface: it has no methods,

but is used to mark the class to give the serialization mechanism permission to serialize objects of the class

• Then there are some clever methods that enable a stream to read and write these whole objects

• We then use these

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This uses a different kind of File Byte Stream as opposed to a Character Stream MACHINE

readable not HUMAN readable

public void writeToFile(String fullPathOfFile) throws IOException { // Lets get a handle on the file to write to File myFile = new File (fullPathOfFile); ObjectOutputStream myStream = new ObjectOutputStream( new BufferedOutputStream( new FileOutputStream(myFile))); System.out.println("Writing to the file now...."); myStream.writeObject(this); // Write the object to file myStream.close(); // Don't forget this!! }

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How USE this method

Person p; //presumably gets filled somewhere……p.writeToFile(“out.ser”);

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Typical Serialised file shown in editor:

This contains the Person record with name “fred” age 41¬ísrPerson—ËÁHrT“IageLnametLjava/lang/String;xp)tfred

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What is actually saved is

1. Class name of the object2. Object’s primitive data3. If attributes are object references, then those

objects are also saved (go to point 1)4. Each object has a unique serial version number

to ensure it’s not saved twice and that the versions read and written match up properly

5. Not saved: Methods or static variables/constants

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But reading has to be done differently

p.readPerson(“out.ser”) would not work because there IS no p until we finish reading.

Instead we need a method that returns a Person:p=Person.readPerson(“out.ser”);

This is an example of a static method (linked directly to the class rather than called for a particular object of the class)

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Here is the code://notice the staticness public static Person readPerson(String fullPathOfFile) throws IOException{ Person tempPerson; File myFile = new File (fullPathOfFile); ObjectInputStream myStream = new ObjectInputStream( new BufferedInputStream(new FileInputStream(myFile))); System.out.println("Reading from the file now...."); // Read the object from file - it needs to be cast try { tempPerson = (Person) myStream.readObject(); } catch(ClassNotFoundException e) { throw new IOException("File had objects of incorrect class"); } myStream.close(); return tempPerson; } 43

Aside: Other static method examples

Collections.sort(someList);

double x=Math.sqrt(9.0);

double x=Math.pow(3.0,2.0);

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What can go wrong?

• NotSerializableException at runtimeThat means that you haven’t made the thing to

serialize AND all the things it points to implement Serializable

• InvalidClassException at runtimeThat happens if you serialize an object and then

change the definition of the object’s class and then try to deserialize it the unique serial version numbers do not match …

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• If you change attributes then we want Java to throw this exception, otherwise we will lose data etc (bugs)

• However, if we just make a small change (such as adding a method) then we don’t care and we don’t want to see the exception

• In your programs you are unlikely to have a problem since you won’t be using files generated many versions ago

If it becomes a problem you will need to investigate the ‘serialver’ command (serial version command)

– You can generate a serialver yourself in your codeprivate static final long serialVersionUID = 1L;

– On the command lineserialver Person2

Do this before you make the change and before serializing the objectNow when you make a minor change and deserialize Java will use the serial version number defined in that static final and no exception will be thrown

– Or through eclipse

Beware trying to serialize Scanner

• If you keep your data separate from your manipulation (so the thing you are serializing does not have a private Scanner instance variable) you won’t have a Scanner in the stuff you are trying to save

But if not …• You should make a Scanner ‘transient’ so that

the system doesn’t try to Serialize itprivate transient Scanner scan;

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3. Another way of I/O whole objects

• This is another nice way to I/O entire objects• XML – have you run across this before?• Extensible Markup Language (XML) is a

markup language created to structure, store, and transport data by defining a set of rules for encoding documents in a format that is both human-readable and machine-readable. (wikipedia)

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This is the result:

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This is how you get it:import java.io.*;import java.beans.XMLEncoder;import java.beans.XMLDecoder;///////////////////////////////////////////////// must have ALL gets and sets – must be a ‘bean’public class PersonXML{ private int age; private String name;//lots deleted

public void write(String pathname) throws IOException{ XMLEncoder encoder = new XMLEncoder(

new BufferedOutputStream(new FileOutputStream(pathname))); encoder.writeObject(this); encoder.close(); }

public static PersonXML read(String pathname) throws IOException { PersonXML result = null; XMLDecoder decoder = new XMLDecoder( new BufferedInputStream( new FileInputStream(pathname))); result = (PersonXML)decoder.readObject(); return result; }}

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Class must be a bean -have gets and sets for all attributes and a default constructor!

• See notes 1-exceptions.doc• And code 1-exceptions-serializable-files

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What have we just done?• Exceptions• A couple of things about files and IO

• Reading/Writing files• Serialization• XMLEncoder and Decoder

• Do you notice that a lotof this depends on inheritance??

• Look at inheritance hierarchy for an Exception• Look at Serializable and XMLEncoderhttp://docs.oracle.com/javase/7/docs/api/

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Save these slides for reference in your Java programming careerI always have to look this stuff up, but if you understand the principles that is not a problem!

Mini Assignment 1 – 10%• Details on Blackboard• You will get 3 mini assignments in which you will be

asked to try out things (also smaller worksheets)• We started this so that you wouldn’t have too many

big assignments but would still get to practise everything we do in this module as it happens – ie. you’d just have one ‘real’ assignment

• Then last year some people didn’t do the first one – 10% is a lot to lose!

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