1. עקרונות תכנות מונחה עצמים תרגול 7: כתיבה לקבצים

2. Outline
Animation
The flickering problem
File I/O
Assignment 3

3. Outline
How to write to a file?
Enforcing policy

4. How to write to a file? Writing to a file sounds simple: Open file
Close file

5. How to write to a file? But what about the exceptions?
File or directory not found
No permissions
No disk space
Cannot close file

6. Write to file – First attempt
import java.io.*; public class BadWriteToFile { public void writeToFile() { System.out.println("Please insert filename:"); BufferedReader tIn = new BufferedReader(new InputStreamReader(System.in)); String tFilename = tIn.readLine(); File tFile = new File(tFilename); PrintWriter tOut = new PrintWriter(new FileWriter(tFile)); tOut.println("Hello, World"); tOut.close(); }

7. Write to file – Second attempt
import java.io.*; public class BadWriteToFile { public void writeToFile() { System.out.println("Please insert filename:"); BufferedReader tIn = new BufferedReader(new InputStreamReader(System.in)); try { String tFilename = tIn.readLine(); File tFile = new File(tFilename); PrintWriter tOut = new PrintWriter(new FileWriter(tFile)); tOut.println("Hello, World"); tOut.close(); } catch (IOException e) { e.printStackTrace(); } }}

8. Write to file – Third attempt
public class GoodWriteToFile { private String promptFile() throws AbortException{…} public void writeToFile() throws GiveUpException{…} private void reportError(); private void reportGiveUp(); } promptFile: Read filename from user, throw AbortException if user cancels writeToFile: Try to write to file, try to recover if fail, give up only if user cancels.

9. The prompt method
private String promptFile() throws AbortException { System.out.println(“please insert filename: “); BufferedReader tIn = new BufferedReader( new InputStreamReader(System.in)); try{ String tFilename = tIn.readLine(); if (tFilename == null || tFilename.isEmpty() ) throw new AbortException() ; return tFilename ; } catch ( IOException e) { }

10. The write method
public void writeToFile() throws GiveUpException{ while(true){ try{ String tFilename = promptFile() ; try { PrintWriter tOut = new PrintWriter( new FileWriter (new File(tFilename))); tOut.println(“Hello, World”); tOut.close(); return ; } catch(IOException e) { reportError(); continue; }

11. The write mthid (cont’d)
… } catch (AbortException e) { reportGiveUp() ; throw new GiveUpException(); } } // end of while loop }// end of function

12. Enforcing Policy Can we make all file access safe ? The OOP way:
Different things to write
Different prompts (CLI, GUI, …)
Different error reports, etc..
The OOP way:
Use a base class for file access
Concrete classes write to files

13. The Base Class
public abstract class WriteToFileBase { protected abstract File promptFile() throws AbortException; protected abstract void writeInner(FileWriter f)throws IOException; protected abstract void reportError (Exception e); protected abstract void reportGiveUp (Exception e);

14. WriteToFileBase – Concrete method
public void writeToFile() throws GiveUpException { while(true){ try{ File tFile = promptFile(); try { FileWriter f = new FileWriter(tFile); writeInner(f); f.close(); return; }catch(AbortException e){ reportGiveUp(e); throw new GiveUpException(); } } }

15. How to use WriteToFileBase?
It is not always possible to extend base class
Other inheritance constrains
Cumbersome
Redundant
Solution
Put the code that write to a file in an inner class
The inner class extends WriteToFileBase

16. Inner Class We have already used Inner Class:
JButton b = new Jbutton(“Ok”);
b.addActionListener( new ActionListener(){
public void actionPerformed(ActionEvent e){ … }

17. Using WriteToFileBase
public class ExampleWriteToFile extends Whatever { public void doSomething() { WriteToFileBase tWrite = new WriteToFileBase() { //other methods as before protected void writeInner(FileWriter f) throws IOException { printWriter tOut = new PrintWriter(f); tOut.println(“Hello, World”); } tWrite.writeToFile();

18. סיכום כתיבה לקובץ Basic Idea: Problem: many exceptions may be thrown
Open file
Write
Close file
Problem: many exceptions may be thrown
Separate responsibilities to different functions
Catch all exception types
Enforcing policy:
Create Abstract Base Class to Enforce policy
Use inner class in concrete classes

19. עבודה מספר 3