‫עקרונות תכנות מונחה עצמים‬
‫תרגול ‪ :7‬כתיבה לקבצים‬
Outline

Animation

The flickering problem

File I/O

Assignment 3
Outline

How to write to a file?

Enforcing policy
How to write to a file?

Writing to a file sounds simple:

Open file

Write

Close file
How to write to a file?

But what about the exceptions?

File or directory not found

No permissions

No disk space

Cannot close file
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();
}
}
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();
}
}}
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.
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) {
throw new AbortException() ;
}
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;
}
The write mthid (cont’d)
…
} catch (AbortException e) {
reportGiveUp() ;
throw new GiveUpException();
}
} // end of while loop
}// end of function
Enforcing Policy


Can we make all file access safe ?

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
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);
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();
}}}
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
Inner Class

We have already used Inner Class:
JButton b = new Jbutton(“Ok”);
b.addActionListener( new ActionListener(){
public void actionPerformed(ActionEvent e){
…
}
}
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();
}
}
‫סיכום כתיבה לקובץ‬



Basic Idea:

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
‫עבודה מספר ‪3‬‬