CSPP : Introduction to Object-Oriented Programming

Transcription

1 CSPP : Introduction to Object-Oriented Programming Harri Hakula Ryerson 256, tel August 7, 2000

2 CSPP : Lecture 15, August 7, Exceptions Files: Text File I/O Project Outline

3 CSPP : Lecture 15, August 7, Error Handling What is the correct way to inform the user of an error? (By users we mean our fellow programmers.) Classic example: Empty stack. In our Stack-example, we wanted to avoid the drastic action of exiting the program when the user wanted to pop an empty stack, so we always returned a value, even though we also reported that the stack was empty.

4 CSPP : Lecture 15, August 7, Exceptions In JAVA there is a way to reroute the flow of control in a consistent and safe manner: Exceptions. Exceptions allow us to divide the program in two parts: the expected and unexpected. The idea is to try to execute the normal flow but in case of something exceptional happening, change to some other routine (error handling). Typical examples: File not found, division by zero, etc.

5 CSPP : Lecture 15, August 7, Pseudocode: try-throw-catch... try { dosomething(); if(error) throw new Exception("I m dying..."); else domore(); catch(exception e) {......

6 CSPP : Lecture 15, August 7, catch The catch-block if executed only if an exception has been thrown before it. They can be specialized: try { catch(ioexception e) {... catch(exception e) {... An exception can only be caught once. There are many exceptions defined by the system, e.g., IOException.

7 CSPP : Lecture 15, August 7, Files: Text File I/O Standard input and output (keyboard and screen) can be thought of as streams of bytes. Files, on the other hand, are streams of bytes, are therefore standard input and output files? Answer: Yes. However, there is a lot that happens behind the screens... Everything we need can be found from the package: java.io. We can pull in the names of the classes by saying import java.io.*; Having done this, java.io.bufferedreader can be written as BufferedReader.

8 CSPP : Lecture 15, August 7, Reading import java.io.*; public class TextFileInputDemo { public static void main(string[] args) { try { BufferedReader inputstream = null; inputstream = new BufferedReader( new FileReader("data.txt")); String line = null; line = inputstream.readline(); System.out.println( "The first line in data.txt is:");

9 CSPP : Lecture 15, August 7, System.out.println(line); catch(filenotfoundexception e) { System.out.println( "File data.txt not found."); catch(ioexception e2) { System.out.println( "Error reading from file data.txt.");

10 CSPP : Lecture 15, August 7, Writing import java.io.*; public class TextFileOutputDemo { public static void main(string[] args) { PrintWriter outputstream = null; try { outputstream = new PrintWriter( new FileOutputStream("out.txt")); catch(ioexception e) { System.out.println( "Error opening the file out.txt.");

11 CSPP : Lecture 15, August 7, System.exit(0); String line = null; int count; for (count = 1; count <= 3; count++) { line = SavitchIn.readLine(); outputstream.println(count + " " + line); outputstream.close();

12 CSPP : Lecture 15, August 7, File I/O Normally all files are binary files. For our purposes text files are sufficient and much easier to debug. Read Write ReadBuffered(FileReader(String filename)) PrintWriter(FileOutputStream(String fname))

13 CSPP : Lecture 15, August 7, File I/O: Exceptions IOException Standard I/O exception. FileNotFoundException Especially useful when opening a file.

14 CSPP : Lecture 15, August 7, Exceptions Revisited Note that some errors are not really errors at all. For instance, file cannot exist unless someone has created it first. It is easier to write our programmes is such a fashion that exceptional circumstances can be handled separately from the main bulk.

15 CSPP : Lecture 15, August 7, We can add our own exceptions: Designing Exceptions public class MyException extends Exception{ public MyException(String message) { super(message);... public void oops() throws MyException { catch(myexception e) { System.out.println(e.getMessage());

16 CSPP : Lecture 15, August 7, Project