Events and Exceptions

Transcription

1 Events and Exceptions Analysis and Design of Embedded Systems and OO* Object-oriented programming Jan Bendtsen Automation and Control

2 Lecture Outline Exceptions Throwing and catching Exceptions creating and throwing exceptions the try-catch() construct Events Simple file I/O Streams, readers and writers The File Class java.io.serializable

3 ...

4 Error-prone activities Consider a (C or similar) function that does the following: void readfile() { // Open a file; // Determine its size; // Allocate that much memory; // Read the file into memory; // Close the file;

5 Error-prone activities Consider a (C or similar) function that does the following: void readfile() { // Open a file; - Oops! The file does not exist // Determine its size; - Oops! The file size cannot be determined // Allocate that much memory; - Oops! Out of memory! // Read the file into memory; - Oops! File corrupted! // Close the file; - Oops! Lost connection

6 Traditional error handling errorcodetype readfile { errorcodetype errorcode = 0; // Variable declarations FILE fp == fopen( MyFile.txt, r ); if (fp!= NULL) { filelength = getfilelength(); if (filelength > 0) { buffer = malloc(filelength*sizeof(int)); if (buffer!= NULL) { // allocation went OK, read // the file into memory; if (readfailed()) { errorcode = -1; else { errorcode = -2; else { errorcode = -3;... return errorcode;

7 Traditional error handling The actual code tends to drown in if (...) {... else { // error handling constructs The logical flow of the code is interrupted => it is easy to lose the overview (the error handling code may itself introduce errors) => it is easy to forget to handle errors (buggy software)

8 Exceptions catch(fileopenfailed) { void readfile() { // Error handling for file opening try { catch(sizedeterminationfailed) { // Open a file; // Error handling for file size // Determine its size; catch(memoryallocationfailed) { // Allocate that much // Error handling for malloc memory; catch(filereadfailed) { // Read the file into memory; // Error handling for read failure catch(fileclosefailed) { // Close the file; // Error handling for lost conn. // etc....

9 Exceptions Exceptions are objects generated when the normal program flow is interrupted They are only generated when needed They can be used to delegate information of a failure or other event to a place in your program that is geared to handle it, thus improving the robustness of your software They don't save you any work... but they can improve the readability and logical flow of your code quite a lot

10 Exceptions as classes java.lang.throwable project.io java.lang.exception IOException #msg: String +IOException() +IOException(String s) +getmessage() FileOpenException #filename: String +getfilename() +getfile()

11 Using exceptions Exceptions are standard objects that must be a (possibly distant) subclass of java.lang.exception When somethings goes wrong, an exception object is created (using a constructor, just like any other object) and then either handled immediately or thrown Once thrown, the program execution is halted The exception is then passed down the call stack until an appropriate code segment is able to process them. This is called catching an exception If an exception is not caught at some point, the program dies

12 Throwing exceptions Generates and throws an exception Forwards exception Method where error occurs Method without error handler Looking for appropriate handler Catches exception Method with error handler Looking for appropriate handler Method calls main

13 Throwing Exceptions in Java Where the exception occurs: public double computeoutput(double input) throws SingularMatrixException { if (D.isSingular()) { throw new SingularMatrixException( D is not invertible! ); // Computations... In the invoking method: public Vector computeoutput(vector inputs) throws SingularMatrixException {... for(double input : inputs) { v.add(computeoutput(input.doublevalue()));

14 Catching Exceptions in Java Error handling code public void compute() {... try { Vector v = system.computeoutput(thevalues);... catch (SingularMatrixException sme) { // Do the necessary to handle singular matrices...

15 Events Events are similar to Exceptions, in that they are generated when something happens They are typically generated in connection with user interaction, e.g., mouse clicks etc. Unlike Exceptions, they are not thrown down the call stack, but rather handed over to an Event Listener which acts as an event handler; If they are not caught, the program typically does not die No try-catch construct, just ordinary method calls

16 The Event Listener model Event sources Event listeners JButton UIWindow JSpinner JMenu SettingsManager Space PlayerCharacter Return Z

17 Events in Java First thing to do is add one or more event listener objects to the event source object(s) involved: public class Simulator extends JFrame implements ActionListener { // Constructor public Simulator() { JButton startbtn = new JButton( Start ); JButton stopbtn = new JButton( Stop ); startbtn.addactionlistener(this); stopbtn.addactionlistener(this);... This way, both startbtn and stopbtn are able to hand over Action events to the Simulator object.

18 Events in Java In order to receive events, an appropriate event handling method must be implemented: public class Simulator extends JFrame implements ActionListener {... public void actionperformed(actionevent ae) { String buttonlabel = ae.getactioncommand(); if (buttonlabel.equals( Start )) { else { startsimulation(); stopsimulation();

19 UML JFrame ActionListener Simulator -startbutton: JButton -endbutton: JButton +actionperformed(actionevent ae) -startsimulation() -stopsimulation()

20 File I/O in Java I/O is abstracted to a high degree in Java I/O is centered around the concept of streams disk files devices other programs memory arrays... They can be of different types byte streams character streams object streams

21 Basic Streams Reading data into a program Writing data from a program

22 Streams in Java Streams are, of course, objects Programs typically use byte streams to perform input and output of 8- bit bytes All byte stream classes inherit from InputStream and OutputStream For instance, if the source/destination are files, one could use FileInputStream FileOutputStream However, if we are working with text (i.e., characters), it is more convenient to use Reader and Writer objects FileReader FileWriter

23 Character-based I/O public static void main(string[ ] args) throws IOException { FileReader in = null; FileWriter out = null; try { in = new FileReader("xanadu.txt"); out = new FileWriter("outagain.txt"); int c; // character by character... while ((c = in.read())!= -1) { out.write(c); finally { if (in!= null) { in.close(); if (out!= null) { out.close();

24 Streams, Readers and Writers Rather than having to work with raw bytes or individual characters, it is often convenient to wrap streams in formatted readers and writers : try { input = new BufferedReader(new FileReader("xanadu.txt")); output = new PrintWriter(new FileWriter("characteroutput.txt")); String line; while ((line = input.readline())!= null) { output.println(line); catch(filenotfoundexception e) { System.out.println( Could not find file! );...

25 Streams, Readers and Writers Program Byte stream Physical file Basic Reader Formatted Reader

26 File objects in Java The File class hides underlying OS-specific details from the user; consider for instance File fil = new File( slides3.pdf ); On a Linux system, fil.getabsolutepath() might return /home/dimon/javacourse/lecture3/slides3.pdf On a Windows system, the same call might return c:\my Documents\java\lecture3\slides3.pdf By manipulating File objects, you can do various things to files existing in the file system Delete files (fil.delete(); fil.deleteonexit(); ) Change the modification date/time fil.setlastmodified(new Date().getTime()); Rename files ( fil.renameto( slides13.ps ); )

27 java.io.serializable The interface java.io.serializable allows writing objects to a (binary) stream: public class Matrix implements Serializable {... Such objects are then read and written via ObjectInputStream and ObjectOutputStream objects, respectively: try { ObjectOutputStream oos = new ObjectOutputStream(new FileWriter( myfile.dat )); oos.writeobject(this);

28 java.io.serializable Serializing objects should be used with some caution, as the writeobject() method automatically includes all object references known to the object in question in the bytes to be written to the stream, thus introducing a lot of overhead If at all in doubt, write your own text-based content-saving methods

29 PlotDemo this time with data loaded from disk paint() PolynomialGraph (JPanel) p(x) FunctionPlotter (JFrame) JButton coefficients Polynomial Click! FileInput

30 Classes (Objects) in PlotDemo paint() PolynomialGraph (JPanel) p(x) FunctionPlotter (JFrame) JButton coefficients Polynomial Click! NumericalInput