COMP1721 Object-Oriented Programming

Transcription

1 School of Computing FACULTY OF ENGINEERING COMP1721 Object-Oriented Programming Lecture 5: Exceptions & File I/O Nick Efford Module Twitter Module Website: Discussion: Quiz 1. Generally, in a class definition, A Fields and methods should be public B Fields should be private, methods should be public C Fields should be static, methods should be public 2. A default constructor for class Foo has a definition looking like A public Foo() B public void Foo() C public new() 3. An instance of class Foo is created like this: A Foo f; B Foo f = new Foo(); C Foo f = Foo(); 2 Today s Objectives To understand exceptions and what makes this approach to error handling different from other approaches To see how exceptions can created and intercepted To explore some simple techniques for reading data from and writing data to text files Signalling Errors Why is this not a good idea in general? public class Account public void withdraw(double amount) if (amount < 0 amount > balance) System.err.println("Error: invalid amount"); System.exit(1); 3 balance -= amount; 4

2 Signalling Errors What are the drawbacks of this approach? public class Account public boolean withdraw(double amount) if (amount < 0 amount > balance) return false; balance -= amount; return true; 5 Exceptions The standard way of representing and dealing with errors in object-oriented languages An exception object is a signal that something has gone wrong during program execution Exception s class conveys information about the type of error, and the object can store additional information in its fields (typically just a descriptive string) Crucial point: an exception is a message that cannot be ignored; it will halt execution unless it is dealt with 6 Java Exception Examples ArithmeticException ArrayIndexOutOfBoundsException NullPointerException InputMismatchException FileNotFoundException IOException direct subclasses of Exception are checked exceptions and require special treatment subclasses of RuntimeException are unchecked exceptions subclasses of Error indicate JVM problems; don t catch these! 7 8

3 Checked vs Unchecked Unchecked Represent typical, programmer-preventable errors Assumed that they can occur virtually anywhere, so no special treatment required by programmer Normally wouldn t want an end-user to see them Checked Represent errors arising from external factors - i.e., cannot prevent by different programming Programmer must catch or specify Effect on Flow of Control Normal flow of control ceases immediately Control is passed up the call stack, until an exception handler for the exception is found If no matching exception handler is found by the time the main method is reached, the program halts and a stack trace is displayed Example: Control.java 9 10 Interception With try & catch Creating Your Own Exceptions one or more catch blocks is required; first match is used put more specific exceptions first; use Exception as a catch all finally block is optional try Code that might throw exceptions catch (FooException error) Code to handle specific FooException errors catch (Exception error) Code to handle all other types of error finally Code that should always execute (see CatchExample1, CatchExample2) public void withdraw(double amount) if (amount < 0 amount > balance) throw new IllegalArgumentException("Invalid amount"); balance -= amount; easiest to use a standard exception class, but you can also define your own 11 12

4 Exception Specifications method doesn t attempt to catch the checked exception, so must declare that it could be thrown public void read() throws FileNotFoundException FileReader reader = new FileReader("myfile.txt"); FileNotFoundException will be thrown if myfile.txt cannot be accessed Useful Tips Create exceptions when you don t know how to deal with an error and want the caller to decide Throwing and catching are supposed to happen in different places - there is no point throwing an exception and then immediately catching it! Don t micro-manage exception handling by wrapping each statement in trycatch Note: for checked exceptions only! Useful Tips Think about what end-users would see! Java I/O A huge and complex topic that could easily consume hours of lectures! - we will focus on one small aspect Main package of relevant classes is java.io Key concept: two parallel class hierarchies, one for handling I/O of binary data, the other for handling text For text files, we can use the Reader and Writer classes; simplest approach to reading is often to use Scanner (which uses Reader classes internally) Simple approach to writing text is to use PrintWriter 15 16

5 Reading Lines: Low-Level Approach public void readfile(string filename) throws IOException BufferedReader input = new BufferedReader( new FileReader(filename)); String line = input.readline(); while (line!= null) line = input.readline(); almost all I/O can result in IOException FileReader wrapped in a BufferedReader, for efficiency Scanning a Text File For Lines public void readfile(string filename) throws FileNotFoundException Scanner input = new Scanner(new File(filename)); while (input.hasnextline()) String line = input.nextline(); input.close(); input.close(); readline will return null at end of file (see ScanLines.java) Scanning a Text File For Numbers Writing to a Text File public void readfile(string filename) throws FileNotFoundException Scanner input = new Scanner(new File(filename)); while (input.hasnextint()) int value = input.nextint(); input.close(); (see ScanNumbers.java) and similarly for double or other numeric types adds formatting capability PrintWriter out = new PrintWriter( new BufferedWriter( new FileWriter("foo.txt"))); Call printf method on this object to do formatted output Call close method on it when you re done adds buffering to improve performance supports output to a file 19 20

6 Other Remarks Can simplify creation: out = new PrintWriter(new File("foo.txt")); Passing in true as the boolean second argument of the constructor enables autoflushing: out = new PrintWriter(new File("foo.txt"), true); each call to println will result in output being delivered to the file Summary We have Introduced the concepts of exceptions as objects that signal errors and halt a program if they are not handled Examined the syntax for throwing and catching exceptions Noted that checked exceptions must either be caught or declared in exception specifications Explored ways of reading from and writing to text files - typically using File, Scanner, PrintWriter Follow-up Work Reading: Chapter 12 of Liang Exercises 12.3 (p.506) (p.507-8) Can leave these until after submitting coursework Put them in ex6 directory of your repository 23