About This Lecture. Outline. Handling Unusual Situations. Reacting to errors. Exceptions

Transcription

1 Exceptions Revised 24-Jan-05 CMPUT Lecture 4 Department of Computing Science University of Alberta About This Lecture In this lecture we will learn how to use Java Exceptions to handle unusual program conditions. D. Szafron, C. Jones 2003 Some code in this lecture is based on code from the book: Java Structures by Duane A. Bailey or the companion structure package CMPUT Lecture 4 Slide 2 Outline When things go wrong Throwables, Errors and Exceptions Throwing exceptions Catching exceptions Defining your own exception classes When things go wrong In the last lecture, we learned about pre/post conditions, and assertions. But what if these fail? Java: originally developed for writing software embedded in specialized devices (cell phones, hand-held computers, appliances etc.) Such software must be able to handle unusual situations and manage appropriately when things go wrong CMPUT Lecture 4 Slide 3 CMPUT Lecture 4 Slide 4 Reacting to errors Handling Unusual Situations What if the user enters a word instead of an integer? We try to pop an empty stack? In many languages, the programmer is solely responsible for dealing with errors no help from the language itself we try to divide by zero? We try to read from a file which doesn t exist? We try to access a remote location but the network is down? Should the program produce bad output, or should it crash, or should it try to recover? Code must be written to help identify kinds of errors and how to deal with them convoluted code! Often, methods will return an unusual value such as 1 or null to indicate a failure often difficult to interpret! Java provides some helpful mechanisms to assist us CMPUT Lecture 4 Slide 5 CMPUT Lecture 4 Slide 6 1

2 Exceptions & Errors Java has 2 classes for modeling problems: Error Exception Errors Java.lang.Error models serious, unrecoverable errors such as fatal linkage problems virtual machine errors These errors are not handled and coped with, they usually terminate execution Note that both are subclasses of Throwable Not as common as Exceptions CMPUT Lecture 4 Slide 7 CMPUT Lecture 4 Slide 8 What is an Exception? An exception is an event that occurs during the execution of a program that disrupts the normal flow of instructions - Sun Microsystems web site tutorial Not as serious as system errors maybe we can try to recover from them, or at least handle them gracefully Why use exceptions? As we will see in this lecture, the advantages of using exceptions include: separating error-handling code from regular code deferring decisions about how to respond to exceptions providing a mechanism for specifying the different kinds of exceptions that can arise in our program to bear these points in mind as we discuss this material CMPUT Lecture 4 Slide 9 CMPUT Lecture 4 Slide 10 The Exception Mechanism Propagating exceptions When a method wants to signal that a problem has arisen (e.g. a precondition is violated) it throws an exception instead of returning normally. The exception goes to the method that called the method that threw the exception. That method can either Catch (handle) the exception Propagate the exception to the method that called it So an exception will bubble up the call stack until it reaches a method with a suitable handler, or it propagates through the main( ) method (the first method on the call stack) If it is not caught by any method the exception is treated like an error: the stack frames are displayed and the program terminates CMPUT Lecture 4 Slide 11 CMPUT Lecture 4 Slide 12 2

3 Example: Throwing an Exception public Ratio(int top, int bottom) throws Exception{ /* pre: bottom!= 0 post: constructs a ratio equivalent to top/bottom if (bottom == 0) throw new Exception("Denominator must not be 0"); this.numerator = top; this.denominator = bottom; Throwing Exceptions What can be thrown? Any instance of Throwable or its sublcasses, including Error and Exception Instances of our own specialized exception classes How do we do it? Create an instance, & throw it! throw new Exception( ); throw new Exception( Explanatory message ); CMPUT Lecture 4 Slide 13 CMPUT Lecture 4 Slide 14 Throws clause in method signature The throws clause tells the compiler that a method is a potential source of that type of exception, and that any method that invokes it must be able to deal with it A method must include a throws clause in its signature if it throws an exception itself it might propagate the exception (because it calls another method that might throw an exception that this method cannot catch/handle) public Ratio add(ratio other) throws Exception { /* pre: other is non-null The throws Clause post: return new fraction - the sum of this & other Assert.pre(other!= null, "Other must not be null"); return new Ratio(this.numerator * other.denominator + this.denominator * other.numerator, this.denominator * other.denominator); CMPUT Lecture 4 Slide 15 CMPUT Lecture 4 Slide 16 Throws clause for the main method The signature of the main method must contain a throws clause for any exception that might be thrown by any method it calls (or methods they call, etc.) and not caught. public static void main(string args[])throws Exception{ Ratio r = new Ratio(1,1); // r == 1.0 r = new Ratio(1,2); // r == 0.5 r.add(new Ratio(1,3)); // r still 0.5 r = r.add(new Ratio(1,4)); // r == 0.75 System.out.println(r.value()); // 0.75 printed r = new Ratio(1, 0); // should be an exception Exceptions not Caught If an exception is not caught by any handler, the exception behaves like an error: the program is halted, a message is displayed, and the stack frames are displayed: 0.75_exceptionOccurred: java.lang.exception (Denominator must not be 0) java.lang.exception: Denominator must not be 0 at Ratio.<init>(Ratio.java) at RatioTest.main(RatioTest.java) at com.apple.mrj.jmanager.jmstaticmethoddispatcher.run(jmawtcontextimpl.java) at java.lang.thread.run(thread.java) CMPUT Lecture 4 Slide 17 CMPUT Lecture 4 Slide 18 3

4 Checked/Unchecked Exceptions Diagram Unchecked exceptions: Any exception that is a subclass of java.lang.runtimeexception java.lang.error Not a compile-time error to ignore the possibility of these exceptions we are free to catch them if we want to, but if we don t we re not required to include a throws clause Throwable Exception RuntimeException Any other subclass Error Any subclass Checked exceptions: All other kinds of exceptions must include a throws clause if an exception may occur somewhere inside it Any subclass Any subclass unchecked checked CMPUT Lecture 4 Slide 19 CMPUT Lecture 4 Slide 20 Java s Exception handling When we write a piece of code that we know might produce an exception and we want to catch (handle) the exception, we encapsulate that code in a try block. If no exception is thrown by code within the try block (or the methods that are called within the try block), the code executes normally. If an exception arises in the try block the execution of the try block terminates execution immediately and a catch-clause is sought to handle the exception. Either 1. An appropriate catch clause is found, in which case it is executed, or 2. The exception is propagated to the calling method CMPUT Lecture 4 Slide 21 Example: try-catch public static void main(string args[])throws Exception { Ratio r; r = new Ratio(1, 0); catch (Exception anexception) { System.out.println(anException); r = new Ratio(); // initializes to 0/1 System.out.println( The value is +r.value()); java.lang.exception: Denominator must not be 0 The value is 0.0 can we removed this? CMPUT Lecture 4 Slide 22 Catch clause A catch clause is associated with a particular try block. It will only be used for exceptions that arise within that try block. A catch clause has one parameter the type of this parameter is the type of the exception that this clause can handle. A try statement can have one or more catch clauses, to catch different types of exceptions. Pause for thought Why not just put the exception-handling code in the method where the exception is thrown instead of throwing the exception? Because, different callers may want to handle the same exception differently. For example, the previous segment binds the variable r to a zero Ratio. A different segment might want to bind the variable r to null if an exception occurs. CMPUT Lecture 4 Slide 23 CMPUT Lecture 4 Slide 24 4

5 block Examples // code inside the try block // no catch clause any exceptions that arise in the try block // will propagate to the calling method // code within the try block catch (Exception anexception) { // code that handles any Exception that arises in the try block CMPUT Lecture 4 Slide 25 Multiple Catch Clauses readfromfile( data.txt ); // readfromfile() might generate an exception catch (FileNotFoundException e) { // deal with file_not_found error catch (IOException e) { // deal with errors that occur while reading from file catch (Exception e) { // deal with all other errors what if we put the Exception clause at the top, in front of FileNotFoundException? CMPUT Lecture 4 Slide 26 Java s built-in exception classes Common run-time problems you ve probably seen already: ArrayIndexOutOfBoundsException NullPointerException ArithmeticException NumberFormatException ClassCastException Java provides more than 2 dozen Exceptions to model different kinds of errors and we can write our own too! User-defined Throwable Classes You can define your own Exception subclasses. This allows you to conditionally match only the Exceptions that you want to match. You can also define your own Error subclasses, although this is unusual because normally the exceptions you define will not be unrecoverable. CMPUT Lecture 4 Slide 27 CMPUT Lecture 4 Slide 28 Example: User-defined Exception To define a new type of exception, simply create a subclass of Exception. It will inherit the property of being throwable For example, if we wanted to define our own exception called ZeroDenominator we would define a class called ZeroDenominator in a file called ZeroDenominator.java It could be as simple as this one line public class ZeroDenominator extends Exception{ Throwing a User-defined Exception public Ratio(int top,int bottom) throws ZeroDenominator { /* pre: bottom!= 0 post: constructs a ratio equivalent to top/bottom if (bottom == 0) throw new ZeroDenominator(); this.numerator = top; this.denominator = bottom; CMPUT Lecture 4 Slide 29 CMPUT Lecture 4 Slide 30 5

6 Catching a User-defined Exception public static void main(string args[]) { Ratio r; r = new Ratio(1, 0); catch (ZeroDenominator anexception) { r = new Ratio(); // initializes to 0/1 System.out.println( The value is +r.value()); Multiple Constructors public class ZeroDenominator extends Exception { public ZeroDenominator(String msg) { super("\nzerodenominator with message " + msg); public ZeroDenominator() { super("\nzerodenominator with no message ); The value is 0.0 CMPUT Lecture 4 Slide 31 CMPUT Lecture 4 Slide 32 finally clause If you want to execute some code whether an exception was raised or not, you can use the optional finally clause Guaranteed to be executed, no matter why we leave the try block It is optional you don t have to have one. Why use finally? The finally clause is useful if you want to perform some kind of clean up operations before exiting the method example: closing down a stream/resource Also avoids duplicating code in each catch clause CMPUT Lecture 4 Slide 33 CMPUT Lecture 4 Slide 34 Full Semantics of try-catch-finally If an expression in the try block throws an exception: the rest of the code in the try block is skipped. If the exception matches the first catch clause the code in its catch block is run the code in the finally clause is run. the rest of the catch clauses are ignored. the rest of the code in the method is run. If the exception does not match the first catch clause, similar actions are taken for the first matching catch clause. CMPUT Lecture 4 Slide 35 Full Semantics of try-catch-finally If the exception does not match any catch clause the code in the finally clause is run all other code in the method is abandoned the exception is thrown to the calling method start looking for some other portion of code that will catch and handle the exception If no exception is thrown at all all code in the try block is executed the code in the finally clause is run normal code in the method following finally is run CMPUT Lecture 4 Slide 36 6

7 Possible Execution Paths (1) Possible Execution Paths (2) 1. No exception occurs 1. Execute the try block 2. Execute the finally clause 3. Execute the rest of the method 1. Exception occurs and is caught 1. Execute the try block until the first exception occurs 2. Execute the first catch clause that matches the exception 3. Execute the finally clause 4. Execute the rest of the method Catch Rest of method Rest of method CMPUT Lecture 4 Slide 37 CMPUT Lecture 4 Slide 38 Possible Execution Paths (3) 3. Exception occurs and is not caught 1. Execute the try block until the first exception occurs 2. Execute the finally clause 3. Propagate the exception to the calling method Throw back Do we deed a different logic? What if we wish to have a cleaner execution Catch Catch Rest of method CMPUT Lecture 4 Slide 39 Q: Can we have either TRY-FINAALY or CATCH-FINALLY? CMPUT Lecture 4 Slide 40 Example: finally & Multiple catches 1 Example: finally & Multiple catches 2 Use a NumberFormatException in the Ratio constructor to signal a zero denominator: public Ratio(int top, int bottom) throws NumberFormatException{ /* pre: bottom!= 0 post: constructs a ratio equivalent to top/bottom if (bottom == 0) throw new NumberFormatException( "Denominator must not be 0"); this.numerator = top; this.denominator = bottom; CMPUT Lecture 4 code based on Bailey Slide pg Consider a divide method in the class Ratio: public Ratio divide(ratio other) throws Exception { /* pre: other is non-null pre: other is not zero post: return new fraction - the division of this and other if (other == null) throw new NullPointerException(); else if (other.numerator == 0) throw new ArithmeticException(); else return new Ratio(this.numerator*other.denominator, this.denominator*other.numerator); CMPUT Lecture 4 code based on Bailey Slide pg

8 Example: finally & Multiple catches 3 Example: finally & Multiple catches 4 Consider some static method in a class called Tester, that calls the divide method and catches all but one exception (the NumberFormatException): private static void calldivide(ratio top, Ratio bottom) throws Exception{ /* Compute the division of top by bottom and report the answer. If top is null then change it to an invalid ratio 1/0 to generate a NumberFormatException that won't be caught. Catch the other exceptions: bottom is zero or bottom is null. Ratio answer; answer = null; if (top == null) top = new Ratio(1, 0); answer = top.divide(bottom); catch (ArithmeticException anexception) { System.out.println("Arithmetic Except: no div by 0"); answer = new Ratio(); catch (NullPointerException anexception) { System.out.println("NullPointerExcept: no div by null"); answer = new Ratio(); finally { System.out.println(" the answer: " + answer); // rest skipped for NumberFormatException since not caught System.out.println("End method answer: + answer.value()); CMPUT Lecture 4 code based on Bailey Slide pg. 943 CMPUT Lecture 4 code based on Bailey Slide pg. 944 Example: finally & Multiple catches 5 main method illustrating exception handling: public static void main(string args[]) { Tester.callDivide(new Ratio(1, 2), new Ratio(3,4)); // should work Tester.callDivide(new Ratio(1, 2), new Ratio(0,4)); // ArithmeticException caught in calldivide() Tester.callDivide(new Ratio(1, 2), null); // NullPointerException caught in calldivide() Tester.callDivide(null, new Ratio(3,4)); // NumberFormatException NOT caught in calldivide() catch (Exception anexception) { System.out.println("NumberFormatException not caught by calldivide()"); CMPUT Lecture 4 code based on Bailey Slide pg