Object-Oriented Programming Lecture 6: Exceptions

Transcription

1 Object-Oriented Programming Lecture 6: Exceptions Dr. Lê H!ng Ph"#ng -- Department of Mathematics, Mechanics and Informatics, VNUH August

2 Content What is an exception? Catching and handling exceptions Specifying the exceptions thrown by a method How to throw exceptions Advantages of exceptions Exercises 2

3 What is an exception? An exception is an event, which occurs during the execution of a program, that disrupts the normal flow of the program s instructions. When an exception occurs within a method, the method creates an exception object and hands it off to the runtime system. The exception object contains information about the error (its type, the state of the program when the error occurred). Creating an exception object and handing it to the runtime system is called throwing an exception. 3

4 What is an exception? When an exception is thrown, the runtime system attempts to find something to handle it. A set of possible somethings is the ordered list of methods that had been called to get to the method where the error occurred. This list of methods is known as call stack. 4

5 What is an exception? The runtime system searches the call stack for a method that can handle the exception. The block of code that handles the exception is called exception handler. The type of exception object thrown must match the type that can be handled by the handler. When the handler is found, the runtime passes the exception to the handler. 5

6 Content What is an exception? Catching and handling exceptions Specifying the exceptions thrown by a method How to throw exceptions Advantages of exceptions Exercises 6

7 Catching and handling exceptions 3 exception handler components: try, catch and finally blocks. An example: ListOfNumbers Creates an ArrayList that contains 10 Integer elements; Specifies a methods named writelist() which writes the list to a text file called output.txt. 7

8 Catching and handling exceptions public class ListOfNumbers { private List<Integer> list; private static final int SIZE = 10; public ListOfNumbers() { list = new ArrayList<Integer>(SIZE); for (int i = 0; i < SIZE; i++) { list.add(new Integer(i)); public void writelist() { PrintWriter out = new PrintWriter(new FileWriter("output.txt")); for (int i = 0; i < SIZE; i++) { out.println("value at: " + i + " = " + list.get(i)); out.close(); 8

9 Checked and unchecked exceptions The constructor new FileWriter("output.txt") initializes an output stream to a file. If the file cannot be opened, the constructor throws an IOException. The call to the ArrayList s get method list.get(i) may throw an IndexOutOfBoundException if the value of its argument is invalid (negative or larger than the size of the list). There are two possible exceptions, however, the compiler reports only the error message thrown by the FileWriter constructor. The first exception is a checked exception. The second exception is an unchecked exception. 9

10 The try block Enclose the code that might throw an exception within a try block. try { // code catch and finally blocks... In our example: PrintWriter out = null; try { out = new PrintWriter(new FileWriter("output.txt")); for (int i = 0; i < SIZE; i++) { out.println("value at: " + i + " = " + list.get(i)); catch... 10

11 The catch blocks Catch blocks are directly after the try block: try { // code catch (ExceptionType e1) { // code catch (ExceptionType e2) { // code Example: try { catch (FileNotFoundException e) { System.err.println("FileNotFoundException: " + e.getmessage()); throw new SampleException(e); catch (IOException e) { System.err.println("Caught IOException: " + e.getmessage()); 11

12 The catch blocks In Java SE 7 and later, a single catch block can handle multiple types of exception. catch (IOException SQLException ex) { logger.log(ex); throw ex; 12

13 The finally block The finally block always executes when the try block exits. This ensures that the finally block is executed even if an unexpected exception occurs. In the example, the program should close the stream before exiting the writelist() method. The try block can exit in one of three ways: The new FileWriter statement fails and throws an IOException. The list.get(i) statement fails and throws an ArrayIndexOutOfBoundsException. Everything succeeds and the try block exits normally. 13

14 The finally block The finally block is a perfect place to perform cleanup regardless of what happens within the try block. finally { if (out!= null) { System.out.println("Closing PrintWriter"); out.close(); else { System.out.println("PrintWriter not open"); 14

15 The complete example public void writelist() { PrintWriter out = null; try { System.out.println("Entering" + " try statement"); out = new PrintWriter(new FileWriter("output.txt")); for (int i = 0; i < SIZE; i++) out.println("value at: " + i + " = " + list.get(i)); catch (ArrayIndexOutOfBoundsException e) { System.err.println("Caught ArrayIndexOutOfBoundsException: " + e.getmessage()); catch (IOException e) { System.err.println("Caught IOException: " + e.getmessage()); finally { if (out!= null) { System.out.println("Closing PrintWriter"); out.close(); else { System.out.println("PrintWriter not open"); 15

16 Content What is an exception? Catching and handling exceptions Specifying the exceptions thrown by a method How to throw exceptions Advantages of exceptions Exercises 16

17 Specifying the exceptions thrown by a method We have seen how exceptions can be handled by a method. Sometimes, it s better to let a method further up the call stack handle the exceptions. In this case, the method must specify that it can throw the exceptions that could occur within its code. public void writelist() throws IOException, ArrayIndexOutOfBoundsException { Recall that ArrayIndexOutOfBoundsException is an unchecked exception, so we can just write the following: public void writelist() throws IOException { 17

18 Content What is an exception? Catching and handling exceptions Specifying the exceptions thrown by a method How to throw exceptions Advantages of exceptions Exercises 18

19 How to throw exceptions Any code can throw an exception by using the throw statement. public Object pop() { Object obj; The Java platform provides many exception classes. All the classes are descendants of the Throwable class. Syntax: throw somethrowableobject; Throwable objects are instances of any subclass Throwable. if (size == 0) { throw new EmptyStackException(); obj = objectat(size - 1); setobjectat(size - 1, null); size--; return obj; 19

20 Throwable class and its subclasses Failures of the JRE Exception: IllegalAccessException, NegativeArraySizeException... RuntimeException: NullPointerException, ArithmeticException... 20

21 Content What is an exception? Catching and handling exceptions Specifying the exceptions thrown by a method How to throw exceptions Advantages of exceptions Exercises 21

22 Advantages of exceptions Advantage 1: Separating error-handling code from regular code. Exceptions separate details of what to do when something out of the ordinary happens from the main logic of a program. Consider the pseudocode method that reads an entire file into memory: readfile { open the file; determine its size; allocate that much memory; read the file into memory; close the file; 22

23 Advantages of exceptions Advantage 1: Separating error-handling code from regular code. Exceptions separate details of what to do when something out of the ordinary happens from the main logic of a program. Consider the pseudocode method that reads an entire file into memory: readfile { open the file; determine its size; allocate that much memory; read the file into memory; close the file; What happens if the file can't be opened? What happens if the length of the file can't be determined? What happens if enough memory can't be allocated? What happens if the read fails? What happens if the file can't be closed? 22

24 errorcodetype readfile { initialize errorcode = 0; open the file; if (thefileisopen) { determine the length of the file; if (gotthefilelength) { allocate that much memory; if (gotenoughmemory) { read the file into memory; if (readfailed) { errorcode = -1; else { errorcode = -2; else { errorcode = -3; close the file; if (thefiledidntclose && errorcode == 0) { errorcode = -4; else { errorcode = errorcode and -4; else { errorcode = -5; return errorcode; 23

25 errorcodetype readfile { initialize errorcode = 0; open the file; if (thefileisopen) { determine the length of the file; if (gotthefilelength) { allocate that much memory; if (gotenoughmemory) { read the file into memory; if (readfailed) { errorcode = -1; else { errorcode = -2; else { errorcode = -3; close the file; if (thefiledidntclose && errorcode == 0) { errorcode = -4; else { errorcode = errorcode and -4; else { errorcode = -5; return errorcode; readfile { try { open the file; determine its size; allocate that much memory; read the file into memory; close the file; catch (fileopenfailed) { dosomething; catch (sizedeterminationfailed) { dosomething; catch (memoryallocationfailed) { dosomething; catch (readfailed) { dosomething; catch (fileclosefailed) { dosomething; 23

26 Advantages of exceptions Advantage 2: Propagating errors up the call stack. Suppose that the readfile() method is the fourth method in a series of nested methods calls made by the main program. method1 { call method2; method2 { call method3; method3 { call readfile; 24

27 Advantages of exceptions Suppose that the method1 is the only method interested in the error that might occur within readfile. Traditional error-notification techniques: method1 { errorcodetype error; error = call method2; if (error) doerrorprocessing; else proceed; errorcodetype method2 { errorcodetype error; error = call method3; if (error) return error; else proceed; errorcodetype method3 { errorcodetype error; error = call readfile; if (error) return error; else proceed; 25

28 Advantages of exceptions In Java, only the methods that care about errors have to worry about detecting errors. method1 { try { call method2; catch (exception e) { doerrorprocessing; method2 throws exception { call method3; method3 throws exception { call readfile; 26

29 Advantages of exceptions Advantage 3: Grouping and differentiating error types. The grouping and categorizing exceptions is a natural outcome of the class hierarchy. catch (ArrayIndexOutOfBoundsException e) { System.err.println("Caught ArrayIndexOutOfBoundsException: " + e.getmessage()); catch (IOException e) { System.err.println("Caught IOException: " + e.getmessage()); 27

30 Content What is an exception? Catching and handling exceptions Specifying the exceptions thrown by a method How to throw exceptions Advantages of exceptions Exercises 28

31 Exercises Exercise 1. Write the program which was asked in the midterm test. Note the use of exceptions in your code. Exercise 2. Is there anything wrong with the following exception handler as written? Will this code compile? try { catch (Exception e) { catch (ArithmeticException a) { 29