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Exceptions. Ensuring program reliability. Program correctness. The term program correctness refers to a program’s working as advertised; that is, it produces correct results for valid inputs
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Exceptions Ensuring program reliability
Program correctness • The term program correctness refers to a program’s working as advertised; that is, it produces correct results for valid inputs • A related concept is program reliability; this is a measure of how well a program performs under a variety of conditions (including invalid inputs) • A program can be correct, but not reliable • A program can be reliable, but not correct • Our goal is to produce programs with both these qualities
Ensuring reliability • We do our best to ensure program correctness through a rigorous testing and debugging process • To ensure reliability, we must anticipate conditions that could cause problems, and try to deal with them before the problems occur • In Java, we have exception handling, a powerful tool for ensuring program reliability
Catching Exceptions • An exception represents an error condition that can occur during the normal course of program execution. • When an exception occurs, or is thrown, the normal sequence of flow is terminated. The exception-handling routine is then executed; we say the thrown exception is caught.
Catching Exceptions • We can increase our programs’ reliability and robustness if we catch the exceptions ourselves using error recovery routines we develop. • One way to do this is to wrap the statements that may throw an exception with the try-catch control statement.
Catching Exceptions - example inputStr = JOptionPane.showInputDialog(null, prompt); try { age = Integer.parseInt(inputStr); // code that might cause error } catch (NumberFormatException e) { JOptionPane.showMessageDialog(null, “” + inputStr + “ is invalid\n” + “Please enter digits only”); }
Catching Exceptions • A try-catch block is similar to a selection structure • If no error occurs (in the example, if the user entered only digits), the catch block is ignored • If there is an error, the catch block executes
Catching Exceptions • Statements in the try block are executed in sequence. • If more than one exception is possible, multiple catch blocks can be written • When one of the statements throws an exception, control is passed to the matching catch block and statements inside the catch block are executed.
Two control flows of the try-catch statement with one catch block.
Catching Exceptions • We must specify which exception we are catching in the catch block’s parameter list. • In Java an exception is represented as an instance of the Throwable class or its subclasses. • The Throwable class has two subclasses: • Error • Exception
Catching Exceptions • The Error class represents serious problems that should not be caught by ordinary applications • The Exception class represents error conditions that should be caught
Propagating Exceptions • When a method may throw an exception, either directly or indirectly, we call the method an exception thrower. • Every exception thrower must be one of two types: • catcher. • propagator.
Propagating Exceptions • An exception catcher is an exception thrower that includes a matching catch block for the thrown exception. • An exception propagator does not contain a matching catch block. • A method may be a catcher of one exception and a propagator of another.
Propagating Exceptions • If a method is an exception propagator, we need to modify its header to declare the type of exceptions the method propagates. • We use the reserved word throws for this declaration. void C( ) throws Exception { ... } void D( ) throws Exception { ... }
Propagating Exceptions • Without the required throws Exception clause, the program will not compile. • However, for the exception of the type called runtime exceptions, the throws clause is optional.
Throwing Exceptions • An exception is thrown using the throw statement. throw <a throwable object> • where <a throwable object> is an instance of the Throwable class or its subclasses.
Throwing Exceptions • If there is a block of code that must be executed regardless of whether an exception is thrown, we use the reserved word finally. inputStr = JOptionPane.showInputDialog(null, “”); try{ number = Integer.parseInt(inputStr); if (num>100) { throw new Exception(“Out of bound”); } } catch (NumberFormatException e) { System.out.println(“Cannot convert to int”); } catch (Exception e) { System.out.println(“Error: ” + e.getMessage()); } finally { System.out.println(“DONE”); }
Throwing Exceptions • When there are multiple catch blocks in a try-catch statement, they are checked in sequence. • It is important to check more specialized exception classes before the more general exception classes. • When an exception is thrown, its matching catch block is executed and the other catch blocks are ignored.
Catching Exceptions • There are two methods of the Throwable class we can call to get information about the thrown exception: • getMessage • printStackTrace
2 possible outcomes of the try-catch statement with multiple catch blocks
Throwing Exceptions • If none of the catch blocks matches the thrown exception, the system will search down the stack trace for a method with a matching catch block. • If none is found, the system will handle the thrown exception.
Throwing Exceptions • Even if there is a return statement inside the try block, the finally block is executed. • When the return statement is encountered in the try block, statements in the finally block are executed before actually returning from the method.
2 possible outcomes of the try-catch statement with multiple catch blocks and the finally block
Propagating Exceptions • Do not catch an exception that is thrown as a result of violating a condition set by the client programmer. • Instead, propagate the exception back to the client programmer’s code and let him or her handle it.
// Class to input age: allows client programmers to specify the // lower and upper bounds of acceptable input values import javax.swing.*; class AgeInput { private static final String DEFAULT_MESSAGE = "Your age:"; private static final int DEFAULT_LOWER_BOUND = 0; private static final int DEFAULT_UPPER_BOUND = 99; private static final int DEFAULT_LOWER_BOUND = 0; private static final int DEFAULT_UPPER_BOUND = 99; private int lowerBound, upperBound; private void setBounds(int low, int high) { lowerBound = low; upperBound = high; } Example
public AgeInput ( ) throws IllegalArgumentException { setBounds(DEFAULT_LOWER_BOUND, DEFAULT_UPPER_BOUND); } public AgeInput (int low, int high) throws IllegalArgumentException { if (low > high) { throw new IllegalArgumentException("Low (" + low + ") was " + "larger than high(" + high + ")"); } else { setBounds(low, high); } } public int getAge() throws Exception { return getAge(DEFAULT_MESSAGE); } Example continued
public int getAge(String prompt)throws Exception { String inputStr; int age; while (true) { inputStr = JOptionPane.showInputDialog(null, prompt); try { age = Integer.parseInt(inputStr); if (age < lowerBound || age > upperBound) { throw new Exception("Input out of bound"); } return age; // input okay so return the value & exit } catch (NumberFormatException e) { JOptionPane.showMessageDialog(null, "'" + inputStr + " is invalid\n“ + "Please enter digits only"); } // end catch block } // end while loop } // end method } // end class
Types of Exceptions • There are two types of exceptions: • Checked. • Unchecked. • A checked exception is an exception that is checked at compile time. • All other exceptions are unchecked, or runtime, exceptions. As the name suggests, they are detected only at runtime.
Types of Exceptions • If a method is a propagator of checked exceptions, the method must have the throws clause. • When calling a method that can throw checked exceptions, use the try-catch statement and place the call in the try block, or modify the method header to include the appropriate throws clause. • If a method is a propagator of runtime exceptions or errors, the throws clause is optional.
