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Exception Handling. Introduction. Users may use our programs in an unexpected ways. Due to design errors or coding errors, our programs may fail in unexpected ways during execution, or may result in an abnormal program termination
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Introduction • Users may use our programs in an unexpected ways. • Due to design errors or coding errors, our programs may fail in unexpected ways during execution, or may result in an abnormal program termination • It is our responsibility to produce robust code that does not fail unexpectedly. • Consequently, we must design error handling into our programs.
Errors and Error Handling • Some typical causes of errors: • File system errors (i.e. disk is full, disk has been removed) • Network errors (i.e. network is down, URL does not exist) • Calculation errors (i.e. divide by 0) • More typical causes of errors: • Array errors (i.e. accessing element –1) • Conversion errors (i.e. convert ‘q’ to a number)
Exceptions • What are they? • An exception is a representation of an error condition or a situation that is not the expected result of a method. • Exceptions are built into the Java language and are available to all program code.
Checked Exceptions • How are they used? • Exceptions fall into two categories: • Checked Exceptions • Unchecked Exceptions • Checked exceptions are inherited from the core Java class Exception. They represent compile time exceptions that are your responsibility to check. • Checked exceptions must be handled in your code. Otherwise, the compiler will issue an error message.
Unchecked Exceptions • Unchecked exceptions are runtime exceptions that result at runtime from conditions that you should not have allowed in the first place (inherited from RuntimeException). • You do not have to do anything with an unchecked exception. • But, if not handled, your program will terminate with an appropriate error message.
Definitions • Stack trace • Name of the exception in a descriptive message that indicates the problem • Complete method-call stack • exceptionObject.printStackTrace(); • Exception message: returns a detailed message which describes the exception. • exceptionObject.getMessage();
Exception Handling • Exception handling is accomplished through • the “try – catch” mechanism: handle the exception by yourself, • or by a “throws” clause in the method declaration: pass exception handling “up the chain”. • If the method contains code that may cause a checked exception, you MUST handle the exception OR pass the exception up the chain.
Coding Exceptions • Try-Catch Mechanism • Wherever your code may trigger an exception, the normal code logic is placed inside a block of code starting with the “try” keyword: • After the try block, the code to handle the exception should it arise is placed in a block of code starting with the “catch” keyword. • If none of the code inside the try block throws an exception, then the program skips the catch clause.
Coding Exceptions • Example • try {… normal program code} catch(ArithmeticException AEx){ … }catch(Exception e) {… exception handling code} finally{ … code to close some files or release resources }
finally Block • Consists of finally keyword followed by a block of code enclosed in curly braces • Optional in a try statement • This block contains code that is ALWAYS executed whether or not an exception is thrown in the corresponding try block or any of its corresponding catch blocks. • If present, is placed after the last catch block • Finally blocks can be used for operations that must happen no matter what (i.e. cleanup operations such as closing a file)
Using the throws Clause • Exceptions can be thrown by statements in method’s body or by methods called in method’s body • Exceptions can be of types listed in throws clause or subclasses
Coding Exceptions • Passing the exception • In any method that might throw an exception, you may declare the method as “throws” that exception, and thus avoid handling the exception yourself. • You are not allowed to add more throws specifiers to a subclass method than are present in the superclass method. • Example • public void myMethod throws IOException {… normal code with some I/O … throw new IOException();}
