INTRODUCTION TO JAVA PROGRAMMING教学（华南理工）11slide-Exceptions.pptVIP

Introduction to Java Programming Chapter11 Exceptions and Assertions Runtime Errors Catch Runtime Errors Exception Classes System Errors Exceptions Runtime Exceptions Checked Exceptions vs. Unchecked Exceptions Unchecked Exceptions Checked or Unchecked Exceptions Declaring, Throwing, and Catching Exceptions Declaring Exceptions Every method must state the types of checked exceptions it might throw. This is known as declaring exceptions. public void myMethod() throws IOException public void myMethod() throws IOException, OtherException Throwing Exceptions When the program detects an error, the program can create an instance of an appropriate exception type and throw it. This is known as throwing an exception. Here is an example, throw new TheException(); TheException ex = new TheException();throw ex; Throwing Exceptions Example /** Set a new radius */ public void setRadius(double newRadius) throws IllegalArgumentException { if (newRadius = 0) radius = newRadius; else throw new IllegalArgumentException( Radius cannot be negative); } Catching Exceptions When an exception is thrown, it can be caught and handled in a try-catch block. try { statements; // Statements that may throw exceptions } catch (Exception1 exVar1) { handler for exception1; } catch (Exception2 exVar2) { handler for exception2; } ... catch (ExceptionN exVar3) { handler for exceptionN; } Catching Exceptions Catch or Declare Checked Exceptions Java forces you to deal with checked exceptions. If a method declares a checked exception (i.e., an exception other than Error or RuntimeException), you must invoke it in a try-catch block or declare to throw the exception in the calling method. For example, suppose that method p1 invokes method p2 and p2 may throw a checked exception (e.g., IOException), you have to write the code as shown in (a) or (b). Example: Declaring, Throwing, and Catching Exceptions Objective: This example demonstrates d