1、Java程序設計,Java Programming Spring, 2009,chapter 8 Exceptions,2/25,Contents,Why Exceptions? What are Exceptions? Handling Exceptions Creating Exception Types,chapter 8 Exceptions,3/25,Why Exceptions?,During execution, programs can run into many kinds of errors; What do we do when an error occurs? Java

2、 uses exceptions to provide the error-handling capabilities for its programs.,chapter 8 Exceptions,4/25,Exceptions,Error Class Critical error which is not acceptable in normal application program. Exception Class Possible exception in normal application program execution; Possible to handle by progr

3、ammer.,chapter 8 Exceptions,5/25,Exceptions,An exception is an event that occurs during the execution of a program that disrupts(使中斷) the normal flow of instructions. Treat exception as an object All exceptions are instances of a class extended from Throwable class or its subclass. Generally, a prog

4、rammer makes new exception class to extend the Exception class which is a subclass of Throwable class.,6,7,異常類的層次結構：,8,Classifying Java Exceptions,Unchecked ExceptionsIt is not required that these types of exceptions be caught or declared on a method. Runtime exceptions can be generated by methods o

5、r by the JVM itself. Errors are generated from deep within the JVM, and often indicate a truly fatal state.,Checked ExceptionsMust either be caught by a method or declared in its signature by placing exceptions in the method signature.,chapter 8 Exceptions,9/25,Exception的分類,檢查性異常(checked exception)：

6、 一般程序中可預知的問題，其產(chǎn)生的例外可能會帶來意想不到的結果，因此Java編譯器要求Java程序必須捕獲或聲明所有的非運行時異常。以IOException為代表的一些類。如果代碼中存在檢查性異常，必須進行異常處理，否則編譯不能通過。如：用戶連接數(shù)據(jù)庫SQLException、FileNotFoundException。 非檢查性異常(unchecked exception)： 以RuntimeException為代表的一些類，編譯時發(fā)現(xiàn)不了，只在能運行時才能發(fā)現(xiàn)。,chapter 8 Exceptions,10/25,Exception的分類,Runtime Exception： Java虛

7、擬機在運行時生成的例外，如被0除等系統(tǒng)錯誤、數(shù)組下標超范圍等，其產(chǎn)生比較頻繁，處理麻煩，對程序可讀性和運行效率影響太大。因此由系統(tǒng)檢測, 用戶可不做處理，系統(tǒng)將它們交給缺省的異常處理程序（當然，必要時，用戶可對其處理）。 Java程序異常處理的原則是： 對于Error和RuntimeException，可以在程序中進行捕獲和處理，但不是必須的。 對于IOException及其他異常，必須在程序進行捕獲和處理。異常處理機制主要處理檢查性異常。,chapter 8 Exceptions,11/25,異常類的層次結構：,chapter 8 Exceptions,12/25,System-Define

8、d Exception(系統(tǒng)定義的異常),Raised implicitly by system because of illegal execution of program; When cannot continue program execution any more; Created by Java System automatically; Exception extended from Error class and RuntimeException class.,13/25,System-Defined Exception,IndexOutOfBoundsException :

9、When beyond the bound of index in the object which use index, such as array, string, and vector ArrayStoreException : When assign object of incorrect type to element of array NegativeArraySizeException : When using a negative size of array NullPointerException : When refer to object as a null pointe

10、r SecurityException : When violate security. Caused by security manager IllegalMonitorStateException : When the thread which is not owner of monitor involves wait or notify method,14,Programmer-Defined Exception,Exceptions raised by programmer Check by compiler whether the exception handler for exce

11、ption occurred exists or not If there is no handler, it is error Sub class of Exception class,chapter 8 Exceptions,15/25,Handling Exceptions,Throwing an exception When an error occurs within a method. An exception object is created and handed off to the runtime system. The runtime system must find t

12、he code to handle the error. Catching an exception The system searches for code to handle the thrown exception. It can be in the same method or in some method in the call stack.,chapter 8 Exceptions,16/25,Keywords for Java Exceptions,throwsDescribes the exceptions which can be raised by a method. th

13、rowRaises an exception to the first available handler in the call stack, unwinding the stack along the way. tryMarks the start of a block associated with a set of exception handlers. catchIf the block enclosed by the try generates an exception of this type, control moves here; watch out for implicit

14、 subsumption. finallyAlways called when the try block concludes, and after any necessary catch handler is complete.,17,Exception Occurrence,throws Statement When programmer-defined exception is raised, if there is no exception handler, need to describe it in the declaration part of method Exception1

15、.java,modifiers returntype methodName(params) throws e1, . ,ek ,18,Example,public class exception1 void Proc(int sel) throws ArithmeticException,ArrayIndexOutOfBoundsException System.out.println(“In Situation + sel ); if(sel=0) System.out.println(no Exception caught); return; if(sel=1) int iArray=ne

16、w int4; iArray10=3; ,19,Exception Occurrence,Raised implicitly by system Raised explicitly by programmer throw Statement ThrowStatement.java,throw ThrowableObject;,Throwable class or its sub class,20,Exception OccurrenceExample,class ThrowStatement extends Exception public static void exp(int ptr) i

17、f (ptr = 0) throw new NullPointerException(); public static void main(String args) int i = 0; ThrowStatement.exp(i); ,運行結果： java.lang.NullPointerException at ThrowStatement.exp(ThrowStatement.java:4) at ThrowStatement.main(ThrowStatement.java:8),21,Throwing Exceptions,You can throw exceptions from y

18、our own methods To throw an exception, create an instance of the exception class and throw it. If you throw checked exceptions, you must indicate which exceptions your method throws by using the throws keyword,public void withdraw(float anAmount) throws InsufficientFundsException if(anAmountbalance)

19、 throw new InsuffientFundsException(Not enough cash); balance = balance - anAmount; ,22,Exceptions Syntax(語法),try / Code which might throw an exception / . catch(FileNotFoundException x) / code to handle a FileNotFound exception catch(IOException x) / code to handle any other I/O exceptions catch(Ex

20、ception x) / Code to catch any other type of exception finally / This code is ALWAYS executed whether an exception was thrown / or not. A good place to put clean-up code. ie. close / any open files, etc. ,23,該語句的基本格式為：,try 可能產(chǎn)生異常的代碼； /不能有其它語句分隔 catch(異常類名 異常對象名) 異常處理代碼； /要處理的第一種異常 catch(異常類名 異常對象名)

21、異常處理代碼； /要處理的第二種異常 finally 最終處理（缺省處理） ,24/25,Handling Exceptions,try-catch 或 try-catch-finally . Three statements help define how exceptions are handled: try identifies a block of statements within which an exception might be thrown; catch must be associated with a try statement and identifies a blo

22、ck of statements that can handle a particular type of exception. The statements are executed if an exception of a particular type occurs within the try block. A try statement can have multiple catch statements associated with it.,chapter 8 Exceptions,25/25,Handling Exceptions,finally must be associa

23、ted with a try statement and identifies a block of statements that are executed regardless of whether or not an error occurs within the try block. Even if the try and catch block have a return statement in them, finally will still run.,26,Exceptions -Defining checked exceptions,Any code which throws

24、 a checked exception MUST be placed within a try block. Checked exceptions are defined using the throws keyword in the method definition:,public class PrintReader extends Reader public int read() throws IOException . ,public void method1() PrintReader aReader; . initialize reader . try int theCharac

25、ter = aReader.read(); catch(IOException x) . ,27,Handling Exceptions,public void replaceValue(String name, Object value) throws NoSuchAttributeException Attr attr=find(name); if(attr=null) throw new NoSuchAttributeException(name); attr.setValue(value); try replaceValue(“att1”, “newValue”); catch(NoS

26、uchAttributeException e) e.printStackTrace(); ,28,Exceptions -throwing multiple exceptions,A Method can throw multiple exceptions. Multiple exceptions are separated by commas after the throws keyword:,public class MyClass public int computeFileSize() throws IOException, ArithmeticException . ,29,Exc

27、eptions -throwing multiple exceptions,public void method1() MyClass anObject = new MyClass(); try int theSize = anOputeFileSize(); catch(ArithmeticException x) / . catch(IOException x) / .,30,Exceptions -catching multiple exceptions,Each try block can catch multiple exceptions. Start with the most s

28、pecific exceptions FileNotFoundException is a subclass of IO Exception It MUST appear before IOException in the catch list,public void method1() FileInputStream aFile; try aFile = new FileInputStream(.); int aChar = aFile.read(); /. catch(FileNotFoundException x) / . catch(IOException x) / . ,31,Exc

29、eption -The catch-all Handler,Since all Exception classes are a subclass of the Exception class, a catch handler which catches Exception will catch all exceptions. It must be the last in the catch List,public void method1() FileInputStream aFile; try aFile = new FileInputStream(.); int aChar = aFile

30、.read(); /. catch(IOException x) / . catch(Exception x) / Catch All Exceptions ,32,Example with try-catch-finally,/* class example FileIn.java */ /* assumes each char is one byte - dangerous import java.io.*; public class FileIn public static void main(String args) try FileInputStream fin = new File

31、InputStream(“c:javatest.java”); int input; while (input = fin.read()!= -1) System.out.print(char)input); catch(IOException ie) System.out.println(e); finally fin.close(); ,chapter 8 Exceptions,33/25,User-defined Exceptions,class UserClass UserErr x = new UserErr(); / . if (val 1) throw x; ,class Use

32、rErr extends Exception ,Creating Exception Types:,chapter 8 Exceptions,34/25,Example 1,public class InsufficientFundsException extends Exception private Bank excepbank; private double excepAmount; InsufficientFundsException(Bank ba, double dAmount) excepbank=ba; excepAmount=dAmount; ,35/25,class Ban

33、k double balance; Bank(double dAmount) balance= dAmount; public void deposite(double dAmount) if(dAmount0.0) balance+=dAmount; public void withdrawal(double dAmount) throws InsufficientFundsException if (balancedAmount) throw new InsufficientFundsException(this,dAmount); balance=balance-dAmount; pub

34、lic String show_balance() String str=The balance is +(int)balance; System.out.println(str); return str; ,36,Example 1,public class ExceptionDemo public static void main(String args) try Bank ba=new Bank(50); ba.withdrawal(100); System.out.println(“Withdrawal successful!”); catch(InsufficientFundsExc

35、eption e) System.out.println(e.toString(); System.out.println(e.excepMessage(); ,37,/Example 2*ThrowExample.java*,class IllegalValueException extends Exception ,class UserTrial int val1,val2; public UserTrial(int a,int b) val1=a; val2=b; void show() throws IllegalValueException if (val10) throw new

36、IllegalValueException(); System.out.println(Value1=+ val1); System.out.println(Value2 =+val2); ,class ThrowExample public static void main(String args) UserTrial values=new UserTrial(-1,1); try values.show(); catch (IllegalValueException e) System.out.println(Illegal Values: Caught in main); ,chapter 8