Java Programming in Java Exceptions JAVA zxmzjut edu

Java程序设计 Programming in Java 第六章 异常处理（Exceptions) 赵小敏 浙江 业大学软件学院 JAVA zxm@zjut. edu. cn 1

先看一个例子 1 2 3 4 5 6 7 8 9 10 11 public class Hello. World{ public static void main(String args[ ]){ int i=0; String greetings[ ]={ “Hello World!”, ”Hello!”, “HELLO WORLD!!”}; while ( i<4){ System. out. println(greetings[i]); i++; } } Hello World! } Hello! HELLO WORLD! Java. lang. Array. Index. Out. Of. Bounds. Exception at Hello. World. main(Hello. World. java: 7) 3

Java中定义的异常类 Java中定义了各种异常类。Java. lang. Throwable 是这些类的父类。 Virtual. Machine. Error Throwable AWTError Runtime. Exception Arithmetic. Exception Null. Pointer. Exception Index. Out. Of. Bounds. Exception EOFException IOException File. Not. Found. Exception 5

p 159例6 -2：List. Of. Numbers 1. import java. io. *; 2. import java. util. *; 3. class List. Of. Numbers { 4. private Array. List list; 5. private static final int size = 10; 6. public List. Of. Numbers () { 7. list = new Array. List(size); 8. for (int i = 0; i < size; i++) 9. list. add(new Integer(i)); 10. } 11. public void write. List() { 12. Print. Writer out = new Print. Writer(new File. Writer("Out. File. txt")); 13. for (int i = 0; i < size; i++) 14. out. println("Value at: " + i + " = " + list. get(i)); 15. out. close(); 16. } 17. } 8

例外处理——try , catch和finally 语句 1 Try{ 2 // code that might throw a partcular exception 3 }catch(My. Exception. Type e){ 4 // code to excute if a My. Exception. Type exception is thrown 5 }catch (Exception e){ 6 // code to execute if a general Exception exception is thrown 7 }finally{ } 13

public void write. List() { Print. Writer out = null; 修改例6 -2得write. List()方法，见例6 -3 try { System. out. println("Entering try statement"); out = new Print. Writer(new File. Writer("Out. File. txt")); for (int i = 0; i <size; i++){ out. println("Value at: " + i + " = " + list. get(i)); } } catch (Array. Index. Out. Of. Bounds. Exception e) { System. err. println("Caught Array. Index. Out. Of. Bounds. Exception: " + e. get. Message()); } catch (IOException e) { System. err. println("Caught IOException: " + e. get. Message()); } finally { if (out != null) { System. out. println("Closing Print. Writer"); out. close(); } else { System. out. println("Print. Writer not open"); } } 14

write. List方法中的try语句块的执行可能有三种情况： • 出现了IOException Entering try statement Caught IOException: Out. File. txt Print. Writer not open • 出现了数组越界错误 Entering try statement Caught Array. Index. Out. Of. Bounds. Exception: 10 >= 10 Closing Print. Writer • 正常退出 Entering try statement Closing Print. Writer 15

捕获与处理异常示例 Public static void main(String args[]){ int i = 0 ; String greetings[]={“Hello World!”, ”Hello!”, ”HELLO!”}; while (i<4){ try { Hello World! System. out. println(greetings[i]); This is always printed }catch(Array. Index. Out. Of. Bounds. Exception e){ Hello! System. out. println(“Re-setting Index Value”); This is always printed i=-1; }finally{ HELLO! System. out. println(“This is always printed”); This is always printed Re-setting Index Value } i++; This is always printed } } 18

异常处理——抛出异常 例： public Object pop() throws Empty. Stack. Exception { Object obj; if (size == 0) throw new Empty. Stack. Exception(); obj = object. At(size - 1); set. Object. At(size - 1, null); size--; return obj; } z 抛出异常的throw语句： throw some. Throwable. Object 20

异常可用方法的例子 //: Exception. Methods. java // Demonstrating the Exception Methods public class Exception. Methods { public static void main(String[] args) { try { throw new Exception("Here's my Exception"); } catch(Exception e) { System. out. println("Caught Exception"); System. out. println("e. get. Message(): " + e. get. Message()); System. out. println("e. to. String(): " + e. to. String()); System. out. println("e. print. Stack. Trace(): "); e. print. Stack. Trace(); } } } 22

自定义异常类, 是Exception类的子类, 可包含普通类的内容。 public class Server. Time. Out. Exception extends Exception{ private String reason; private int port ; public Server. Time. Out. Exception(String reason, int port){ this. reason = reason; this. port = port; } public String get. Reason(){ return reason; } public int get. Port(){ return port; } 23 }

抛出产生的异常 public void connect. Me(String server. Name) throws Server. Time. Out. Exception{ int success ; int port. To. Connect = 80; success = open(server. Name, port. To. Connect); if(success= -1){ throw new Server. Timed. Out. Exception( “Could not connect”, 80); } } 24

获得异常并处理 public void find. Server(){ … try{ connect. Me(default. Server); }catch(Server. Time. Out. Exception e){ System. out. println(“Server timed out, try another”); try{ connect. Me(alternate. Server); }catch(Server. Time. Out. Exception e 1){ System. out. println(“No server avaliable”); } } 25

Java异常处理程序的基本形式 try //执行时程序块 catch Exception. Type 1 e //对Exception. Type 1的处理 catch Exception. Type 2 e //对Exception. Type 2的处理 throw e //再抛出这个“异常” finally try程序块和catch语句 28

1. public class Using. Exceptions { 2. public static void main( String args[] ){ 3. try{ 4. method 1(); // 1 5. }catch(Exception e){ 6. System. err. println(e. get. Message()); // 2 7. } 8. finally{ 9. System. out. println("Program is end!"); // 3 10. } 11. } 12. public static void method 1() throws Exception { 13. method 2(); //4 14. } 15. public static void method 2() throws Exception { 16. method 3(); //5 17. } 18. public static void method 3() throws Exception { 19. throw new Exception( "Exception thrown in method 3" ); //6 20. } 21. } 29

1. class Test. Exception{ 2. public static void main(String []args){ 3. try{ 4. mb_method(); 5. }catch(Exception e){ 6. System. out. print('m'); 7. } 8. System. out. println('n'); 9. } 10. 11. static void mb_create. Exception(){ 12. throw new Array. Index. Out. Of. Bounds. Exception(); 13. } 14. 15. static void mb_method(){ 16. try{ 17. mb_create. Exception(); 18. System. out. print('a'); 19. }catch(Arithmetic. Exception e){ catch(Array. Index. Out. Of. Bounds. Exception e){ 20. 21. System. out. print('b'); 22. } 23. finally{ 24. System. out. print('c'); 25. } 26. System. out. print('d'); 27. } 28. } 30