1. Introduction to OO Program Design
Software College of SCU
Instructor: Shu, Li
week03

2. Unit 1.1 Java Applications
Topics Covered Today
Unit 1.1 Java Applications
1.1.5 Exception Objects
1.1.6 Code Convention

3. Program Errors Syntax (compiler) errors Run-time errors Logic errors
Errors in code construction (grammar, types)
Detected during compilation
Run-time errors
Operations illegal / impossible to execute
Detected during program execution
Treated as exceptions in Java
Logic errors
Operations leading to incorrect program state
May (or may not) lead to run-time errors
Detect by debugging code

4. Exceptions
An exception is an event, which occurs during the execution of a program, that disrupts the normal flow of the program's instructions.
Examples
Division by zero
Access past end of array
Out of memory
Number input in wrong format (float vs. integer)
Unable to write output to file
Missing input file

5. Method readInteger()
Pseudo-code for a method that reads an integer from the standard input:
This pseudo-code ignores the failures that may occur:
The string cannot be read from the standard input. For example, the standard input may be a damaged file.
The string does not contain an integer. For example, the user may type "2r" instead of "25".
int readInteger () {
Read a string from the standard input
Convert the string to an integer value
Return the integer value }

6. The Traditional Approach
Include conditional statements to detect and handle program failures.
The code is difficult to read and maintain.
int readInteger () {
while (true) {
read a string from the standard input;
if (read from the standard input fails) {
handle standard input error;
} else {
convert the string to an integer value;
if (the string does not contain an integer) {
handle invalid number format error; }
else {
return the integer value;

7. each failure is handled in a catch block
Exception Handling
Exception handling is a mechanism that allows failures to be handled outside the normal flow of the code.
将程序运行中的所有错误都看成一种异常，通过对语句块的检测，一个程序中的所有异常被集中起来放到程序中的某一段中进行处理.
normal flow of the code
int readInteger () {
while (true) {
try {
read a string from the standard input;
convert the string to an integer value;
return the integer value;
} catch (read from the standard input failed) {
handle standard input error;
} catch (the string does not contain an integer) {
handle invalid number format error;
} } }
each failure is handled in a catch block

8. An Example of Exception Handling
// DivideByZero.java
public class DivideByZero {
public static void main(String[] args)
System.out.println(5/0);
System.out.println("this will nor be printed.");
System.out.println("Division by zero.");
} // main }
Exception in thread "main" java.lang.ArithmeticException: / by zero
at DivideByZero.main(DivideByZero.java:9)

9. Using try-catch Statement
public class DivideByZero {
public static void main(String[] args)
try {
System.out.println(5/0);
} catch (Exception e) {
System.out.printf("Caught runtime exception = %s", e); }
Caught runtime exception = java.lang.ArithmeticException: / by zero

10. Exception Object.
In Java, an exception is an object that describes an abnormal situation.
public class DivideByZero {
public static void main(String[] args)
try {
System.out.println(5/0);
} catch (Exception e) {
System.out.printf("Caught runtime exception = %s", e); }

11. An exception object contains the following information:
The kind of exception
A call stack which indicates where the exception occurred
A string with additional information about the exception

12. Method of Exception. String getMessage(). String toString().
Obtains the argument that was passed to the constructor of the exception object.
String toString().
The name of the exception (its type), followed by a colon ( : ), followed by the String returned by method getMessage.
void printStackTrace().
This method displays, in the standard error stream, the String returned by method toString, followed by information that shows where the exception was thrown. The information includes a list of all the methods that were called before the exception occurred.

13. Method of Exception.. public class DivideByZero {
public static void main(String[] args)
try {
System.out.println(5/0);
} catch (Exception e) {
System.out.printf("%s\n", e.getMessage());
System.out.printf("%s\n", e.toString());
e.printStackTrace(); }
F:\my teaching\2014S\week03>java DivideByZero
/ by zero
java.lang.ArithmeticException: / by zero
at DivideByZero.main(DivideByZero.java:6)

14. What kinds of exceptions occurred in your labs?
Type of Exceptions
What kinds of exceptions occurred in your labs?
java.lang.ArithmeticException: / by zero
java.lang.NumberFormatException: For input string: "4.0"
java.lang.ArrayIndexOutOfBoundsException: 0
java.lang.ClassNotFoundException: Helloworld …

15. Exception Class Hierarchy

16. Exception Class Hierarchy
Only objects that are instances of this class (or one of its subclasses) are thrown by the Java Virtual Machine or can be thrown by the Java throw statement.
Similarly, only this class or one of its subclasses can be the argument type in a catch clause.

17. Exception Class Hierarchy
Class Error is used for serious problems from which an application is unlikely to recover.
An example is the "out of memory" error.

18. Exception Class Hierarchy
Class Exception is used for abnormal conditions that an application can be expected to handled.

19. Checked & Unchecked Exception
Two types of exceptions  checked & unchecked by compiler

20. Error & RunTimeException Example
Unchecked Exceptions
Error & RunTimeException
Example
NullPointerException
IndexOutOfBoundsException
Catching unchecked exceptions is optional
Handled by Java Virtual Machine if not caught

21. Class Exception (except RunTimeException)
Checked Exceptions
Class Exception (except RunTimeException)
Errors typical program should handle
Example
IOException, FileNotFoundException
Compiler requires “catch or declare”
Catch and handle exception in method, OR
Declare method can throw exception, force calling function to catch or declare exception in turn.

22. CheckedExceptionDemo.java import java.io.*;
public class CheckedExceptionDemo {
public static void main(String[ ] args) {
FileInputStream stream = new FileInputStream (args[0]);
// Process file …
// ...
stream.close(); }

23. Code Compile Errors
F:\My Teaching\ssd3f07\>javac CheckExceptionDemo.java
CheckExceptionDemo.java:4: 未报告的异常 java.io.FileNotFoundException；必须对其进行捕捉或声明以便抛出
FileInputStream stream = new FileInputStream(args[0]); ^
CheckExceptionDemo.java:7: 未报告的异常 java.io.IOException；必须对其进行捕捉或声明以便抛出
stream.close();
2 错误

24. Catching Multiple Exceptions
import java.io.*;
public class CheckExceptionDemo {
public static void main(String[] args) {
try {
FileInputStream stream = new FileInputStream(args[0]);
stream.close();
} catch (ArrayIndexOutOfBoundsException aiobe) {
System.err.println("The program needs a command argument.");
} catch (FileNotFoundException exception) {
System.err.println("Cannot find file '" + args[0] + "'");
} catch (SecurityException exception) {
System.err.println("No permissions for '" + args[0] + "'");
} catch (IOException ioe) {
System.err.println("Close file error.");
} } }

25. Throws Exceptions
Checked exceptions could be declared in the method header using throws.
import java.io.*;
public class CheckExceptionDemo {
public static void main(String[] args)
throws IOException // declares exception {
FileInputStream stream = new FileInputStream(args[0]);
stream.close(); }

26. Raised implicitly by system Raised explicitly by programmer
Exception Occurrence
Raised implicitly by system
System-Defined Exceptions
Raised explicitly by programmer
User-Defined Exceptions

27. System-Defined Exception
IndexOutOfBoundsException :
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 pointer
SecurityException :
When violate security. Caused by security manager
IllegalMonitorStateException :
When the thread which is not owner of monitor involves wait or notify method

28. User-Defined Exceptions
A new checked exception class can be defined by extending the class Exception.
A new unchecked exception class can be defined by extending the class RuntimeException.

29. OutOfRangeException.java
public class OutOfRangeException extends Exception {
public OutOfRangeException() { }
public OutOfRangeException(String message) {
super(message); }

30. The throw Statement.
User-defined methods can also throw exceptions. To throw an exception, use the keyword throw, following by an exception object.
For example:
throw new OutOfRangeException();
throw new OutOfRangeException("Not a valid number");

31. The throw Statement..
public void printAge (int age) throws OutOfRangeException {
if (age<0 || age>200) {
  OutOfRangeException e =
new OutOfRangeException(“age exception”);   throw e;  //抛出异常
} System.out.println(age); }
int age; age = -100;
if (age<0) {
  Exception e = new Exception("Age Exception");  //创建异常对象   throw e;  //抛出异常
} System.out.println(age);

32. Throw Exception A( ) throws OutOfRangeException {
if (error) throw new OutOfRangeException(); }
B( ) {
try {
A( );
catch (OutOfRangeException e) { ...action... }

33. Chained Exceptions
An application often responds to an exception by throwing another exception.
In this example, when an IOException is caught, a new SampleException exception is created with the original cause attached and the chain of exceptions is thrown up to the next higher level exception handler.
try {
} catch (IOException e) {
throw new SampleException("Other IOException", e); }
链式异常, 异常链

34. Exception Handling

35. ExceptionDemo.java public class ExceptionDemo {
public static void main(String[] args)   {
methodA();
System.out.println("MethodA passed");   }
public static void   methodA()   { try { methodB(); System.out.println("MethodB passed"); } catch (Exception e) {    e.printStackTrace();      System.exit(1);     }

36. ExceptionDemo.java (Cont.)
public static void methodB() throws Exception {
methodC();  
System.out.println("MethodC passed"); }
public static void methodC() throws Exception {
methodD();
System.out.println("MethodD passed");
} public static void methodD() throws Exception { throw new Exception("This is an Exception Message");
} }

37. Execution Result of ExceptionDemo.java
public class ExceptionDemo {
public static void main(String[] args){
methodA();
System.out.println("MethodA passed"); }
public static void methodA() {
try {
methodB();
System.out.println("MethodB passed");
} catch (Exception e) {
e.printStackTrace();
System.exit(1);
public static void methodB() throws Exception {
methodC();
System.out.println("MethodC passed"); }
public static void methodC() throws Exception {
methodD();
System.out.println("MethodD passed");
public static void methodD() throws Exception {
throw new Exception("This is an Exception Message");

38. Execution Result of ExceptionDemo.java
Exception Name
Call Stack

39. The finally Block FileInputStream stream1 = null;
FileOutputStream stream2 = null;
try {
stream1 = new FileInputStream(name1);
stream2 = new FileOutputStream(name2);
...
} catch (...) {
} finally {
if (stream2 != null) stream2.close();
if (stream1 != null) stream1.close(); }

40. Procedure for using exceptions
Summary.
Java primitives
try
throw
catch
finally
throws
Procedure for using exceptions
Enclose code generating exceptions in try block
Use throw to actually generate exception
Use catch to specify exception handlers
Use finally to specify actions after exception
违例控制就是在程序中提供这样一种能力： 1)监视程序中的异常情况 2)当异常情况发生时，将控制权交给你自己编写的违例控制代码

41. Summary.. try { // try block encloses throws
throw new eType1(); // throw jumps to catch }
catch (eType1 e) { // catch block 1
...action // run if type match
catch (eType2 e) { // catch block 2
finally { // final block
...action // optional, always be executed

42. Tutorial of Java Exception

43. Unit 1.1 Java Applications
Topics Covered Today
Unit 1.1 Java Applications
1.1.5 Exception Objects
1.1.6 Code Convention

44. Why need Code Convention?
专业编码工作的产品将会是你的源代码。对于任何商业的产品，应该确定你的代码有最高的质量。
要是代码要保存很长一段时间，你必须使得你的代码可读以及便于其他人所理解。难于理解的代码可能要舍弃和重写。
作为开发团队的一部分，你应该致力于你的代码和你的队友的保持一致。
编码的风格一般反映你是什么样的程序员。清晰的和整齐的代码常常反映出一个拥有清晰头脑，组织能力很强的程序员。

45. Java Code Conventions
Java Code Conventions Quick Reference