Object-Oriented Programming (Java)

Object-Oriented Programming (Java)

Topics Covered Today • Unit 1.1 Java Applications • 1.1.5 Exception Objects • 1.1.6 Code Convention • 1.1.7 Javadoc

Program Errors • Syntax (compiler) 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

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

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 }

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; } } } }

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 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; } } } normal flow of the code each failure is handled in a catch block

Run-time Exception publicclass DivideByZero { publicstaticvoid main(String[] args) { System.out.println(5/0); System.out.println("this will not be printed."); System.out.println("Division by zero."); } } Exception in thread "main" java.lang.ArithmeticException: / by zero at DivideByZero.main(DivideByZero.java:9)

try-catch publicclass DivideByZero1 { publicstaticvoid 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

Exception Object • In Java, an exception is an object that describes an abnormal situation. • 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

Method of Exception • String getMessage(). • 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.

Exception Class Hierarchy

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.

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

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

Unchecked Exceptions • Error & RunTimeException • Serious errors not handled by typical program • Example • NullPointerException • IndexOutOfBoundsException • Catching unchecked exceptions is optional • Handled by Java Virtual Machine if not caught

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.

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

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

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."); • } } }

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(); • } • }

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.

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

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");

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

Exception Handling

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); } }

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"); } }

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"); } }

Execution Result of ExceptionDemo.java Exception Name Call Stack

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(); }

Generating & Handling Exceptions • Java primitives • try • throw • catch • finally • 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

Java Syntax 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 ...action... // run if type match } finally { // final block ...action... // optional, always be executed }

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。

Java Code Conventions • http://java.sun.com/docs/codeconv/html/CodeConvTOC.doc.html • Java Code Conventions Quick Reference

JAVA Comments • // single line comment • /* */ multi-line comment • Javadoc

JavaDoc and Javadoc Program • Javadoc 解析 Java 源文件中的声明和文档注释，并产生相应的 HTML 页（缺省），描述公有类、保护类、内部类、接口、构造函数、方法和域。 • 维护软件比开发软件花费的更多，如果软件很好的归档，维护工作将很容易。很多程序员在软件开发完后才归档，这是错误的。编码完成时归档也应该结束。 • 使用Javadoc可增强归档。 • 整个Java API的文档使用Javadoc形式。

Javadoc Syntax • Javadoc comments open with a "slash-asterisk-asterisk" sequence ( /** ) and close with an "asterisk-slash" sequence ( */ ) on the same line or on separate lines. • Each internal line begins with a single asterisk ( * ) . • For example: /** * body text * body text * body text */ /** body text */

Some Useful Tags • @author – programmer's name; used for classes • @version – version of the program; used for classes • @param – description of a formal parameter; used for methods and constructors • @return – description of a return value; used for methods • @exception (or @throws) – description of an exception; used for methods and constructors • @see – reference to a related entity; used for classes, constructors, methods, and data fields • http://java.sun.com/j2se/javadoc/index.html

Including HTML Tags

Javadoc Command • A program that automatically extracts documentation from your Java sources. • For each source file MyClass.java,it will create a MyClass.html,plus a couple of additional pages. • Example: • Unit1.1.7 TwoInts.java

Running Javadoc

Options to the javadoc program javadoc -private -author -d docs TwoInts.java

Use JavaDoc with Eclipse

Object-Oriented Programming (Java)