Exceptions Handling Errors using Exceptions

Transcription

1 Java Programming in Java Exceptions Handling Errors using Exceptions

2 Exceptions Exception = Exceptional Event Exceptions are: objects, derived from java.lang.throwable. Throwable Objects: Errors (Java Structure Violation) Exceptions (Java Logic Violations) Runtime Exceptions

3 Exceptions Creating an Exception Throw an Exception Exception handler appropriate to handle the thrown exception: Catch the Exception Exception Throw Catch

4 Exception Types Object Throwable Error Exception... RuntimeException......

5 Exception Types Errors: Dynamic Linking Error, hard failure in the virtual machine VM throws an Error. Exceptions: Indication of a not very serious systematic problem Program throws an Exception. Runtime Exceptions: Exceptions that occur within the Java virtual machine during runtime (e.g. NullPointerException). VM throws a Runtime Exception.

6 Exception Types Advantages of Java's Error Handling 1. Error Handling Code is separated from regular code. 2. Error Propagation up the Call Stack. 3. Grouping Error Types and Error Differentiation.

7 Call Stack method1 x = method2(...) method2 y = method3(...) method3 z = method4(...)

8 Error Differentiation Exception Instances of Throwable or subclass of Throwable Object Throwable ArrayException InvalidIndexException NoSuchElementException ElementTypeException

9 Error Differentiation Leaf Class (class without subclass): represents a specific type of exception, specialized handler. Node Class (class with one or more subclass): represents a group of related exceptions, general handler. Leaf Class: Node Class: catch (InvalidIndexException e)... catch (ArrayException e)...

10 Catch or Specify Requirement Catch Specify Checked Exception Exception Types: Runtime Exceptions I/O-Exceptions Own Exceptions A method either has to catch or specify all checked exceptions that can be thrown within it's scope. Runtime Exceptions: Don't have to be caught or specified. Checked Exceptions: Not runtime exceptions, checked by the compiler; checked exceptions must be caught or specified.

11 Catch or Specify Requirement Checked Exceptions: Due to external circumstances that the programmer cannot prevent Compiler checks that these exceptions are handled All IOExceptions are checked exceptions Unchecked Exceptions: Programmers fault, can be prevented Included exceptions: e.g. NullPointerException, ArithmeticException e.g. to avoid the NullPointerException the reference can be checked for null

12 Try and Catch Critical statements are capsuled in try-catch statements try <some dangerous action> catch (Exception e) <some reasonable behaviour>... finally <something absolutely necessary>

13 Try and Catch: Syntax Diagram Syntax Diagram: try block catch finally (object) block block catch (object) block finally block

14 Exception Handler Parameter e: exception handler parameter (variable) type of exception, exception message, stack trace. Available methods: e.tostring() e.getmessage() e.printstacktrace()

15 Try and Catch Example: HelloWorld with Exception Handling // Hello program with exception handling class ExceptionalHello2 public static void main (String args[]) try /* Now let's say hello */ System.out.println("Hello " + args[0]); // line 9 catch (Exception e) System.out.println("Hello! Who are you?!"); // e.printstacktrace(); continue Java

16 Try and Catch continued java ExceptionalHello2 Output: Hello! Who are you?! with (inside catch block): e.printstacktrace(); Output: Hello! Who are you?! java.lang.arrayindexoutofboundsexception: at ExceptionalHello2.main(ExceptionalHello2.java:9)

17 Writing your own Exception / Example class SampExcept extends Exception SampExcept (String Message) // Constructor super(message); // pass message to parent public class ExTest static void range(int numb) throws SampExcept if(numb < 0 numb > 100) throw new SampExcept("0-100 expected"); // line 15 else System.out.println("Number is " + numb); continue

18 Writing your own Exception / Example public static void main (String args[]) try range(75); range(250); // line 28 catch(sampexcept e) System.out.println("Err: " + e.getmessage()); Version 1 Output Number is 75 Err: expected Java

19 Writing your own Exception / Example Version 2 public static void main (String args[]) try range(75); range(250); // line 28 catch(sampexcept e) System.out.println("\n\nError Messages:"); System.out.println("\nErr1:\n" + e.getmessage()); System.out.println("\nErr2:\n" + e.tostring()); System.out.println("\nErr3: " ); e.printstacktrace();

20 Writing your own Exception / Example Output Version 1 Number is 75 Class Method Error Messages: Class Class Err1: expected Class Class Object Object Err2: SampExcept: expected Err3: SampExcept: expected at ExTest2.range(ExTest2.java:15) at ExTest2.main(ExTest2.java:28)

21 I/O-Programmierung catch Java import java.io.*; public class IOTest2 public static void main(string[] args) try byte barray[] = new byte[128]; System.out.print("Texteingabe: "); System.in.read(bArray); String s = new String(bArray,0); System.out.print("Eingabe: "); System.out.println(s); catch(ioexception ioe) System.out.println(ioe.toString()); ioe.printstacktrace();

22 I/O-Programmierung specify Java import java.io.*; public class IOTest22 public static void main(string[] args) throws IOException byte barray[] = new byte[128]; System.out.print("Texteingabe: "); System.in.read(bArray); String s = new String(bArray,0); System.out.print("Eingabe: "); System.out.println(s);