'99 정보과학회 튜토리얼 1

Advanced Flow of Control : Introduction

• This chapter focuses on:– exception processing– catching and handling exceptions– creating new exceptions– exception analysis

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Exceptions( 예외상황 )

• An exception is an object that describes an

unusual situation

– Exceptions are thrown by a program or runtime

environment, and

– Thrown exceptions may be caught and handled by

another part

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예외상황 발생

• Runtime exception

– irrecoverable exception

– thrown by the runtime library code, not your code

• User-defined exception

– by throw statement.

– you have to make provision for catching thrown exceptions

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예외상황 정의

• Exception object is a first-class object

– 일반 object 처럼 클래스를 이용하여 정의되고

– 일반 object 처럼 사용될 수 있다 .

• 일반 object 와 차이점

– throw 될 수 있다 .

• 예외상황을 정의하는 클래스

– Exception class 를 상속받아서 정의해야 한다 .

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Object

Throwable

Exception Error

RuntimeExceptionClassNotFoundException

IllegalAcessException

InstantiationException

InterruptedException

NoSuchMethodException

…..

ArithmeticException

NagativeArraySizeException

ArrayIndexOutOfBoundsException

SecurityException

…

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Execution Flow

• A program can be separated into

– a normal execution flow and

– an exception execution flow

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Exception Handling

• A program can deal with an exception in one of

three ways:

(1) ignore it

(2) handle it where it occurs

(3) handle it an another place in the program

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Exception Handling

• If an exception is ignored by the program,

– The program will terminate and produce a message

– The message includes a call stack trace

– main 메쏘드부터 호출과정

• See Zero.java

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public class Zero { public static void main (String[] args) {

int numerator = 10; int denominator = 0;

System.out.println(numerator/denominator);

// cann’t reach hereSystem.out.println(“Cann’t reach here”);

} // method main

} // class Zero

Example: Zero.java

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The throw Statement

• A programmer can define an exception by

extending the appropriate class

• Exceptions are thrown using throw statement:

throw exception-object;

• Usually a throw statement is nested inside an if

statement

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Example: Throw_Demo.java

import java.io.IOException;public class Throw_Demo { public static void main (String[] args) throws Ooops { Ooops problem = new Ooops ("Alert!"); throw problem;

// execution never gets to this point } // method main} // class Throw_Demo

class Ooops extends IOException { Ooops (String message) { super (message); } // constructor Ooops} // class Ooops

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The try-catch Statement

• To process an exception when it occurs

try { … } catch (E1 x) { … }

…

catch (En x) { … }

• When an exception occurs in the try block,

processing continues at the first catch clause

that matches the exception type

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Example: Adding.java

import java.io.*;public class Adding { public static void main (String[] args) { int num1 = User_Reader.get_integer ("Enter a number:"); int num2 = User_Reader.get_integer ("Enter another number: ");

System.out.println ("The sum is " + (num1+num2)); } // method main} // class Adding

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class User_Reader { public static int get_integer (String prompt) { BufferedReader stdin = new BufferedReader

(new InputStreamReader(System.in)); int number = 0; boolean valid = false;

while (! valid) { System.out.print (prompt); System.out.flush (); try { number = Integer.parseInt (stdin.readLine()); valid = true; } catch (NumberFormatException exception) { System.out.println ("Invalid input. Try again."); } catch (IOException exception) { System.out.println ("Input problem. Terminating."); System.exit(0); } } return number;

} // method get_integer} // class User_Reader

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The finally Clause

• A try statement can have an optional clause finally

try { … } catch (E1 x) { … }

…

catch (En x) { … }

finally { … }

• The statements in the finally clause are executed

whether exception is generated or not

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class ThrowDemo{ public static void main(String arg[]){ try{

System.out.println("a() 메서드 이전 "); a(); System.out.println("a() 메서드 이후 ");}catch(Exeception e){ System.out.println("main:" + e);}finally{ System.out.println("main:finally 블록 ");}

}

Example: ThrowDemo.java

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public static void a(){ try{

System.out.println("throw 문 이전 "); throw new ArithmeticException();

} catch(Exception e){

System.out.println("a: " + e); } finally{

System.out.println("a:finally 블록 "); } }//method main

}//class ThrowDemo

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Exception Propagation

• Exceptions propagate up through the method

calling hierarchy

– until they are caught and handled or

– until they reach the outermost level

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Exception Propagation

Chain of callat runtime Main()

Apple();

Orange()

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Source program

Main(){ apple();}

apple(){ orange();}orange(){ int i, j =0; ... i=i/j;} Exception occur!

Runtime system look for try…catch statement in orange

Then here surrounding the call to orange()

Then here enclosing the call to apple()

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Example: Propagation_Demo.java

public class Propagation_Demo {

static public void main (String[] args) { Exception_Scope demo = new Exception_Scope(); System.out.println("program beginning"); demo.level1(); System.out.println("program ending"); } // method main

} // class Propagation_Demo

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class Exception_Scope { public void level3 (int adjustment) { int current = 1; System.out.println("level3 beginning"); current = current / adjustment; System.out.println("level3 ending"); } // method level3 public void level2() { System.out.println("level2 beginning"); level3 (0); System.out.println("level2 ending"); } // method level2 public void level1() { System.out.println("level1 beginning"); try { level2(); } catch (ArithmeticException problem) { System.out.println (problem.getMessage()); problem.printStackTrace(); } System.out.println("level1 ending"); } // method level1} // class Exception_Scope

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Exceptions

• An exception is either checked or unchecked

– The compiler will complain if a checked exception

is not caught appropriately

– An unchecked exception does not require explicit

handling

• A checked exception can only be thrown

– within a try block or

– within a method specified to throw the exception

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Specification of uncaught exceptions

• Specify uncaught exceptions in method

definition

m(...) throws A, B, …, C

{ … }

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import java.io.*;

public class Spec{public static void main (String[] args)throws IOException{

System.out.println("jylee....!!");

int ch = System.in.read();System.out.println((char)ch);

}}

Example: Spec.java

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Uncaught Exception Analysis in JDK

• Intraprocedural analysis

– Based on programmer’s specifications of method

definitions.

– Check whether thrown exceptions are caught or s

pecified in the method definition