Lecture 3.3. Java Exception Handling

Java Programming

Transparency No. 1-1

Lecture 3.3. Java Exception Handling

Cheng-Chia Chen

Basic Java Syntax


Contents

1. Traditional approach to program errors/exceptions

2. Java’s exception handling

3. The try statement

4. Exception propagation

5. The catch clause

6. The finally clause

Basic Java Syntax


Unexpected conditions which may arise during program execution called exceptions Examples:

System file or i/o errors, disk full or unavailable, socket connection closed.

Program file or i/o errors, unexpected EOF, file not found. Programmer errors, array subscript out of bounds, System errors, unable to load class file, problems with JVM.

Traditional approach to exceptions: mechanism: return value indicating occurrence of exceptions + special

error_code variable storing exact type or details of errors problems: 1. Exceptions are not forced to be handled and error checking may be

ignored entirely. 2. difficult to deal with when there are many types of exceptions that

need to be handled.

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Example 1:

// this call may raise exceptions

dbh = dbm_open(dbname, 0, GDBM_WRCREAT, … );

// check/handle all possible exceptions raised from previous call.

if (dbh == NULL) { // NULL indicates exceptions

logTime(); // error code stored in dbm_errno

const char *ret = dbm_strerror(dbm_errno);

fprintf(stderr, "ERROR Unable to open database: %s.\n", ret);

fflush(stderr);

return -1;

}

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Example 2:

num = write(fd, buffer, ioSize); if (num == -1) { if (errno == EWOULDBLOCK) { // This condition is unlikely, however, if the internal system buffers // fill it may occur. In this case, do nothing and retry later. logTime(); fprintf(stderr,"WARNING unable to write on fd: %d without blocking.\n", fd); fflush(stderr); return 0; } else { logError("ERROR error writing to fd"); clearErrorFd(fd, fdState); return 0; } }

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Problems found from both examples

1. programmer must remember to check for errors each time a statement which can potentially raise a exception is executed. ex1: if-statement after the first dbm_open() call. ex2: if-statement after the first write() call. error handling codes interleaved with normal program statements => re

duces the readability and maintainability of the code.

2. different ways of representing error codes / error messages with different global error variables. The programmer must be able to remember all inconsistant error indications of each function. ex1: null => exception; error code put on dbm_errono ex2: -1 => exception; error code put on errorno. same value in errorno and dbm_errono represent different errors.

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Java’s Exceptions

An exception is an object that describes an unusual or erroneous situation

Exceptions are thrown by a program, and may be caught and handled by another part of the program

A program can therefore be separated into a normal execution flow and an exception execution flow

An error is also represented as an object in Java, but usually represents a unrecoverable situation and should not be caught

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3

Exception Handling

A program can deal with an exception in one of three ways: ignore it handle it where it occurs handle it an another place in the program

The manner in which an exception is processed is an important design consideration

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4

Exception Handling

If an exception is ignored by the program, the program will terminate and produce an appropriate message

The message includes a call stack trace that indicates on which line the exception occurred

The call stack trace also shows the method call trail that lead to the execution of the offending line

See Zero.java

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Java’s call stack on executing Zero

public class Zero { static int divide(int x,int y) { return x/y; // exception occurs here

}

public static void main(String[] args) {

int x = 10; int y = 0; System.out.println ( divide(x,

y)); // and here } // method main} // class Zero

java Zero java.lang.ArithmeticException: / by zero at Zero.divide(Zero.java:3) at Zero.main(Zero.java:8)

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5

The try Statement

To process an exception when it occurs, the line that throws the exception is executed within a try block

A try block is followed by one or more catch clauses, which contain code to process an exception

Each catch clause has an associated exception type

When an exception occurs, processing continues at the first catch clause that matches the exception type

See Product.java

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import java.io.*;public class Product { public static void main (String[] args) { int x = UserReader.readInt(“Enter a number: "); int y = UserReader.readInt("Enter another number: "); System.out.println ("The product is: " + x*y ); } // method main} // class Adding

class UserReader { public static int readInt (String prompt) { BufferedReader stdin = new BufferedReader (new InputStreamReader(System.in)); int number = 0; boolean valid = false;

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while (! valid) { System.out.print (prompt); System.out.flush (); try { // statements possibly throwing exceptions put here!! 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 readInt} // class UserReader

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6

Exception Propagation

If it is not appropriate to handle the exception where it occurs, it can be handled at a higher level

Exceptions propagate up through the method calling hierarchy until they are caught and handled or until they reach the outermost level

A try block that contains a call to a method in which an exception is thrown can be used to catch that exception

See PropagationDemo.java

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

class ExceptionScope { public void level3 (int a) { int c = 1; System.out.println("level3 beginning"); current = c / a; // divided by zero System.out.println("level3 ending"); } // method level3

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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 p) { System.out.println (p.getMessage()); p.printStackTrace(); } System.out.println("level1 ending"); } // method level1} // class ExceptionScope

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main() :

…demo.level1()…

level1() :…try{ …level2() …}catch(ArithmeticException p){…} ….

level2() :

…level3()…

level3() :…

c = c / 0; // divided by zero…

normal flow exception flow

Exception propagation

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7

Types of Exceptions

An exception is either checked or unchecked A checked exception can only be thrown within a try blo

ck or within a method that is declared (at the throws-clause of the method header) to throw that exception

The compiler will complain if a checked exception is not handled appropriately

An unchecked exception does not require explicit handling, though it could be processed that way In previous example, ArithmeticException is an unchecked e

xception, hence it need not be declared even it is not catched at method level3() and level2().

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8

The throw Statement

A programmer can define an exception by extending the appropriate class

Exceptions are thrown using the throw statement:

throw exception-object;

See ThrowDemo.java Usually a throw statement is nested inside an if stateme

nt that evaluates the condition to see if the exception should be thrown

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import java.io.IOException;public class ThrowDemo { public static void main (String[] args) throws MyExp { MyExp p = new MyExp ("Alert!"); throw p; // execution never gets to this point } // method main} // class ThrowDemo

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

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9

The finally Clause

A try statement can have an optional clause designated by the reserved word finally

If no exception is generated, the statements in the finally clause are executed after the statements in the try block complete

Also, if an exception is generated, the statements in the finally clause are executed after the statements in the appropriate catch clause complete

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9

The finally Clause

Exception Handling:

try { code that may generate exception;

} catch (Exception1 e1) {

code to handle exception of type Exception1;

} catch( ….

} catch (ExceptionN eN) {

code to handle exception of type ExceptionN;

} finally {

code that are guaranteed to be executed before (normally or abnormally) leaving this try-catch statement;

}

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(Part of) the Exception Hierarchy

Object Throwable Eorror :

LinkageError, ThreadDeath, VirtualMachineError, AWTError Exception:

RunTimeError ArithmeticException IndexOutOfBoundException NullPointerException …

IllegalAccessException NoSuchMethodException ClassNotFoundException IOException …

Note: Checked exception must be either catched or declared at the throws-clause of the method header.

All Throwable but Error and RuntimeException are checked Exceptions.

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What can we get from an Exception

java.lang.Throwable: Constructor Summary

Throwable() : Constructs a new Throwable with null as its error message string. Throwable([ [String msg] [, Throwable cause ]]) : Constructs a new Throwable with th

e specified error message and/or cause. Method Summary

Throwable fillInStackTrace() : Fills in the execution stack trace. String getLocalizedMessage() : Creates a localized description of this Throwable. String getMessage() : Returns the error message string of this throwable object. Throwable getCause(); void printStackTrace(), printStackTrace(PrintStream s), printStackTrace(Printer

Writer s) : Prints this Throwable and its backtrace to the standard error stream or s.

StackTraceElement[] getStackTrace() void setStackTraceElement(StackTrace[])

String toString() : Returns a short description of this throwable object.

java.lang.Exception: Constructors: Exception(), Exception(String). All methods are inherited from Throwable.

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StackTraceElement

An element in a stack trace, as returned by Throwable.getStackTrace().

Each element represents a single stack frame. All stack frames except for the one at the top of the stack represent a method invocation.

The frame at the top of the stack represents the the execution point at which the stack tr

ace was generated. Typically, this is the point at which the throwable corresponding to the stack trace was c

reated.

Methods boolean equals(Object obj) String getClassName() StringgetFileName() int getLineNumber() String getMethodName() int hashCode() boolean isNativeMethod()

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Lecture 3.3. Java Exception Handling