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Lecture 3
Exceptions, Singleton,
Bridge & Logging
Error handling in
procedural languages
int foo() {
if ( !doSomething1() )
return –1;
…
if ( !doSomething2() )
return –2;
…
}
Exceptions
public interface Algorithm {
public void run() throws
ExecutionException,
IllegalDataException;
}
Example
int[] iArray = randomSortedIntArray();
IAlgorithm alg = new BinarySearch( iArray, 29 );
try {
alg.run();
} catch ( IllegalDataException e1 ) {
System.err.println( "Illegal Data Exception
while running Binary Search: " +
e.getMessage() );
e.printStackTrace();
} catch ( ExecutionException e ) {
System.err.println( "Execution Exception while
running Binary Search " + e.getMessage() );
e.printStackTrace();
}
How the catch block works
if ( thrown exception instance of
IllegalDataException ) {
…
} else if ( thrown exception instanceof
ExecutionException ) {
…
}
printStackTrace method
java.io.FileNotFoundException: fred.txt
at java.io.FileInputStream.<init>(FileInputStream.java)
at java.io.FileInputStream.<init>(FileInputStream.java)
at ExTest.readMyFile(ExTest.java:19)
at ExTest.main(ExTest.java:7)
Heap/Stack
Swallowing exceptions is is
a major NO-NO!
try {
anObject.method( arg1 );
} catch ( Exception e ) {}
Runtime error #3256 at
0x34AD546
Exceptions allow for extra info…
public class ParserException extends Exception {
public ParserException( File f, int iLine ) {
this.f = f;
this.iLine = iLine;
}
public final File getFile() { return f; }
public final int getLine() { return iLine; }
private File f;
private int iLine;
}
Creating your own
exception classes
public class ApplicationException
extends Exception {}
public class ApplicationOpenFailedException
extends ApplicationException {}
Throwing multiple
exceptions
public interface IApplication {
public void open() throws
ApplicationOpenFailedException, IOException;
public void close() throws
ApplicationCloseFailedException, IOException;
}
finally
public class AlwaysDoThatWithStreams {
public static void main( String[] args ) {
FileWriter fw = null;
try {
fw = new FileWriter( "args.txt" );
for ( int i = 0; i &lt; args.length; i++ ) {
os.println( "ARG[" + i + "] = [" +
args[ i ] + "]" );
}
} catch ( IOException e ) {
System.out.println( "Error while writing args: " +
e.getMessage() );
} finally {
if ( fw != null ) fw.close();
}
}
}
public static int parseInt(String s,
int radix)
throws NumberFormatException
Logging
01/01/2002 9:45:03 [EntityReader] ERROR: class StrangeEntity not found
01/01/2002 9:45:05 [EntityReader] DEBUG: created instance of BasicEntity
Log.error( this, "class StrangeEntity not found" );
Log.debug( this, "created instance of BasicEntity" );
Singleton
public class Singleton {
public static Singleton getInstance() {
return instance;
}
private Singleton() {}
// create instance
private static Singleton instance = new Singleton();
public static void main( String[] args ) {
Singleton handle1 = Singleton.getInstance();
Singleton handle2 = Singleton.getInstance();
System.out.println( "Handle1 [" + handle1 + "]" );
System.out.println( "Handle2 [" + handle2 + "]" );
}
}
Handle1 [Singleton@1c9f7e]
Handle2 [Singleton@1c9f7e]
Log class
public final class Log {
public
public
public
public
public
public
static
static
static
static
static
static
void
void
void
void
void
void
debug( String msg );
debug( Object this, String msg );
error( String msg );
error( Object this, String msg );
warning( String msg );
warning( Object this, String msg );
public static void setOptionsOn( int options );
public static void setOptionsOff( int options );
public static void setPrinter( ILogPrinter lp );
public static ILogPrinter getPrinter();
public static void registerClass( Class c );
public static void unregisterClass( Class c );
}
Log as utility class
Log.getInstance().error( "Something is wrong" );
Log.error( "Something is wrong" );
Basic Log methods
public final class Log {
public static void error( String msg );
public static void error( Object o, String msg );
public static void debug( String msg );
public static void debug( Object o, String msg );
}
Passing the object which
reports the message
public static void error( Object o, String msg ) {
Log( "ERROR: " + o.getClass().getName() +
":" + msg );
}
Factoring out the
implementation
public class ILogImpl {
public void print( String sMessage );
}
Factoring out the
implementation
public class Log {
…
public static void setLogImpl( ILogImpl lm ) {
logImpl = lm;
}
…
private void print( String s ) {
logImpl.print( s );
}
private static ILogImpl logImpl;
}
StdOutLogImpl
public final class StdOutLogImpl
implements ILogImpl {
public StdOutLogImpl () {}
public void print( String s ) {
System.out.println( s );
}
}
Complete Log class
public class Log {
/**
* @param Object o - the object, which signals the error
* @param String sMsg - the error message
* @param Throwable - the exception which occurred
*/
public static void error( Object o, String sMsg, Throwable e );
public static void error( String sMsg, Throwable e );
public static void error( Object o, String sMsg );
public static void error( String sMsg );
public static void debug( Object o, String sMsg );
public static void debug( String sMsg );
public static void warning( Object o, String sMsg );
public static void warning( String sMsg );
/**
* @param Boolean bDebug - indicates if the debugging messages
* should be printed or not
*/
public static void setDebug( boolean bDebug );
public static boolean getDebug();
/**
* @param ILogImpl logImpl - the settable delegate which
* actually does the printing of the messages.
* It is specific to the medium where the messages go.
*/
public static void setLogImpl( ILogImpl logImpl );
private Log();
Assignment
Please read
Arnold & Gosling Java Programming
language Chapters on Exception and
Garbage collection
Chapters 16 and 25 from Agile Software
Development Book
Assignment 1
Define & implement Log, ILogImpl,
StdOutLogImpl, FileLogImpl,
MultiLogImpl