Exceptions

1
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

2
Exception Hierarchy

• Errors
• Errors are caused by bugs in the Java VM, or by
  the program running out of memory or other
  resources.
• Errors are usually not handled by programs.
• Exceptions
• Exceptions can be caused by bugs in the program
  or improper data supplied to the program
• Exceptions should be handled, or caught
• An exception is either checked or unchecked
• Checked exceptions must be caught or declared as
  thrown.
• You are not required to handle unchecked
  exceptions.

3
Exception Handling

• Try-catch block and finally block

try   // code may cause/throw exceptions
catch (Exception1 e1)   // handle Exception1
catch (Exception2 e2)   // handle
Exception2 finally   // optional   // code
always executed whether an exception // is
thrown or not
4
Define Your Own Exception
public class MyException extends Exception
  public MyException(String msg)    
super(msg)  
5
Throw Statement and Throws Clause
public class MyClass   public void
aMethod(int i, int j)     throws MyException
    // ...     throw new MyException(reason)
    // ...  
6
I/O Streams

• A stream is a sequence of bytes.
• In a program, we read information from an input
  stream and write information to an output stream
• An I/O stream is either a
• character stream, which deals with text data
• byte stream, which deal with binary data

7
Two Types of I/O

• Stream-based I/O supports reading or writing data
  sequentially.
• A stream may be opened for either reading or
  writing, but not both.
• Random access I/O supports reading and writing
  data at any position of a file.
• A random access file may be opened for both
  reading and writing.

8
Standard I/O Streams

• There are three standard I/O streams
• standard input System.in
• standard output System.out
• standard error System.err

9
Some classes used for I/O
10
Common Text Input Combinations
11
I/O Program Examples

• ReadCharacters.java
• ReadLines.java
• ReadWords.java

12
Reading/Writing Text Files

• The reader and writer classes can handle text
  I/O. Handle conversion between Unicode and native
  character encoding
• May throw IOException

BufferedReader in    new BufferedReader(
       new FileReader("foo.in")) PrintWriter
out    new PrintWriter(        new
BufferedWriter(          new FileWriter("foo.out"
)))
13
Example InventoryItem.java
import java.text.DecimalFormat public class
InventoryItem private String name private
int units private float price private
DecimalFormat fmt public InventoryItem(String
itemName, int numUnits,
float cost) name itemName units
numUnits price cost fmt new
DecimalFormat ("0.") public String
toString() return name "\t" units "
at " price " " fmt.format ((units
price))
14
Example Inventory.java
import java.util.StringTokenizer import
java.io. public class Inventory public
static void main (String args) final int
MAX 100 InventoryItem items new
InventoryItemMAX StringTokenizer
tokenizer String line, name,
file"inventory.dat" int units, count 0
float price try FileReader fr
new FileReader (file) BufferedReader
inFile new BufferedReader(fr) line
inFile.readLine()
15
Example Inventory.java
while (line ! null) tokenizer new
StringTokenizer (line) name
tokenizer.nextToken() try units
Integer.parseInt(tokenizer.nextToken())
price Float.parseFloat(tokenizer.nextToken())
itemscount new InventoryItem(name,
units, price) catch (NumberFormatException
exception) System.out.println("Error in
input. Line ignored") System.out.println(lin
e) line inFile.readLine() inFile.close
()
16
Example Inventory.java
for (int scan 0 scan lt count scan)
System.out.println(itemsscan) catch
(FileNotFoundException exception)
System.out.println("The file " file
" was not found.") catch
(IOException exception)
System.out.println(exception)
17
Example Inventory

• Input

• Output

Widget 14 at 3.35 46.9 Spoke 132 at 0.32
42.24 Wrap 58 at 1.92 111.36 Thing 28 at
4.17 116.76 Brace 25 at 1.75 43.75 Clip
409 at 0.12 49.08 Cog 142 at 2.08 295.36
Widget 14 3.35 Spoke 132 0.32 Wrap 58 1.92 Thing
28 4.17 Brace 25 1.75 Clip 409 0.12 Cog 142 2.08
18
Example TestData.java
final int MAX 10 int value String file
"test.dat" FileWriter fw new
FileWriter(file) BufferedWriter bw new
BufferedWriter(fw) PrintWriter outFile new
PrintWriter(bw) for (int line1 line lt MAX
line) for (int num1 num lt MAX num)
value (int) (Math.random() 100)
outFile.print(value " ")
outFile.println() outFile.close()
19
Example TestData Output
12 92 80 25 40 10 25 82 89 17
20 12 34 69 27 4 43 50 39 64
43 4 13 83 68 63 12 50 36 20
32 41 35 20 7 50 89 67 68 49
90 25 54 59 30 88 61 92 28 1
45 57 50 6 95 90 66 17 6 27
0 86 19 70 75 21 98 30 80 19
54 93 54 31 43 54 74 35 10 92
47 12 79 2 82 33 22 81 44 26
95 90 77 76 63 16 12 30 89 60
20
JDBC

• Java Database Connectivity
• Steps
• 1. Load a JDBC driver
• 2. Establish a connection with a data base
• 3. Send queries and update statements
• 4. Process the results

21
Setting up an Access Database Driver for Windows

• Open the ODBC Data Sources (32 bit) folder
• StartSettingsControl PanelODBC Data Sources
  (32 bit)
• Select the System DSN tab (top of the dialog box)
• Click on the Add button (at the right)
• Select Microsoft Access Driver(.mdb) from the
  list box
• Click Finish
• A dialog box with the title ODBC Microsoft Access
  Setup will appear,
• Type a description in the Description Text field
  (optional)
• In the Database section,
• click on the Select button,
• find and select your saved version of the
  access.mdb database,
• Click OK
• Click OK in the ODBC Microsoft Access Setup
  Dialog
• The System DSN section of the initial ODBC Data
  Source Administrator dialog should now have the
  new mdb database name in the list of system data
  sources.

22
The UserPass Table
23
Example BuildUserDB_Access.java
import java.sql. import java.io. import
java.util. public class BuildUserDB_Access
public static final String database
"Access 2000" public static final String
jdbcDriver "sun.jdbc.odbc.JdbcOdbcDriver"
public static final String dataSource
"jdbcodbc" public static void main(String
args) String dbName "Users"
String tableName "UserPass" Connection
conn null Statement stmt null
24
Example BuildUserDB_Access.java
try Class.forName(jdbcDriver)
catch(ClassNotFoundException e)
System.exit(1) try String url
dataSource dbName conn DriverManager.getCon
nection(url) stmt conn.createStatement()
catch (SQLException se) System.exit(1)
25
Example BuildUserDB_Access.java
try String dropString "DROP TABLE "
tableName stmt.executeUpdate(dropString)
catch (SQLException se) try String
createString "CREATE TABLE " tableName
" (username VARCHAR(128) NOT NULL PRIMARY
KEY," " password VARCHAR(128))"
stmt.executeUpdate(createString)
26
Example BuildUserDB_Access.java
String insertString "INSERT INTO "
tableName " VALUES ('Scott McNealy',
'lavender')" stmt.executeUpdate(insertString) in
sertString "INSERT INTO " tableName "
VALUES ('Steve Jobs', 'aqua')" stmt.executeUpdate
(insertString) insertString "INSERT INTO "
tableName " VALUES ('Bill Gates',
'blue')" stmt.executeUpdate(insertString)
27
Example BuildUserDB_Access.java
ResultSet rset
stmt.executeQuery("SELECT FROM " tableName)
while( rset.next() ) System.out.println(rs
et.getString("username")
"" rset.getString("password"))
stmt.close() rset.close()
conn.close() catch (SQLException se)

28
Thread

• A sequential flow of control within a program
• All programs have at least one thread
• In Java, the thread is
• Application main()
• Applet Browser is main thread
• Java is a multi-threaded language

29
Creating Threads in Java

• Create an object of Thread by
• Subclass Thread
• Provide definition of run() method
• Implement Runnable interface
• Declare run() method
• Create Thread object when needed

30
Subclassing the Thread Class

• public class SimpleThread extends Thread
• ..
• public void run()
• // provide an override for run() method

31
Implement Runnable Inteface

• public class Test extends Applet implements
  Runnable
• private Thread testThread null
• // when needed, create instance of Thread
• testThread new Thread(this, Test)

32
Two types of thread

• Daemon thread
• background thread subordinate to creator thread
• ends when creator thread ends
• threads that run indefinitely usually are
  created as daemon threads
• User thread
• independent of creator thread
• has life of its own!
• run() method returns after done or it must be
  explicitly stopped or destroyed

33
Threads and priority scheduling

• Can run in parallel if there are multiple
  processors.
• On a single CPU, threads are scheduled
• Threads are scheduled based on priority relative
  to other threads
• system chooses the runnable thread with highest
  priority which runs until
• it yields (to thread with same priority) or exits
  run() or is preempted

34
Thread Priority

• Threads inherit priority from their creator
• can be modified afterwards
• MIN_PRIORITY (1) to MAX_PRIORITY (10)
• NORM_PRIORITY (5) is default

35
Time Slicing and Priority Scheduling

• Time-slicing
• implemented by some OSs to fight selfish thread
  behavior
• each process gets a quantum of time, executes in
  a round-robin fashion i.e., each in turn
• Priority Scheduling
• threads at same priority can share as above
• threads at lower priority must wait
• threads at higher priority preempt lower threads
• but lower priority thread may be run to avoid
  starvation
• up to the thread scheduler in JVM

36
Stopping Threads

• Stop when the task is finished (a natural
  death)
• thread arranges for own termination run() method
  terminates naturally
• interrupt()
• one thread signals another thread that it should
  stop
• just sets a flag does not stop the thread
• isInterrupted()
• checks to see if a thread has been interrupted
  (may still be running)
• isAlive()
• checks to see if a thread is still operating
• if true, thread is started and not stopped may
  be Runnable or Not Runnable

37
Thread Synchronization

• At times it is necessary to limit resources to
  one user at a time.
• use an internal lock to ensure only one thread
  among competitors gains access to critical
  sections of code
• A thread acquires the lock by executing
• a synchronized instance method of that object.
• body of a synchronized statement that
  synchronizes on the object.
• a synchronized static method of a class.

38
Synchronization at the Method level

• synchronized public void method1() ...
• // at the code block level
• synchronized(theObject) statement

39
Waiting for a Thread

• join()
• waits until thread dies no synchronization used
• thread1.join() // waits until thread1 dies
• thread1.join(1000) // wait up to 1 second
• sleep(long millisec)
• suspend execution for specified time
• can throw InterruptedException,
• so must be in try block or must indicate calling
  method throws this exception