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 fairly rare and usually fatal. They
  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 reading
  or writing, but not reading and writing.
• Random access I/O supports reading and writing
  data at any positions of a file. A random access
  file may be opened for 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
Reading/Writing Text Files

• Use reader and and writer classes for text I/O.
  Handles 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"
)))
10
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))
11
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()
12
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
()
13
Example Inventory.java
for (int scan 0 scan System.out.println(itemsscan) catch
(FileNotFoundException exception)
System.out.println("The file " file
" was not found.") catch
(IOException exception)
System.out.println(exception)
14
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
15
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 line) for (int num1 num value (int) (Math.random() 100)
outFile.print(value " ")
outFile.println() outFile.close()
16
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
17
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

18
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
19
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)
20
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)
21
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)
22
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)

23
The UserPass Table