CST252 Network Software Design

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CST252 Network Software Design

• Lecture 5 Exceptions and I/O

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Resources

• Java Tutorial (java.sun.com)
• Essential Java Classes/IO Reading and Writing
• Essential Java Classes/Handling Errors with
  Exceptions
• Course Website
• users.wmin.ac.uk/lancasd/CCTM91
• Book
• Cay Horstmann and Gary Cornell. Core Java
• Volume 1 chapters 11, 12 (Vol 2 in older editions)

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Admin

• Today - last lecture on general Java
• Phase test next week
• 1300 to 1400 11th March
• Rooms on 4th Floor of A block
• Followed by lecture
• starting at 1430
• on Networking

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Lecture 4 Review

• Inheritance
• Manager extends Employee
• Only need code for differences.
• super keyword
• Interface
• A specification
• Employee implements Comparable
• EmployeeSortTest.java
• ArrayLists (Wrapper classes)
• Sorting and Searching - postponed to today

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L4 Inheritance

• Superclass and subclass
• Polymorphism
• Dynamic Binding
• Object class, final, and abstract keywords
• Using Interfaces

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Quiz

• A) Interface int
• boolean wakeUp()
• B) final class Executive extends Manager
• Executive(....) super(...) sp 0.0
• void NegotiateSeverancePay(double d)
• sp d bonus
• double sp
• C) abstract class person
• public abstract String getDescription()
• D) class Student extends person
• public String getDescription()
• return a student at Wmin

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L5 Exceptions and I/O Overview

• Complete lecture on sorting/searching
• Exceptions and errors
• try, catch blocks
• Files, streams
• text, byte files

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Sorting

• Utility methods in
• java.util.Arrays,
• java.util.Collections
• Arrays of primitive type
• Arrays.sort(int arraytobesorted)
• Arrays.sort(double arraytobesorted)
• Lists (ArrayLists, and other more exotic such as
  LinkedList)
• Collections.sort(ArrayList altobesorted)

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Sorting Arrays of basic types

• For arrays of primitive types
• int, long, short, char, byte, boolean, float,
  double
• sort() using Quicksearch algorithm
• import java.util.
• int a new int10000
• // fill array
• Arrays.sort(a)
• See SortTest1.java (and SortTest2.java for
  ArrayList version)
• An in-place method

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Sorting Arrays/ArrayLists of Objects

• Need some way of comparing the elements which are
  now objects
• Elements must implement the comparable interface
• or can pass a Comparator to the sort method
• Strings have a default compareTo() method that is
  case-sensitive
• to override for case-insensitive use a Comparator

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Sorting ArrayLists of Strings

• import java.util.
• ArrayList s new ArrayList()
• s.add(the)
• s.add(quick)
• s.add(brown)
• ...
• Collections.sort(s)
• See SortTest3.java

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Search

• Generally must go though all the elements
• Short cut if the list is sorted - binarysearch
• Must act on something already sorted
• Arrays of primitive type
• int i Arrays.binarySearch(int a,int key)
• Objects need comparable interface
• int i Collections.binarySearch(c, element)
• Returns the index of the matching object, or a
  negative number if there is no matching element
• Access the matching object using ai or c.get(i)

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L5 Errors and Exceptions

• Exception handling in Java
• Kinds of Exception - hierarchy
• checked and unchecked
• Advertising exceptions with throws
• try catch blocks to deal with exceptions

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Errors

• Bugs in code
• Attempting to open files that do not exist
• Trying network connections without a link
• Expect the program to act sensibly and not just
  crash immediately
• graceful termination
• return to a safe state and wait for user input

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Exception Handling

• Two logical parts
• Code to detect an error
• Code to return the system to a safe state, or to
  terminate gracefully
• These bits of code should be in separate places.
• When an error occurs, normal execution terminates
  and control is transferred from where the error
  occurred to where error-handling code can deal
  with the error

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Handling Exceptions

• Try Catch blocks - schematically
• try
• code
• more code
• catch (ExceptionType e)
• handler for this type

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Try Catch Blocks

• If any code inside the try block throws an
  Exception of the type specified in the catch
  clause
• The program skips the remainder of the code in
  the try block
• The code in the handler block is executed
• If no exception is thrown
• The program skips the catch clause

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Various kinds of errors

• User input errors
• wrong format, spelling
• Device errors
• hardware errors, printer may be turned off
• Physical limitations
• hard disk fills up
• Code errors
• a bug in the program

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Catching Multiple Exceptions

• try
• code that might throw exceptions
• catch (MalformedURLException e1)
• handler for this type
• catch (UnknownHostException e2)
• handler for this type
• catch (IOException e3)
• handler for all other IO problems

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Simplified Java Exception Hierarchy
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Kinds of Exception in Java

• Exceptions are instances of classes derived from
  Throwable
• Error describes internal errors in Java runtime
  system - little you can do, unchecked
• RuntimeException You made a programming error,
  unchecked
• IOException An abnormal situation, checked

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Examples

• ClassNotFoundException eg main()
• ArrayIndexOutOfBoundsException
• OutOfMemoryError the JVM
• ArithmeticException divide by 0
• FileNotFoundException checked IOException

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Checked vs Unchecked

• Some kinds of error can be forseen but are not
  under your control, such as the IOExceptions, for
  these, the compiler insists that you supply a
  handler, and will complain if you dont. These
  are known as checked exceptions.
• For other, unchecked exceptions, you have the
  choice of supplying a handler or not.

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Which exceptions to catch?

• Checked exceptions
• the compiler insists that you provide a handler
• the kind of exception a method may throw is
  advertised in the header of the method and
  documentation
• readLine() throws IOException
• Not internal Java errors inheriting from Error
• Put time into fixing RuntimeException errors
  rather than catching them - the fact they occur
  is your fault

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Propagating Exceptions

• Instead of try catch at this level, leave
  (propagate) to whoever uses the method.
• public void read(BufferedReader reader) throws
  IOException
• boolean done false
• while(!done)
• String line reader.readLine()
• if (line null) //EOF
• done true
• else
• ... // process line

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Using Exceptions

• Making your own exceptions or methods that throw
  predefined exceptions
• Performance - do not overuse
• ExceptionalTest - timing results
• Do not wrap every statement in a separate try
  block
• big unreadable code
• Do not ignore exceptions
• catch( Exception e) // bad
• Propagating exceptions is OK

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L5 I/O Overview

• Streams
• Generality of I/O
• Bytes versus higher level (doublesetc)
• I/O class hierarchy
• Layering of functionality
• IOExceptions are checked and must be handled

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Streams

• A stream of electrical (optical) impulses, a
  stream of bits from
• read head of a disk
• a network connection
• a microphone
• a camera
• Treat files and network connections similarly

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Binary Streams from Files

• File input from a disk file using FileInputStream
• Give the file name in constructor
• FileInputStream fin new FileInputStream(employe
  e.dat)
• Then can read bytes

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Binary Streams from Network

• Sockets or URL connections
• Similar syntax to file case
• URL url new URL(protocolresource)
• InputStream uin url.openStream()
• Then can read bytes

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Text/Binary Formats

• Two fundamentally different ways of storing data
• For example the integer 12345 as
• text (chars) 1 2 3 4 5
• binary (bytes) 0 0 48 57 (since
  123454825657)
• Java has two hierarchies of classes to deal with
  each situation
• text Reader and Writer class hierarchy
• binary InputStream, OutputStream etc

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Reading Char/Byte Streams

• Low level facilities allow you to look directly
  at the byte stream
• Reader reads single characters
• public int read() //reads next char (as int
  0-65535)
• InputStream reads single bytes
• public int read() //reads next byte (as int
  0-255)
• Read as an int so that it can return -1 when you
  get to the end of the stream
• Also write methods

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Higher level I/O

• Rarely program at the level of chars/bytes
• Use layering to obtain more convenient interfaces
  
• pass an existing stream into the constructor of
  another, with different capabilities
• eg Rather than read bytes, want to assemble them
  to read at a higher level
• DataInputStream combines bytes into numeric
  types
• eg Buffered streams (efficiency)

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Layering Streams

• FileInputStream retrieves bytes from a file
• DataInputStream assembles bytes into more useful
  forms

• Combine the functionalities
• FileInputStream fin new FileInputStream(employe
  e.dat)
• DataInputStream din new DataInputStream(fin)
• double x din.readDouble()

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Reading and Writing Text

• Concentrate on this
• Reader and Writer hierarchies
• To attach a file to a reader/writer FileReader
  and FileWriter
• FileWriter out new FileWriter(output.txt)
• FileReader in new FileReader(input.txt)
• To read keystrokes from the console
  InputStreamReader in new
  InputStreamReader(System.in)

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Writing Text

• To write Strings (rather than chars) layer
• Combine a PrintWriter with a FileWriter
• PrintWriter out new PrintWriter
• (new FileWriter(employee.txt))
• out.println(Harry hacker)
• double salary 34000
• out.println(salary)
• Use the same println and print methods that you
  are familiar with from System.out (System.out is
  a PrintWriter class)

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End of Line characters

• The character used to signify the end of a line
  is different on different platforms
• Windows \r\n
• UNIX \n
• Mac \r
• println() automatically picks the right one
• Obtain using System.getProperty(line.separator)
• Similar call for directory separator

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Reading Text

• Use the readline() method of the BufferedReader
  class
• BufferedReader in new BufferedReader
• (new FileReader(employee.txt))
• String line
• while ((line in.readLine()) ! null)
• //do something with line
• Returns a String, use Integer.parseInt() etc to
  change to numeric datatype
• Returns null when there is no more data
• Must catch IOExceptions

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Typical code for reading a file

• try
• BufferedReader in new BufferedReader
• (new FileReader(myfile.txt))
• boolean done false
• while(!done)
• String line in.readLine()
• if (line null) //EOF
• done true
• else
• ... // process line
• catch(IOException e)
• e.printStackTrace()

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StringTokenizer

• When reading with readLine(), each line is
  returned as one long string
• Often want to split it into smaller individual
  strings (parsing)
• Use the StringTokenizer helper class
• Split on delimiters (blank space, commas, ...)
• Get individual strings with nextToken() method

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Delimited text

• Store employee records as
• Harry Hacker355001989101
• Carl Cracker7500019871215
• Tony Tester380001990315
• Break up lines with
• StringTokenizer t new StringTokenizer(line,)
  
• Use nextToken() method
• name t.nextToken()
• salary Double.parseDouble(nextToken())
• int y Integer.parseInt(nextToken())
• DataFileTest.java

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An Object Oriented Approach

• Want to save objects
• Not all of the same type, some Employees, some
  Managers
• Could save
• type of object
• data defining the object
• Object serialization does this for you

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Serialization

• ObjectOutputStream out new ObjectOutputStream
• (new FileOutputStream(employee.dat)
• Employee harry new Employee(Harry
  Hacker,50000,1989,10,1)
• Manager boss new Manager(Carl
  Cracker,75000,1987,1,13)
• out.writeObject(harry)
• out.writeObject(boss)
• When reading back in, cast to correct type
• Any class that you want to save in this way must
  implement Serializable

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More

• Files and directories
• creating files
• Flush
• sometimes want to explicitly ensure that data is
  actually written (from possible buffers) to file
• Serialized objects
• file format
• object reference problem

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Packages and jar