Java Features

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Java Features

• If you understand this, you will be able to
• Read code containing exceptions
• Important Exceptions make programs more robust
• Java makes you handle exceptions
• Outline the benefits problems of thread use
• Important communication is usually concurrent

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Key Features of Java

• Exceptions
• Separate normal code and error-handling code
• Handlers respond to exceptional occurrences
• Uniformly handles all sorts of error
• Concurrency built into the language
• Several methods executing at the same time
• Not an operating-system dependent add-on

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Exceptions

• Respond to Illegal situations from
• Hardware / Operating System
• Language (generated by run-time system)
• Software (explicit use of throw statement)
• An exception object
• Contains information about the situation
• A handler
• Catches the exception
• Performs appropriate programming

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Examples of Exceptions

• Divide by zero
• Reading from a file, but disk corrupt
• Writing, but disk full
• Out of memory
• Data communication link failure

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Why use exceptions

• Java goal portability and robustness
• Alternative to
• Automatic program termination
• Undefined (unpredictable) response
• Returning funny values - often ignored 

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Handling an Exception

• Handlers are catch clauses
• try
• // statements that may throw an exception
• catch (ExceptionType e)
• ... // Catches only exceptions of this type

Information about the exception, e.g. an error
message
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Steps

• Exception object thrown
• Abort blocks and look for a handler for that
  class of exception object.
• If no handler is found, abort program.
• Otherwise execute the catch block.
• Carry on with the following statement.

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Exception Example

• class MyException extends Exception
• Define a new exception class
• To hold information about a particular problem
• e.g. noticing that a user has left a multi-player
  game
• Can add properties relevant to this situation
• Can catch just this class of exception

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Typical use of exceptions

• try
• link new Link(OpponentDetails)
• // play the game
• average score / numTries
• link.send("Average "average)
• catch(DivideByZero ex)
• // handle divide by zero
• catch(CommsError ex)
• // handle communications problem

Normal code
Handler
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Deallocating Resources

• Must return resources even if an exception occurs
• But the block is terminated skipping statements
• A finally clause is always executed
• Allowing resources to be deallocated.
• // Allocate resource
• try
• // code that could generate an exception
• finally
• // return the resource

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De-allocating Resources

• try
• link new Link(OpponentDetails)
• try
• // play the game
• finally
• link.close()
• catch(DivideByZero ex)
• catch(CommsError ex)

We can be sure that the link will be closed
Unless someone nukes the computer
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Compile-Time Checking

• Compiler checks exception handlers
• A method must either
• handle all checked exceptions or
• name them in the header with a throws clause
• (part of contract with user)
• Some exceptions arent checked
• Very severe or very common
• Why these?
• Ensures exceptions handled why?

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Threads

• Single thread of execution
• Code executes one statement at a time
• Multiple Threads
• Independently execute code at the same time
• using data in shared main memory.
• The Java Virtual Machine supports many threads
• Implemented by
• multiple processors,
• time-slicing a single hardware processor
• Treat as if each has its own processor

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Why Threads

• Allow parallel processing
• Multiple tasks of varying duration
• Logical division of tasks
• Break problem into modules
• Receive task, Send task
• Keep user interface responsive
• Other threads deal with data communications

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Declaring a Thread

• class MouseChase implements runnable
• void run()
• while (true)
• // move to next position
• repaint()
• sleep(100)

This code will be executed in parallel with rest
of program after the thread is started
A Thread uses a class with a run() method
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Starting a thread

• Create a runnable object.
• MouseChase myMC new MouseChase()
• Create a thread using the runnable object
• Thread myThread new Thread(myMC)
• Start the thread
• myThread.start()
• The thread now has a life of its own.
• What if start() is replace by run()?

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Concurrency Issues

• Synchronisation
• Ensure threads are ready to interact
• Imagine a waiter writing orders on a blackboard
• The chef copies them into his notebook
• Communication
• Exchanging data safely
• Requires synchronisation
• Deadlocks
• Cycle of threads holding resources others need

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Problems

• Using a bank account example, explain
• How deadlock can occur transferring money
• Why synchronisation is needed

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Summary

• Exceptions
• Make program more robust
• Compile time checking enforces use
• Threads
• Useful to keep user interface responsive
• Increase
• Likelihood of errors
• Complexity of debugging