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Advanced Flow of Control

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Advanced Flow of Control - Introduction -ExceptionsException HandlingSlide 4The try StatementException PropagationSlide 7The throw StatementThe finally ClauseThreadsSlide 11Shared DataSlide 13SynchronizationControlling Threads1Advanced Flow of Control - Introduction - Two additional mechanisms for controlling process execution are exceptions and threadsChapter 14 focuses on:exception processingcatching and handling exceptionscreating new exceptionsseparate process threadssynchronizing threads2Exceptions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A program can therefore be separated into a normal execution flow and an exception execution flowAn error is also represented as an object in Java, but usually represents a unrecoverable situation and should not be caught3Exception HandlingA program can deal with an exception in one of three ways:ignore ithandle it where it occurshandle it in another place in the programThe manner in which an exception is processed is an important design consideration4Exception HandlingIf an exception is ignored by the program, the program will terminate and produce an appropriate messageThe message includes a call stack trace that indicates on which line the exception occurredThe call stack trace also shows the method call trail that lead to the execution of the offending lineSee Zero.java5The try StatementTo process an exception when it occurs, the line that throws the exception is executed within a try blockA try block is followed by one or more catch clauses, which contain code to process an exceptionEach catch clause has an associated exception typeWhen an exception occurs, processing continues at the first catch clause that matches the exception typeSee Adding.java6Exception PropagationIf it is not appropriate to handle the exception where it occurs, it can be handled at a higher levelExceptions propagate up through the method calling hierarchy until they are caught and handled or until they reach the outermost levelA try block that contains a call to a method in which an exception is thrown can be used to catch that exceptionSee Propagation_Demo.java7ExceptionsAn exception is either checked or uncheckedA checked exception can only be thrown within a try block or within a method that is designated to throw that exceptionThe compiler will complain if a checked exception is not handled appropriatelyAn unchecked exception does not require explicit handling, though it could be processed that way8The throw StatementA programmer can define an exception by extending the appropriate classExceptions are thrown using the throw statement: throw exception-object;See Throw_Demo.java Usually a throw statement is nested inside an if statement that evaluates the condition to see if the exception should be thrown9The finally ClauseA try statement can have an optional clause designated by the reserved word finallyIf no exception is generated, the statements in the finally clause are executed after the statements in the try block completeAlso, if an exception is generated, the statements in the finally clause are executed after the statements in the appropriate catch clause complete10ThreadsProcessing can be broken into several separate threads of control which execute at the same time"At the same time" could mean true parallelism or simply interlaced concurrent processingA thread is one sequential flow of execution that occurs at the same time another sequential flow of execution is processing the same programThey are not necessarily executing the same statements at the same time11ThreadsA thread can be created by deriving a new thread from the Thread classThe run method of the thread defines the concurrent activity, but the start method is used to begin the separate thread process See Simultaneous.javaA thread can also be created by defining a class that implements the Runnable interfaceBy implementing the interface, the thread class can be derived from a class other than Thread12Shared DataPotential problems arise when multiple threads share dataSpecific code of a thread may execute at any point relative to the processing of another threadIf that code updates or references the shared data, unintended processing sequences can occur that result in incorrect results13Shared DataConsider two withdrawals from the same bank account at the same timetask: withdraw 300Is amount <= balanceYESbalance -= 300balance531231-69task: withdraw 300Is amount <= balanceYESbalance -= 30014SynchronizationMultiple threads of control can be made safe if areas of code that use shared data are synchronizedWhen a set of code is synchronized, then only one thread can be using that code at a timeThe other threads must wait until the first thread is completeThis is an implementation of a synchronization mechanism called a monitorSee ATM_Accounts.java15Controlling ThreadsThread processing can be temporarily suspended, then later resumed, using methods from the Thread classA thread can also be put to sleep for a specific amount of timeThese mechanisms can be quite helpful in certain situations, like controlling animationsSee


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