NoSuchElementException thrown when an attempt is made to access the next in Java

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NoSuchElementException thrown when an attempt is made to access the next
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element of an enumeration, but all elements have been exhausted made, but the reference was null
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NullPointerException thrown when an attempt to reference an object has been SecurityException thrown by the current security manager when an attempt to
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access a resource, object, or method has been made but not permitted
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Dealing with exceptions can involve either catching them or throwing the exception to a calling method There are many different types of exceptions, and many of these have subclasses of exceptions For example, when dealing with TCP sockets, it is possible for a SocketException to be thrown The SocketException class has several subclasses, however, that indicate more specific error conditions A ConnectException may have been thrown, to indicate that the connection was refused, or a NoRouteToHost connection may have been thrown, to indicate that the network connection to the host is down or blocked by a firewall As readers work through the various classes that make up the Java API, and their methods, an enormous range of exceptions will be encountered Rather than provide an exhaustive list here, when methods of a class are discussed in later chapters, the methods likely to be thrown will be covered 2713 Handling Exceptions To deal with errors that occur at runtime, programmers must write an exception handler This is a special section of code that catches any thrown exceptions How programmers choose to handle such code is not prescribed (for example, an error message could be output to the user or the exception ignored), but an exception must be caught and dealt with, unless a method indicates that it will throw an exception itself Java provides three statements for handling exceptions 1 The try statement indicates a block of code that can generate exceptions Unless a method that throws a specific type of exception instead of handling it is being written, any code that generates an exception within a try block must be enclosed For example, if the programmer is dealing with network I/O and his or her method doesn't declare that it
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throws an IOException, then the network I/O code must be enclosed within a try statement 2 The catch statement is used to catch exceptions thrown within a try block of code A catch statement must specify which exception it is dealing with; many catch statements can be applied to a single try statement 3 The finally statement is a generic catchall for cleaning up after a try block Whether exceptions were thrown or not, the code within a finally statement will be executed Exceptions: Try Statements Unless a method indicates that it throws an exception, the compiler will enforce exception handling Any code that uses a method that indicates it throws an exception must be enclosed within a try block If an exception does occur, execution of the code within the try block stops immediately, and the exception will be caught by a catch block or calling method
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// Code outside of try block should not throw an exception try { // do something that could generate an exception } // Handle exception
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Exceptions: Catch Statements The purpose of the catch statement is to provide error-handling code that handles an error gracefully One or more catch statements may follow a try block, dealing with specific types of exceptions
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// Try block can generate exceptions try { // generate an exception } catch (SocketException se) { Systemerrprintln ("Socket error reading from host : " + se); Systemexit(2); } catch (Exception e) { Systemerrprintln ("Error : " + e); Systemexit(1); }
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Exceptions: Finally Statements Sometimes the programmer will want to execute statements regardless of whether or not an error occurs Putting cleanup code within a try block is not appropriate, as it will not be executed if an exception is thrown Instead, the finally statement should be used; it comes after any catch statements and the try block
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// Try block can generate exceptions try { // generate an exception } catch (SomeException some) { // handle some exception
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} finally { // clean up after try block, regardless of any // exceptions that are thrown }
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2714 Causes of Exceptions Despite the best-laid plans, no application is foolproof and exceptions will be encountered at runtime This will, it is hoped, occur during debugging, rather than exceptions being accidentally discovered by users By catching and handling exceptions, applications will be robust enough to recover, and if not, to retire gracefully with an appropriate message In networking, the most common cause of exceptions is related to the state of the network connection Users connect to network services over a variety of communication mechanisms, of varying degrees of speed and quality When a network loses its connection to the Internet, or that connection becomes highly congested with traffic, it may be impossible to connect to a particular host and thus to its network services Or the user may be behind a firewall, preventing a direct connection from being established In our experience, these are the two most common causes of exceptions in network applications The next most likely cause is a security restriction, either imposed by the browser on applets or by a Java security policy or security manager When writing applets or software that uses security (such as distributed clients and servers), programmers may want to consider trying to catch any Security Exception that is thrown Though not mandatory, in that the compiler does not enforce it, it may be advantageous to do so if users are likely to change their security settings or run software within restricted environments However, a SecurityException is thrown only if code attempts to do something prohibited by the JVM external security measures such as firewalls will simply result in an inability to connect to certain hosts
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