Java Programing

while(true) { blinkState = !blinkState; repaint( ); Thread.sleep(500); } } catch (InterruptedException ie) {} } Basically, run( ) is an infinite while loop. This means the method will run continuously until the thread is terminated by a call to the controlling Thread object’s interrupt( ) method. The body of the loop does three things on each pass: Flips the value of blinkState to its opposite value using the not operator, “!” Calls repaint( ) to redraw the text Sleeps for 500 milliseconds (half a second) sleep( ) is a static method of the Thread class. The method can be invoked from anywhere and has the effect of putting the current thread to sleep for the specified number of milliseconds. The effect here is to give us approximately two blinks per second. The try/catch construct, described in the next section, traps any errors in the call to the sleep( ) method of the Thread class. 2.4.6 Exceptions The try/catch statement in Java is used to handle special conditions called exceptions . An exception is a message that is sent, normally in response to an error, during the execution of a statement or a method. When an exceptional condition arises, an object is created that contains information about the particular problem or condition. Exceptions act somewhat like events. Java stops execution at the place where the exception occurred, and the exception object is said to be thrown by that section of code. Like an event, an exception must be delivered somewhere and handled. The section of code that receives the exception object is said to catch the exception. An exception causes the execution of the instigating section of code to stop abruptly and transfers control to the code that receives the exception object. The try/catch construct allows you to catch exceptions for a section of code. If an exception is caused by any statement inside of a try clause, Java attempts to deliver the exception to the appropriate catch clause. A catch clause looks like a method declaration with one argument and no return type. If Java finds a catch clause with an argument type that matches the type of the exception, that catch clause is invoked. A try clause can have multiple catch clauses with different argument types; Java chooses the appropriate one in a way that is analogous to the selection of overloaded methods. You can catch multiple types of exceptions from a block of code. Depending on the type of exception thrown, the appropriate catch clause will be executed. If there is no try/catch clause surrounding the code, or a matching catch clause is not found, the exception is thrown up the call stack to the calling method. If the exception is not caught there, it’s thrown up another level, and so on until the exception is handled. This provides a very flexible error-handling mechanism, so that exceptions in deeply nested calls can bubble up to the surface of the call stack for handling. As a programmer, you need to know what exceptions a particular statement can generate, so methods in Java are required to declare the exceptions they can throw. If a method doesn’t handle an exception itself, it must specify that it can throw that exception, so that its calling method knows that it may have to handle it. See Chapter 4, for a complete discussion of exceptions and the try/catch clause. - 52

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