visual basic 6 barcode generator The transient and volatile Modifiers in Java

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The transient and volatile Modifiers
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Java defines two interesting type modifiers: transient and volatile These modifiers are used to handle somewhat specialized situations When an instance variable is declared as transient, then its value need not persist when an object is stored For example: class T { transient int a; // will not persist int b; // will persist } Here, if an object of type T is written to a persistent storage area, the contents of a would not be saved, but the contents of b would The volatile modifier tells the compiler that the variable modified by volatile can be changed unexpectedly by other parts of your program One of these situations involves multithreaded programs (You saw an example of this in 11) In a multithreaded program, sometimes, two or more threads share the same instance variable For efficiency considerations, each thread can keep its own, private copy of such a shared variable The real (or master) copy of the variable is updated at various times, such as when a synchronized method is entered While this approach works fine, it may be inefficient at times In some cases, all that really matters is that the master copy of a variable always reflects its current state To ensure this, simply specify the variable as volatile, which tells the compiler that it must always use the master copy of a volatile variable (or, at least, always keep any private copies up to date with the master copy, and vice versa) Also, accesses to the master variable must be executed in the precise order in which they are executed on any private copy Note volatile in Java has, more or less, the same meaning that it has in C/C++
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Using instanceof
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Sometimes, knowing the type of an object during run time is useful For example, you might have one thread of execution that generates various types of objects, and another thread that processes these objects In this situation, it might be useful for the processing thread to know the type of each object when it receives it Another situation in which knowledge of an object's type at run time is important involves casting In Java, an invalid cast causes a run-time error Many invalid casts can be caught at compile time However, casts involving class hierarchies can produce invalid casts that can be detected only at run time For example, a superclass called A can produce two subclasses, called B and C Thus, casting a B object into type A or casting a C object into type A is legal, but casting a B object into type C (or vice versa) isn't legal Because an object of type A can refer to objects of either B or C, how can you know, at run time, what type of object is actually being referred to before attempting the cast to type C It could be an object of type A, B, or C If it is an object of type B, a run-time exception will be thrown Java provides the run-time operator instanceof to answer this question The instanceof operator has this general form: object instanceof type Here, object is an instance of a class, and type is a class type If object is of the specified type or can be cast into the specified type, then the instanceof operator evaluates to true Otherwise, its result is false Thus, instanceof is the means by which your program
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can obtain run-time type information about an object The following program demonstrates instanceof: // Demonstrate instanceof operator class A { int i, j; } class B { int i, j; } class C extends A { int k; } class D extends A { int k; } class InstanceOf { public static void main(String args[]) { A a = new A(); B b = new B(); C c = new C(); D d = new D(); if(a instanceof A) Systemoutprintln("a is instance of A"); if(b instanceof B) Systemoutprintln("b is instance of B"); if(c instanceof C) Systemoutprintln("c is instance of C"); if(c instanceof A) Systemoutprintln("c can be cast to A"); if(a instanceof C) Systemoutprintln("a can be cast to C"); Systemoutprintln(); // compare types of derived types A ob; ob = d; // A reference to d Systemoutprintln("ob now refers to d"); if(ob instanceof D) Systemoutprintln("ob is instance of D"); Systemoutprintln(); ob = c; // A reference to c Systemoutprintln("ob now refers to c"); if(ob instanceof D) Systemoutprintln("ob can be cast to D"); else Systemoutprintln("ob cannot be cast to D"); if(ob instanceof A) Systemoutprintln("ob can be cast to A");
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Systemoutprintln(); // all objects can be cast to Object if(a instanceof Object) Systemoutprintln("a may be cast to if(b instanceof Object) Systemoutprintln("b may be cast to if(c instanceof Object) Systemoutprintln("c may be cast to if(d instanceof Object) Systemoutprintln("d may be cast to
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Object"); Object"); Object"); Object");
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The output from this program is shown here: a b c c is instance is instance is instance can be cast of of of to A B C A
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ob now refers to d ob is instance of D ob now refers to c ob cannot be cast to D ob can be cast to A a b c d may may may may be be be be cast cast cast cast to to to to Object Object Object Object
The instanceof operator isn't needed by most programs, because, generally, you know the type of object with which you are working However, it can be very useful when you're writing generalized routines that operate on objects of a complex class hierarchy
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