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| Core Java - Interview Questions and Answers for 'Instanceof' - 2 question(s) found - Order By Newest | ||||
 Usually asked in relation to casting and ClassCastException. | ||||
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| Ans. The operator instanceOf is used to verify if the specified object is the instance of specified class or interface. Syntax if(x instanceOf ABC) where x is an object reference and ABC could be a class name or interface name. The above statement will be true if x holds an object that is an instance of ABC or any of the child class of ABC or if x holds an object that implements ABC. instanceOf operator is used to verify in case of downcasting. For ex - DerivedClass extends BaseClass x is the reference of BaseClass but holds DerivedClass object ( Polymorphism ) There is an operation that is defined in Derived Class, let's say derivedClassMethod() We cannot call derivedClassMethod() directly using x as x is reference of BaseClass and not DerivedClass and hence can only access methods that are defined in BaseClass and overridden in derived class. Though we can cast it to DerivedClass as following ((DerivedClass)x).derivedClassMethod(); But it may throw ClassCastException in case x doesn't hold an instance of DerivedClass at that point. So before casting it to DerivedClass we may like to make sure that it is an instance of DerivedClass and hence won't throw ClassCastException. So we make a check for it if(x instanceOf DerivedClass) { ((DerivedClass)x).derivedClassMethod(); } | ||||
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 Discuss Correct / Improve  instanceOf instanceOf operator Why we need instanceOf operator use of instanceOf operator  Asked in 2 Companies  Intermediate  frequent | ||||
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 In which cases isn't instanceof operator a bad practice? | ||||
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| Ans. To avoid ClassCastException. Though the following code will compile fine but will result in ClassCastException during runtime. Fruit fruit = new Apple(); Banana banana = Banana(fruit); // ClassCastException This code will not give compile time error as Banana and Fruit are related as Banana either extends or implement Fruit, So downcasting is acceptable. With this code we assume that the Fruit handler will have the Apple object at that point, violating which the code will throw the exception. This exception can be avoided by following code. Fruit fruit = new Apple(); if(fruit instanceOf Banana){ Banana banana = Banana(fruit); // ClassCastException } | ||||
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| Discuss Correct / Improve java instanceof classcastexception runtime exceptions instanceOf operator | ||||
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