To do this we call the takeForWalk() method that only exists on Dog, or the play() method which only exists on Cat. All the dogs need to be taken for a walk, and all the cats need to be played with using a bird-shaped toy. Imagine a method that receives a list of animals for processing. The cast operates on the reference it doesn't transform an object to a different type.Ĭan you give an example of when casting (Dog doggy = (Dog) myAnimal) makes sense? K's doIt() for the same reasons as above. Suppose I have a doIt() method implemented in both M and K. Many Code Analysis tools such as FindBugs will notify you of redundant casts so you can remove them. A Dog already is an Animal and it has all the methods of an Animal as well as its own. In fact, casting to a superclass is redundant. If you cast a Dog to an Animal and ask it what animal it is it'll still answer that it's a dog. k's class is always K, regardless of whather you cast its reference to M or not. You've casted k to M, but all classes have a getClass() method. Why is that? It was casted to the more general M! If the cast is not possible then a ClassCastException is thrown at runtime. That Animal might well be a Cat with properties unique to it (purring, rigorous regime of self-cleaning, etc.). Otherwise it would force the Universe to infer properties unique to a dog (wagging tail, barking, etc.) onto an Animal. public class Cat extends AnimalĬasting an Animal to a Dog can only be done if the Animal in question is indeed a Dog. To give you an idea about classes and objects, let's create a Cat class that represents states and behaviors of real world Cat.Consider a real-world example: public class Dog extends AnimalĪll dogs are animals, but not all animals are dogs. Instance variables are also known as member variables. Source.Ĭlasses define states as instance variables and behaviors as instance methods. Figure 1: Illustrates class and object relationship through cookie-cutter and cookies. Imagine a class as a cookie-cutter and objects as cookies. What Are Classes?Ī class is a template or blueprint from which objects are created. Polymorphism allows the suprclass reference object to call methods on the subclass methods. We’ll see what a reference type is later in this article. Polymorphism is what makes inheritance powerful. The important thing to remember when creating an object is: the reference type should be the same type or a super type of the object type. Instantiating a class means the same thing as creating an object. Memory is allocated in RAM whenever creating a logical object.Īn object is also referred to an instance of a class. Software objects are the actual representation of real world objects. We can actually represent a real world dog in a program as a software object by defining its states and behaviors. State tells us how the object looks or what properties it has. To illustrate, a dog has states like age, color, name, and behaviors like eating, sleeping, and running. In this contrived example one would define an interface DoSonething with a method bad(). Eg if object is type X invoke foo if its a Y invoke bar and so. What Are Objects?Īn object is an entity that has states and behaviors.įor example, dog, cat, and vehicle. Polymorphism can only replace if tests when the if test is basically dispatchi g to a variety of methods depending on the 'type' of an object. OOP focuses on each object’s states and behaviors. Each object decides what to do with a received message. Object-oriented programming is a programming paradigm where everything is represented as an object. ― Paul Graham Fundamentals of object-oriented programming It lets you accrete programs as a series of patches. Object-oriented programming offers a sustainable way to write spaghetti code.
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