polymorphism

问题 Suppose you have the following code class A { int i = 4; A() { print(); } void print () { System.out.println("A"); } } class B extends A { int i = 2; //"this line" public static void main(String[] args){ A a = new B(); a.print(); } void print () { System.out.println(i); } } this will print 0 2 Now, if you remove line labeled "this line" the code will print 4 4 I understand that if there was no int i=2; line, A a = new B(); will call class A, initializes i as 4, call constructor, which gives

问题 Suppose you have the following code class A { int i = 4; A() { print(); } void print () { System.out.println("A"); } } class B extends A { int i = 2; //"this line" public static void main(String[] args){ A a = new B(); a.print(); } void print () { System.out.println(i); } } this will print 0 2 Now, if you remove line labeled "this line" the code will print 4 4 I understand that if there was no int i=2; line, A a = new B(); will call class A, initializes i as 4, call constructor, which gives

问题 Using Automapper 3.3.1.0 there is a different mapping behavior between the usage of Mapper.Map<IEnumerable<TDestination>>(someEnumerable) compared to someEnumerable.AsQueryable().Project().To<TDestination>() This does not appear to be a limitation of a SQL LINQ provider or other as this is witnessed in an in-memory collection. As with many things this is best explained by example: Note: the following code can be found at https://gist.github.com/kmoormann/b3949d006f4083ab6ee4 using System

问题 I am confused about the concepts of inheritance and polymorphism. I mean, what is the difference between code re-usability and function overriding? Is it impossible to reuse parent class function using inheritance concept or else is it impossible to override parent class variables using Polymorphism. There seems little difference for me. class A { public: int a; virtual void get() { cout<<"welcome"; } }; class B:public A { a =a+1; //why it is called code reuse void get() //why it is called

问题 This question already has an answer here : Closed 7 years ago . Possible Duplicate: SWIG Java Retaining Class information of the objects bouncing from C++ Question : Why is my C++ swigged object losing its type when passed to a Java callback function? Setup : I've taken the Swig Java example for doing callbacks and added an object to be passed to the callback run(Parent p) . The callback works as expected but when I pass a Child object the Java seems to lose its type and think its of type

问题 I have the abstract parent class Animal: public abstract class Animal { public abstract <T extends Animal> T copyAnimal(T animal); } I then want to create a subclass Duck but to override the copyAnimal I want to use Duck as the parameters such that: public class Duck extends Animal { @Override public Duck copyAnimal(Duck duck) { return copyOfDuck; } } This of course gives me a compiler error saying that the method is not overridden. That being said how can I adjust this code so that I don't

问题 I have a couple of classes that share a common base class, with the exception that they differ in the way their methods work. So in the example below, Adder and Multiplier are the same except for the way in which their calculation is performed. Is there a way to change "a" to a Multiplier on the fly? Do I need to implement methods which convert derived classes to each other? e.g. something like a = a.asMultiplier() ? As you can see in the code below I tried reinterpret_cast to a Multiplier,

问题 This question already has answers here : Closed 8 years ago . Possible Duplicate: Why can't you reduce the visibility of a method in a java subclass? How come I can override a private method in superclass with a public when in a subclass, but I cannot override a public method in the superclass into private method in subclass? Why? Thank you in advance. 回答1: Overriding a method can't ever reduce the visibility. Allowing that would violate the Liskov Substitution Principle, which states

问题 I'm trying to set up polymorphic behaviour using Hibernate with JPA annotations. It seems sensible (maybe even necessary) to create an (abstract) class encapsulating the state and behaviours required for the inheritance hierarchy to participate in persistence; for example I need to annotate an Id property, which I cannot do in an interface without making the field public, in order to avoid a 'No identifier specified on Entity' exception on the superclass I need subclasses to provide a

问题 I've seen both used interchangebly but do they really mean the same? From my understanding, Polymorphism stretches the fact that you could exchange an instance of a class by an instance of a subclass, and Late Binding means that when you call a method of an instance, the type decides which method (subclass/superclass) gets called. 回答1: Wikipedia has a very nice article about this: http://en.wikipedia.org/wiki/Polymorphism_in_object-oriented_programming Summary: Late binding is a way to