overriding

问题 How can I control the background image and colour of a body element within an iframe ? Note, the embedded body element has a class, and the iframe is of a page that is part of my site. The reason I need this is that my site has a black background assigned to the body, and then a white background assigned to divs that contain text. A WYSIWYG editor uses an iframe to embed content when editing, but it doesn\'t include the div, so the text is very hard to read. The body of the iframe when in the

问题 Suppose I have two C++ classes: class A { public: A() { fn(); } virtual void fn() { _n = 1; } int getn() { return _n; } protected: int _n; }; class B : public A { public: B() : A() {} virtual void fn() { _n = 2; } }; If I write the following code: int main() { B b; int n = b.getn(); } One might expect that n is set to 2. It turns out that n is set to 1. Why? 回答1: Calling virtual functions from a constructor or destructor is dangerous and should be avoided whenever possible. All C++

问题 Consider the code : #include <stdio.h> class Base { public: virtual void gogo(int a){ printf(\" Base :: gogo (int) \\n\"); }; virtual void gogo(int* a){ printf(\" Base :: gogo (int*) \\n\"); }; }; class Derived : public Base{ public: virtual void gogo(int* a){ printf(\" Derived :: gogo (int*) \\n\"); }; }; int main(){ Derived obj; obj.gogo(7); } Got this error : >g++ -pedantic -Os test.cpp -o test test.cpp: In function `int main()\': test.cpp:31: error: no matching function for call to

问题 I have a Wicket page class that sets the page title depending on the result of an abstract method. public abstract class BasicPage extends WebPage { public BasicPage() { add(new Label(\"title\", getTitle())); } protected abstract String getTitle(); } NetBeans warns me with the message \"Overridable method call in constructor\", but what should be wrong with it? The only alternative I can imagine is to pass the results of otherwise abstract methods to the super constructor in subclasses. But

问题 Given the following class public class Foo { public int FooId { get; set; } public string FooName { get; set; } public override bool Equals(object obj) { Foo fooItem = obj as Foo; if (fooItem == null) { return false; } return fooItem.FooId == this.FooId; } public override int GetHashCode() { // Which is preferred? return base.GetHashCode(); //return this.FooId.GetHashCode(); } } I have overridden the Equals method because Foo represent a row for the Foo s table. Which is the preferred method

问题 This post is a Community Wiki . Edit existing answers to improve this post. It is not currently accepting new answers. What issues / pitfalls must be considered when overriding equals and hashCode ? 回答1: The theory (for the language lawyers and the mathematically inclined): equals() (javadoc) must define an equivalence relation (it must be reflexive , symmetric , and transitive ). In addition, it must be consistent (if the objects are not modified, then it must keep returning the same value).

问题 In terms of Java, when someone asks: what is polymorphism? Would overloading or overriding be an acceptable answer? I think there is a bit more to it than that. IF you had a abstract base class that defined a method with no implementation, and you defined that method in the sub class, is that still overridding? I think overloading is not the right answer for sure. 回答1: The clearest way to express polymorphism is via an abstract base class (or interface) public abstract class Human{ ... public

问题 Why is it not possible to override static methods? If possible, please use an example. 回答1: Overriding depends on having an instance of a class. The point of polymorphism is that you can subclass a class and the objects implementing those subclasses will have different behaviors for the same methods defined in the superclass (and overridden in the subclasses). A static method is not associated with any instance of a class so the concept is not applicable. There were two considerations driving

问题 I\'ve created my own theme as a separate Maven project, and it is loaded correctly. Now I want to change the size of an component. For example, a <p:orderList>. It has a class called ui-orderlist-list which is defined in primefaces.css with a fixed 200x200 dimension. No matter what I do in my theme.css , it is overwritten by this attribute and there is no way I can make the content part of a <p:orderList> wider. For other components I might want to override just one instance of a component,