我听说利斯科夫替换原则(LSP)是面向对象设计的基本原则。它是什么?它的一些使用例子是什么?
当前回答
LSP的这种形式太强大了:
如果对于每个类型为S的对象o1,都有一个类型为T的对象o2,使得对于所有用T定义的程序P,当o1取代o2时,P的行为不变,那么S是T的子类型。
这基本上意味着S是t的另一个完全封装的实现,我可以大胆地认为性能是P行为的一部分……
因此,基本上,任何延迟绑定的使用都违反了LSP。当我们用一种类型的对象替换另一种类型的对象时,获得不同的行为是OO的全部意义所在!
维基百科引用的公式更好,因为属性取决于上下文,并不一定包括程序的整个行为。
其他回答
罗伯特·马丁有一篇关于利斯科夫替换原理的优秀论文。它讨论了可能违反原则的微妙和不那么微妙的方式。
论文的一些相关部分(注意,第二个例子被大量压缩):
A Simple Example of a Violation of LSP One of the most glaring violations of this principle is the use of C++ Run-Time Type Information (RTTI) to select a function based upon the type of an object. i.e.: void DrawShape(const Shape& s) { if (typeid(s) == typeid(Square)) DrawSquare(static_cast<Square&>(s)); else if (typeid(s) == typeid(Circle)) DrawCircle(static_cast<Circle&>(s)); } Clearly the DrawShape function is badly formed. It must know about every possible derivative of the Shape class, and it must be changed whenever new derivatives of Shape are created. Indeed, many view the structure of this function as anathema to Object Oriented Design. Square and Rectangle, a More Subtle Violation. However, there are other, far more subtle, ways of violating the LSP. Consider an application which uses the Rectangle class as described below: class Rectangle { public: void SetWidth(double w) {itsWidth=w;} void SetHeight(double h) {itsHeight=w;} double GetHeight() const {return itsHeight;} double GetWidth() const {return itsWidth;} private: double itsWidth; double itsHeight; }; [...] Imagine that one day the users demand the ability to manipulate squares in addition to rectangles. [...] Clearly, a square is a rectangle for all normal intents and purposes. Since the ISA relationship holds, it is logical to model the Square class as being derived from Rectangle. [...] Square will inherit the SetWidth and SetHeight functions. These functions are utterly inappropriate for a Square, since the width and height of a square are identical. This should be a significant clue that there is a problem with the design. However, there is a way to sidestep the problem. We could override SetWidth and SetHeight [...] But consider the following function: void f(Rectangle& r) { r.SetWidth(32); // calls Rectangle::SetWidth } If we pass a reference to a Square object into this function, the Square object will be corrupted because the height won’t be changed. This is a clear violation of LSP. The function does not work for derivatives of its arguments. [...]
到目前为止,我发现LSP最清晰的解释是“利斯科夫替换原则说,派生类的对象应该能够替换基类的对象,而不会给系统带来任何错误,也不会修改基类的行为”。文中给出了违反LSP的代码示例并进行了修复。
LSP说“对象应该被它们的子类型替换”。 另一方面,这一原则指向
子类永远不应该破坏父类的类型定义。
通过以下示例,可以更好地理解LSP。
没有太阳能发电:
public interface CustomerLayout{
public void render();
}
public FreeCustomer implements CustomerLayout {
...
@Override
public void render(){
//code
}
}
public PremiumCustomer implements CustomerLayout{
...
@Override
public void render(){
if(!hasSeenAd)
return; //it isn`t rendered in this case
//code
}
}
public void renderView(CustomerLayout layout){
layout.render();
}
LSP修复:
public interface CustomerLayout{
public void render();
}
public FreeCustomer implements CustomerLayout {
...
@Override
public void render(){
//code
}
}
public PremiumCustomer implements CustomerLayout{
...
@Override
public void render(){
if(!hasSeenAd)
showAd();//it has a specific behavior based on its requirement
//code
}
}
public void renderView(CustomerLayout layout){
layout.render();
}
以Board数组的形式实现ThreeDBoard会有用吗?
也许你想把不同平面上的ThreeDBoard切片作为一个板。在这种情况下,您可能希望为Board抽象出一个接口(或抽象类),以允许多种实现。
就外部接口而言,您可能希望为TwoDBoard和ThreeDBoard提取一个Board接口(尽管上述方法都不适合)。
该原则由Barbara Liskov在1987年提出,并通过关注超类及其子类型的行为来扩展开闭原则。
当我们考虑违反它的后果时,它的重要性就变得显而易见了。考虑一个使用以下类的应用程序。
public class Rectangle
{
private double width;
private double height;
public double Width
{
get
{
return width;
}
set
{
width = value;
}
}
public double Height
{
get
{
return height;
}
set
{
height = value;
}
}
}
想象一下,有一天,客户要求除了矩形之外还能操作正方形。因为正方形是矩形,所以square类应该派生自rectangle类。
public class Square : Rectangle
{
}
然而,这样做会遇到两个问题:
一个正方形不需要从矩形继承高度和宽度变量,如果我们必须创建成千上万个正方形对象,这可能会造成严重的内存浪费。 从矩形继承的width和height setter属性不适用于正方形,因为正方形的宽度和高度是相同的。 为了将height和width设置为相同的值,我们可以创建两个新属性,如下所示:
public class Square : Rectangle
{
public double SetWidth
{
set
{
base.Width = value;
base.Height = value;
}
}
public double SetHeight
{
set
{
base.Height = value;
base.Width = value;
}
}
}
现在,当有人设置一个正方形物体的宽度时,它的高度将相应地改变,反之亦然。
Square s = new Square();
s.SetWidth(1); // Sets width and height to 1.
s.SetHeight(2); // sets width and height to 2.
让我们继续考虑另一个函数:
public void A(Rectangle r)
{
r.SetWidth(32); // calls Rectangle.SetWidth
}
如果我们将一个方形对象的引用传递给这个函数,我们将违反LSP,因为该函数对其参数的导数不起作用。属性width和height不是多态的,因为它们在矩形中没有被声明为虚的(正方形对象将被损坏,因为高度不会被改变)。
然而,通过将setter属性声明为virtual,我们将面临另一个违反,即OCP。事实上,派生类正方形的创建会导致基类矩形的变化。