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
A square is a rectangle where the width equals the height. If the square sets two different sizes for the width and height it violates the square invariant. This is worked around by introducing side effects. But if the rectangle had a setSize(height, width) with precondition 0 < height and 0 < width. The derived subtype method requires height == width; a stronger precondition (and that violates lsp). This shows that though square is a rectangle it is not a valid subtype because the precondition is strengthened. The work around (in general a bad thing) cause a side effect and this weakens the post condition (which violates lsp). setWidth on the base has post condition 0 < width. The derived weakens it with height == width.
因此,可调整大小的正方形不是可调整大小的矩形。
其他回答
利斯科夫替换原则(来自Mark Seemann的书)指出,我们应该能够在不破坏客户端或实现的情况下,用另一个接口的实现替换一个接口的实现。正是这一原则使我们能够解决未来出现的需求,即使我们今天不能预见它们。
If we unplug the computer from the wall (Implementation), neither the wall outlet (Interface) nor the computer (Client) breaks down (in fact, if it’s a laptop computer, it can even run on its batteries for a period of time). With software, however, a client often expects a service to be available. If the service was removed, we get a NullReferenceException. To deal with this type of situation, we can create an implementation of an interface that does “nothing.” This is a design pattern known as Null Object,[4] and it corresponds roughly to unplugging the computer from the wall. Because we’re using loose coupling, we can replace a real implementation with something that does nothing without causing trouble.
一些补充:我想知道为什么没有人写基类的不变量、前提条件和后置条件,这些派生类必须遵守。 对于派生类D来说,基类B完全可转换,类D必须服从某些条件:
基类的内变体必须由派生类保留 派生类不能加强基类的先决条件 派生类不能削弱基类的后置条件。
因此派生类必须知道基类施加的上述三个条件。因此,子类型的规则是预先确定的。这意味着,只有当子类型遵守某些规则时,才应该遵守'IS A'关系。这些规则,以不变量、前置条件和后置条件的形式,应该由正式的“设计契约”来决定。
关于这个问题的进一步讨论可以在我的博客:利斯科夫替换原理
设q(x)是关于类型为T的x的对象的可证明属性,那么q(y)对于类型为S的对象y应该是可证明的,其中S是T的子类型。
实际上,公认的答案并不是利斯科夫原理的反例。正方形自然是一个特定的矩形,因此从类矩形继承是完全有意义的。你只需要以这样的方式实现它:
@Override
public void setHeight(double height) {
this.height = height;
this.width = height; // since it's a square
}
@Override
public void setWidth(double width) {
setHeight(width);
}
所以,提供了一个很好的例子,然而,这是一个反例:
class Family:
-- getChildrenCount()
class FamilyWithKids extends Family:
-- getChildrenCount() { return childrenCount; } // always > 0
class DeadFamilyWithKids extends FamilyWithKids:
-- getChildrenCount() { return 0; }
-- getChildrenCountWhenAlive() { return childrenCountWhenAlive; }
在这个实现中,DeadFamilyWithKids不能从FamilyWithKids继承,因为getChildrenCount()返回0,而从FamilyWithKids它应该总是返回大于0的值。
我在每个答案中都看到了矩形和正方形,以及如何违反LSP。
我想用一个真实的例子来展示LSP是如何符合的:
<?php
interface Database
{
public function selectQuery(string $sql): array;
}
class SQLiteDatabase implements Database
{
public function selectQuery(string $sql): array
{
// sqlite specific code
return $result;
}
}
class MySQLDatabase implements Database
{
public function selectQuery(string $sql): array
{
// mysql specific code
return $result;
}
}
这种设计符合LSP,因为无论我们选择使用哪种实现,行为都不会改变。
是的,你可以在这个配置中违反LSP,做一个简单的改变,像这样:
<?php
interface Database
{
public function selectQuery(string $sql): array;
}
class SQLiteDatabase implements Database
{
public function selectQuery(string $sql): array
{
// sqlite specific code
return $result;
}
}
class MySQLDatabase implements Database
{
public function selectQuery(string $sql): array
{
// mysql specific code
return ['result' => $result]; // This violates LSP !
}
}
现在子类型不能以同样的方式使用,因为它们不再产生相同的结果。
LSP是关于类的契约的规则:如果基类满足契约,则LSP派生的类也必须满足该契约。
在Pseudo-python
class Base:
def Foo(self, arg):
# *... do stuff*
class Derived(Base):
def Foo(self, arg):
# *... do stuff*
如果每次在派生对象上调用Foo,它给出的结果与在Base对象上调用Foo完全相同,只要arg是相同的。