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

使用指向基类的指针或引用的函数必须能够在不知道它的情况下使用派生类的对象。

当我第一次阅读LSP时,我认为这是一个非常严格的含义,本质上等同于接口实现和类型安全强制转换。这意味着语言本身要么保证LSP,要么不保证LSP。例如,在严格意义上,ThreeDBoard当然可以取代Board,就编译器而言。

在阅读了更多关于LSP的概念之后,我发现LSP的解释通常比这更广泛。

简而言之,对于客户端代码来说,“知道”指针后面的对象是派生类型而不是指针类型的含义并不仅限于类型安全。对LSP的遵守也可以通过探测对象的实际行为进行测试。也就是说,检查对象的状态和方法参数对方法调用结果或从对象抛出的异常类型的影响。

再次回到示例,理论上Board方法可以在ThreeDBoard上很好地工作。然而,在实践中,在不妨碍ThreeDBoard打算添加的功能的情况下,防止客户端可能无法正确处理的行为差异是非常困难的。

掌握了这些知识后,评估LSP粘附性可以成为一个很好的工具,可以确定何时组合机制更适合扩展现有功能,而不是继承。

其他回答

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。

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。

以下是这篇文章的摘录,很好地澄清了事情:

(. .为了理解一些原则,重要的是要意识到它什么时候被违反了。这就是我现在要做的。

违反这一原则意味着什么?它意味着对象不履行用接口表示的抽象所施加的契约。换句话说,这意味着您错误地识别了抽象。

考虑下面的例子:

interface Account
{
    /**
     * Withdraw $money amount from this account.
     *
     * @param Money $money
     * @return mixed
     */
    public function withdraw(Money $money);
}
class DefaultAccount implements Account
{
    private $balance;
    public function withdraw(Money $money)
    {
        if (!$this->enoughMoney($money)) {
            return;
        }
        $this->balance->subtract($money);
    }
}

是否违反LSP?是的。这是因为帐户合同告诉我们帐户将被提取,但情况并非总是如此。那么,我该怎么做才能解决这个问题呢?我只是修改了合同:

interface Account
{
    /**
     * Withdraw $money amount from this account if its balance is enough.
     * Otherwise do nothing.
     *
     * @param Money $money
     * @return mixed
     */
    public function withdraw(Money $money);
}

Voilà,现在合同已得到满足。

这种微妙的违反通常会使客户有能力区分所使用的具体对象之间的差异。例如,给定第一个Account的契约,它看起来像下面这样:

class Client
{
    public function go(Account $account, Money $money)
    {
        if ($account instanceof DefaultAccount && !$account->hasEnoughMoney($money)) {
            return;
        }
        $account->withdraw($money);
    }
}

而且,这自动违反了开闭原则(即取款要求)。因为你永远不知道如果违反合同的对象没有足够的钱会发生什么。它可能什么都不返回,可能会抛出异常。所以你必须检查它是否hasEnoughMoney()——这不是接口的一部分。因此这种强制的依赖于具体类的检查违反了OCP。

这一点也解决了我经常遇到的关于LSP违反的误解。它说:“如果父母的行为在孩子身上改变了,那么它就违反了LSP。”然而,事实并非如此——只要孩子不违反父母的契约。

简单来说,LSP是指同一超类的对象应该能够在不破坏任何东西的情况下相互交换。

例如,如果我们有一个从Animal类派生的Cat和Dog类,那么任何使用Animal类的函数都应该能够使用Cat或Dog,并且行为正常。

这里有一个清单来确定你是否违反了利斯科夫法则。

如果你违反了以下项目之一->,你违反了里斯科夫。 如果你不违反任何->不能得出任何结论。

检查表:

No new exceptions should be thrown in derived class: If your base class threw ArgumentNullException then your sub classes were only allowed to throw exceptions of type ArgumentNullException or any exceptions derived from ArgumentNullException. Throwing IndexOutOfRangeException is a violation of Liskov. Pre-conditions cannot be strengthened: Assume your base class works with a member int. Now your sub-type requires that int to be positive. This is strengthened pre-conditions, and now any code that worked perfectly fine before with negative ints is broken. Post-conditions cannot be weakened: Assume your base class required all connections to the database should be closed before the method returned. In your sub-class you overrode that method and left the connection open for further reuse. You have weakened the post-conditions of that method. Invariants must be preserved: The most difficult and painful constraint to fulfill. Invariants are sometimes hidden in the base class and the only way to reveal them is to read the code of the base class. Basically you have to be sure when you override a method anything unchangeable must remain unchanged after your overridden method is executed. The best thing I can think of is to enforce these invariant constraints in the base class but that would not be easy. History Constraint: When overriding a method you are not allowed to modify an unmodifiable property in the base class. Take a look at these code and you can see Name is defined to be unmodifiable (private set) but SubType introduces new method that allows modifying it (through reflection): public class SuperType { public string Name { get; private set; } public SuperType(string name, int age) { Name = name; Age = age; } } public class SubType : SuperType { public void ChangeName(string newName) { var propertyType = base.GetType().GetProperty("Name").SetValue(this, newName); } }

还有2项:方法参数的逆变性和返回类型的协方差。但这在c#中是不可能的(我是c#开发人员),所以我不关心它们。