谁能给我解释一下模板方法模式和策略模式的区别是什么?

据我所知,它们99%是一样的——唯一的区别是 模板方法模式有一个抽象类作为基础 类,而策略类使用已实现的接口 由每个具体的策略类。

然而,就客户端而言,它们是以完全相同的方式被消费的——这是正确的吗?


当前回答

两者都非常相似,客户端代码以类似的方式使用它们。与上面最流行的答案不同,两者都允许在运行时选择算法。

The difference between the two is that while the strategy pattern allows different implementations to use completely different ways of the achieving the desired outcome, the template method pattern specifies an overarching algorithm (the "template" method) which is be used to achieve the result -- the only choice left to the specific implementations (sub-classes) are certain details of the said template method. This is done by having the the template method make call(s) to one or more abstract methods which are overridden (i.e. implemented) by the sub-classes, unlike the template method which itself is not abstract and not overridden by the sub-classes.

客户端代码使用抽象类类型的引用/指针调用模板方法,该引用/指针指向具体子类之一的实例,该实例可以在运行时确定,就像使用策略模式时一样。

其他回答

我建议你读一下这篇文章。它解释了一个实际案例的差异。

引用自文章

"As one can see implementing classes also depend upon the template method class. This dependency causes to change the template method if one wants to change some of the steps of the algorithm. On the other side strategy completely encapsulates the algorithm. it gives the implementing classes to completely define an algorithm. Therefore if any change arrives one does need to change the code for previously written classes. This was the primary reason I choose strategy for designing up the classes. One feature of template method is that template method controls the algorithm. Which can be a good thing in other situation but in my problem this was restricting me to design the classes. On the other side strategy does not control the steps of an algorithm which enables me to add completely different conversion methods. Hence in my case strategy helps me for implementation. One drawback of strategy is that there is too much code redundancy and less code sharing. As it is obvious in the presented example of this article I have to repeat the same code in four classes again and again. Therefore it is hard to maintain because if the implementation of our system such as step 4 which is common to all is changed then I will have to update this in all 5 classes. On the other side, in template method, I can only change the superclass and the changes are reflected into the sub classes. Therefore template method gives a very low amount of redundancy and high amount of code sharing among the classes. Strategy also allows changing the algorithm at run-time. In template method one will have to re-initialize the object. This feature of strategy provide large amount of flexibility. From design point of view one has to prefer composition over inheritance. Therefore using strategy pattern also became the primary choice for development."

不,它们的消费方式不一定相同。“模板方法”模式是为未来的实现者提供“指导”的一种方式。您告诉他们,“所有Person对象都必须有一个社会安全号码”(这是一个微不足道的例子,但它正确地传达了思想)。

策略模式允许在多个可能的实现之间切换。它(通常)不是通过继承实现的,而是通过让调用者传入所需的实现来实现的。例如,允许为ShippingCalculator提供几种不同的计算税收的方法之一(可能是NoSalesTax实现和PercentageBasedSalesTax实现)。

有时候,客户端会告诉对象使用哪种策略。就像在

myShippingCalculator.CalculateTaxes(myCaliforniaSalesTaxImpl);

但是客户端永远不会为基于模板方法的对象这样做。事实上,客户端甚至可能不知道对象是基于模板方法的。模板方法模式中的那些抽象方法甚至可能受到保护,在这种情况下,客户端甚至不知道它们的存在。

模板方法模式善于阐明算法的整体步骤,而策略模式适合灵活性和可重用性,因此如果需要,可以将策略组合在一起,例如:jdk8中的许多功能接口,如Comparator.reversed().那么比较(Comparator)是策略的一个角色。

模板方法模式侧重于更高的内聚性,而策略模式则与上下文对象松散耦合以分离关注点。

策略易于维护,因为上下文不知道具体的策略,无论主要算法在上下文中发生什么变化都不会影响策略。另一方面,如果在抽象模板类中改变算法的框架,可能会影响其子类的升级。

在策略模式中,子类起主导作用,它们控制算法。这里的代码是跨子类复制的。算法的知识和如何实现它分布在许多类中。

在模板模式中,基类具有算法。它最大化了子类之间的重用。由于算法在一个地方,基类保护它。

You probably mean template method pattern. You are right, they serve very similar needs. I would say it is better to use template method in cases when you have a "template" algorithm having defined steps where subclasses override these steps to change some details. In case of strategy, you need to create an interface, and instead of inheritance you are using delegation. I would say it is a bit more powerful pattern and maybe better in accordance to DIP - dependency inversion principles. It is more powerful because you clearly define a new abstraction of strategy - a way of doing something, which does not apply to template method. So, if this abstraction makes sense - use it. However, using template method may give you simpler designs in simple cases, which is also important. Consider which words fit better: do you have a template algorithm? Or is the key thing here that you have an abstraction of strategy - new way of doing something

模板方法的例子:

Application.main()
{
Init();
Run();
Done();
}

这里你继承了application,并替换了init, run和done的操作。

策略的例子:

array.sort (IComparer<T> comparer)

在这里,当编写比较器时,您不继承数组。数组将比较算法委托给比较器。