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The original object, called context, holds a reference to a strategy object. The context delegates executing the behavior to the linked strategy object. In order to change the way the context performs its work, other objects may replace the currently linked strategy object with another one.

Strategy pattern is also known as Policy Pattern. We define multiple algorithms and let client application pass the algorithm to be used as a parameter. One of the best example of strategy pattern is Collections.sort() method that takes Comparator parameter. Based on the different implementations of Comparator interfaces, the Objects are getting sorted in different ways. For our example, we will try to implement a simple Shopping Cart where we have two payment strategies - using Credit Card or using PayPal. First of all we will create the interface for our strategy pattern example, in our case to pay the amount passed as argument. PaymentStrategy.java

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This is wrong implementation of strategy, you either put an if else loop based on a string which says which payment method to use and then call that objects pay method, or pass the interface reference of payment Strategy in the pay method.

The strategy options mentioned in the Java Deflater originated in the zlib (C) implementation of ZLIB and (RFC 1950) and DEFLATE (1951), I believe. They are present in virtually all compression libraries that implement DEFLATE.

The bottom line is that, for the 80% case, you don't care to tweak strategy. You might be interested in fiddling with window bits, just to see what happens. But only do that when you've done everything else you need to do in your app.

In absence of Context, the caller has to fetch concrete strategy before executing strategy. If you are calling Add or Subtract strategy 100 times in your application, you have to expose particular concrete strategy.

Context decides at runtime which strategy has to be executed. If you create a concrete strategy instance yourself and invoke it then it becomes a compile time thing and not exactly what the strategy pattern aims for.

Before dependency injection came into picture, a class was needed at runtime which would do an if-else based on client needs and select the appropriate concrete strategy implementation. This class was called as Context.

When dependency injection came into picture then scenario specific concrete strategy instances could be inserted at runtime. So, the container doing this dependency injection started playing the role of the Context.

The purpose of using any design pattern is maximum cohesion and loose coupling.In the Strategy design pattern, the client interacts with the context and set the algorithm name or strategy name. So It maintains loose coupling between client and implementation.

I am currently learning design patterns on my own. As I studied the Strategy pattern I found something that looks strange for me. I looked for discussions on this pattern but none answered my question... which is how can I implement the Strategy pattern to let it be clean, to maintain encapsulation and to make adding a new strategy easy. To explain my problem here is the "canonical" strategy pattern:

I understand what is going on, but I feel strange to let the client knows something about the different strategies that can be applied. For me it would be cleaner to let Context instantiate the different strategies and not Client, since Context deals with strategies it should (at least in my mind) be the only one to be able to instantiate the strategy.

Through the gui a user can choose the strategy that he wants to use to sort the list. The user can click on a button to launch a sort. A call is done through the main class (not shown) to inst_field.mpv_sortContext.sortElements(a_string) with a string as parameters that describes the strategy to use. This string is then used in sortElements to select the instance of the strategy that the user wants to apply.

With my implementation I have on one side the client and on the other side all the code that deals with Strategies (Context, interface and concrete classes). If I want to add a new strategy I just have to add an instanciation of the new strategy in the Context class and a description of this new strategy in the gui to let the user knows about it.

I am aware that in the implementation I done is also not so nice because Context contains an instance for each possible strategy and because of that I don't need a reference to the interface, but I find it cleaner than letting the Field and the client knows about these classes.

Well... Am I totally wrong? Is there something I missed in the "canonical" strategy pattern. Is the "canonical" way the one and only way to implement the Strategy pattern? or Is there a better way to implement this pattern in a way that only the classes which should know are aware of the strategy instances and in a way that adding a new strategy can be easily done?

Your "strategy" isn't necessarily unclean, as you describe it and I think you may be getting bogged down with the idea of who the client is. Your client is providing the implementation to use but that may be a necessary implementation detail. For instance, the java RMI tutorial's ComputeEngine essentially uses just this pattern. The "compute" implementation is passed by the client - as only the client knows the computation to execute.

However, more commonly, the strategy is used to provide a way for logic to be made configurable within some context or to allow a common context to be tailored to specific uses. It also has the benefits of hiding internal structures from clients as required. Often to do this, the strategy to use will be configured internally to the context. This may be provided by:

I wold say that you are not wrong. It really depends on the purpose behind the use of the strategy. If it's an internal system concern then some rules should drive the selection (behind a factory). If it is configurable for whatever reason then it should be driven by configuration and the management hidden inside the context (class that manages the overall logic that uses the strategy). However, if it's dependent on user data or behaviour then either the data drives the selection internally or you have to accept that the client will have to pass you your strategy.

Note also that a goal behind this pattern is to remove conditional logic while keeping alternative implementations. So, if your strategy causes you to do lots of conditional logic then you may need to rethink if it clarifies your code.

You hold all the strategies in heap - it is not good. Firstly, strategy pattern often provides functionality for a long term, or even for all time app is running. So you don't need any another strategy except chosen. So, in case if you have a very big number of very big strategies, you'll hold in heap a lot of objects, that you don't need.

Let's say you're developing a turn-based strategy game with two kind of Units: Infantry and Tank (subclasses of Unit). Your terrain could be Plains, Railroad or Forests.

Units also are able to move, and have a field baseMovement that holds the number of hexagons it can walk. We're developing a strategy game, not a real world simulation, so we don't care how they move, we just want to add a value to their coordinates (in my example I only use X coordinate in order to get a simpler code). If all the terrain was the same, we wouldn't need any Strategy object... but we need to change the behaviour of move() method at runtime!

Polymorphism is a much broader concept in programming and yes the strategy pattern in Java uses a form of polymorphism catalogued as inclusion polymorphism to accomplish its intent, but this is by no means the only type of polymorphism that exists, neither it is the only way to implement the strategy pattern as I will demonstrate soon.

Also polymorphism is not something that only exists in Java or in object-oriented programming languages. Different forms of polymorphism exist in all the programming paradigms and not in all languages you are forced to use polymorphism to implement a strategy pattern (e.g. functional languages).

From my point of view the relations between these two concepts are: that the strategy pattern leverages the power of polymorphism available in languages like Java to implement its intent, and that polymorphism can be considered in itself a pattern.

As such, this would be an strategy pattern implementation without resorting to inclusion polymorphism at all. In a function composition strategy we may still be using other forms of polymorphism (e.g. parametric), but that is not a requirement for the strategy pattern

As discussed above, in Java we are probably forced to use inclusion polymorphism to implement a Strategy pattern, but as explained above as well, patterns are not something that belong to a specific language, so if we think of the strategy pattern as a concept living outside any language boundaries then you will easily see other languages implement this in different ways.

In this example, a straightforward polymorphism approach will result in massive code duplication. A more complex approach which places a intermediate class between animal and say robin fail to take in to consideration that how a animal moves is not really what defines it. Furthermore, it is possible that a animal has other strategy objects and they cannot all be made polymorphic through intermediate classes.

In Strategy pattern, we create objects which represent various strategies and a context object whose behavior varies as per its strategy object. The strategy object changes the executing algorithm of the context object. e24fc04721