What are the differences between the Strategy design pattern and the State design pattern? I was going through quite a few articles on the web but could not make out the difference clearly.
Can someone please explain the difference in layman's terms?
Honestly, the two patterns are pretty similar in practice, and the defining difference between them tends to vary depending on who you ask. Some popular choices are:
A "classic" implementation would match either State or Strategy for every item on the list, but you do run across hybrids that have mixes of both. Whether a particular one is more State-y or Strategy-y is ultimately a subjective question.
getStatus() method that will return different statuses based on the
state of the object, but the caller of the method doesn't have to be
coded differently to account for each potential state.The difference simply lies in that they solve different problems:
The constructs for achieving these different goals are however very similar; both patterns are examples of composition with delegation.
Some observations on their advantages:
By using the State pattern the state-holding (context) class is relieved from knowledge of what state or type it is and what states or types that are available. This means that the class adheres to the open-closed design principle (OCP): the class is closed for changes in what states/types there are, but the states/types are open to extensions.
By using the Strategy pattern the algorithm-using (context) class is relieved from knowledge of how to perform a certain task (-- the "algorithm"). This case also creates an adherence to the OCP; the class is closed for changes regarding how to perform this task, but the design is very open to additions of other algorithms for solving this task.
This likely also improves the context class' adherence to the single responsibility principle (SRP). Further the algorithm becomes easily available for reuse by other classes.
Can somebody please explain in layman's terms?
Design patterns are not really "layman" concepts, but I'll try to make it as clear as possible. Any design pattern can be considered in three dimensions:
Let's compare State and Strategy.
State is used in one of two cases [GoF book p. 306]:
- An object's behavior depends on its state, and it must change its behavior at run-time depending on that state.
- Operations have large, multipart conditional statements that depend on the object's state. This state is usually represented by one or more enumerated constants. Often, several operations will contain this same conditional structure. The State pattern puts each branch of the conditional in a separate class. This lets you treat the object's state as an object in its own right that can vary independently from other objects.
If you want to make sure you indeed have the problem the State pattern solves, you should be able to model the states of the object using a finite state machine. You can find an applied example here.
Each state transition is a method in the State interface. This implies that for a design, you have to be pretty certain about state transitions before you apply this pattern. Otherwise, if you add or remove transitions, it will require changing the interface and all the classes that implement it.
I personally haven't found this pattern that useful. You can always implement finite state machines using a lookup table (it's not an OO way, but it works pretty well).
Strategy is used for the following [GoF book p. 316]:
- many related classes differ only in their behavior. Strategies provide a way to configure a class with one of many behaviors.
- you need different variants of an algorithm. For example, you might define algorithms reflecting different space/time trade-offs. Strategies can be used when these variants are implemented as a class hierarchy of algorithms [HO87].
- an algorithm uses data that clients shouldn't know about. Use the Strategy pattern to avoid exposing complex, algorithm-specific data structures.
- a class defines many behaviors, and these appear as multiple conditional statements in its operations. Instead of many conditionals, move related conditional branches into their own Strategy class.
The last case of where to apply Strategy is related to a refactoring known as Replace conditional with polymorphism.
Summary: State and Strategy solve very different problems. If your problem can't be modeled with a finite state machine, then likely State pattern isn't appropriate. If your problem isn't about encapsulating variants of a complex algorithm, then Strategy doesn't apply.
State has the following UML class structure:
Strategy has the following UML class structure:
Summary: in terms of the static structure, these two patterns are mostly identical. In fact, pattern-detecting tools such as this one consider that "the structure of the [...] patterns is identical, prohibiting their distinction by an automatic process (e.g., without referring to conceptual information)."
There can be a major difference, however, if ConcreteStates decide themselves the state transitions (see the "might determine" associations in the diagram above). This results in coupling between concrete states. For example (see the next section), state A determines the transition to state B. If the Context class decides the transition to the next concrete state, these dependencies go away.
As mentioned in the Problem section above, State implies that behavior changes at run-time depending on some state of an object. Therefore, the notion of state transitioning applies, as discussed with the relation of the finite state machine. [GoF] mentions that transitions can either be defined in the ConcreteState subclasses, or in a centralized location (such as a table-based location).
Let's assume a simple finite state machine:
Assuming the subclasses decide the state transition (by returning the next state object), the dynamic looks something like this:
To show the dynamics of Strategy, it's useful to borrow a real example.
Summary: Each pattern uses a polymorphic call to do something depending on the context. In the State pattern, the polymorphic call (transition) often causes a change in the next state. In the Strategy pattern, the polymorphic call does not typically change the context (e.g., paying by credit card once doesn't imply you'll pay by PayPal the next time). Again, the State pattern's dynamics are determined by its corresponding fininte state machine, which (to me) is essential to correct application of this pattern.
The Strategy Pattern involves moving the implementation of an algorithm from a hosting class and putting it in a separate class. This means that host class does not need to provide the implementation of each algorithm itself, which is likely to lead to unclean code.
Sorting algorithms are usually used as an example as they all do the same kind of thing (sort). If each differing sorting algorithm is put into its own class, then the client can easily choose which algorithm to use and the pattern provides an easy way to access it.
The State Pattern involves changing the behaviour of an object when the state of the object changes. This means that the host class does not have provide the implementation of behaviour for all the different states that it can be in. The host class usually encapsulates a class which provides the functionality that is required in a given state, and switches to a different class when the state changes.
Consider an IVR (Interactive Voice Response) system handling customer calls. You may want to program it to handle customers on:
To handle this situation you can use a State Pattern.
This process of connecting a customer to a support executive can itself be implemented using a Strategy Pattern where the executives are picked based on either of:
The strategy pattern decides on 'how' to perform some action and state pattern decides on 'when' to perform them.
Strategy represents objects that "do" something, with the same begin and end results, but internally using different methodologies. In that sense they are analogous to representing the implementation of a verb. The State pattern OTOH uses objects that "are" something - the state of an operation. While they can represent operations on that data as well, they are more analogous to representation of a noun than of a verb, and are tailored towards state machines.
Strategy: the strategy is fixed and usually consists of several steps. (Sorting constitutes only one step and thus is a very bad example as it is too primitive in order to understand the purpose of this pattern). Your "main" routine in the strategy is calling a few abstract methods. E.g. "Enter Room Strategy", "main-method" is goThroughDoor(), which looks like: approachDoor(), if (locked()) openLock(); openDoor(); enterRoom(); turn(); closeDoor(); if (wasLocked()) lockDoor();
Now subclasses of this general "algorithm" for moving from one room to another room through a possible locked door can implement the steps of the algorithm.
In other words subclassing the strategy does not change the basic algorithms, only individual steps.
THAT ABOVE is a Template Method Pattern. Now put steps belonging together (unlocking/locking and opening/closing) into their own implementing objects and delegate to them. E.g. a lock with a key and a lock with a code card are two kinds of locks. Delegate from the strategy to the "Step" objects. Now you have a Strategy pattern.
A State Pattern is something completely different.
You have a wrapping object and the wrapped object. The wrapped one is the "state". The state object is only accessed through its wrapper. Now you can change the wrapped object at any time, thus the wrapper seems to change its state, or even its "class" or type.
E.g. you have a log on service. It accepts a username and a password. It only has one method: logon(String userName, String passwdHash). Instead of deciding for itself whether a log on is accepted or not, it delegates the decision to a state object. That state object usually just checks if the user/pass combination is valid and performs a log on. But now you can exchange the "Checker" by one that only lets priviledged users log on (during maintanace time e.g.) or by one that lets no one log on. That means the "checker" expresses the "log on status" of the system.
The most important difference is: when you have choosen a strategy you stick with it until you are done with it. That means you call its "main method" and as long as that one is running you never change the strategy. OTOH in a state pattern situation during the runtime of your system you change state arbitrarily as you see fit.
Strategy pattern is used when you have multiple algorithm for a specific task and client decides the actual implementation to be used at runtime.
UML diagram from wiki Strategy pattern article:
Key features:
Refer to this post for more info & real world examples:
Real World Example of the Strategy Pattern
State pattern allows an object to alter its behaviour when its internal state changes
UML diagram from wiki State pattern article:
If we have to change the behavior of an object based on its state, we can have a state variable in the Object and use if-else condition block to perform different actions based on the state. State pattern is used to provide a systematic and lose-coupled way to achieve this through Context and State implementations.
Refer to this journaldev article for more details.
Key differences from sourcemaking and journaldev articles:
In layman's language,
in Strategy pattern, there are no states or all of them have same state. All one have is different ways of performing a task, like different doctors treat same disease of same patient with same state in different ways.
In state Pattern, subjectively there are states, like patient's current state(say high temperature or low temp), based on which next course of action(medicine prescription) will be decided.And one state can lead to other state, so there is state to state dependency( composition technically).
If we technically try to understand it , based on code comparison of both, we might lose the subjectivity of situation,because both look very similar.
State comes with a little bit dependencies within the state derived classes: like one state knows about other states coming after it. For example, Summer comes after winter for any season state, or Delivery state after the Deposit state for shopping.
On the other hand, Strategy has no dependencies like these. Here, any kind of state can be initialized based on the program/product type.
Both patterns delegate to a base class that has several derivative, but it's only in the State pattern that these derivative classes hold a reference back to context class.
Another way to look at it is that the Strategy pattern is a simpler version of the State pattern; a sub-pattern, if you like. It really depends if you want the derived states to hold references back to the context or not (i.e: do you want them to call methods on the context).
For more info: Robert C Martin (& Micah Martin) answer this in their book, "Agile Principles, Patterns and Practices in C#". (http://www.amazon.com/Agile-Principles-Patterns-Practices-C/dp/0131857258)
Both Strategy and State pattern has the same structure. If you look at the UML class diagram for both patterns they look exactly same, but their intent is totally different. State design pattern is used to define and manage state of an object, while Strategy pattern is used to define a set of interchangeable algorithm and lets client to choose one of them. So Strategy pattern is a client driven pattern while Object can manage there state itself.
This is a pretty old question, but still, I was also looking for the same answers and this is what I have discovered.
For State pattern lets consider an example of Medial Player Play button. When we do play it starts playing and makes the context aware that it is playing. Every time the client wants to perform play operation he checks the current state of the player. Now the client knows the state of the object is playing via the context object so he calls the pause state objects actions method. The part of the client realizing the state and on what state it needs to do action can be automated.
https://www.youtube.com/watch?v=e45RMc76884 https://www.tutorialspoint.com/design_pattern/state_pattern.htm
In the case of Strategy pattern, the arrangement of the class diagram is same as state pattern. The client comes to this arrangement to do some operation. That is instead of the different states there are different algorithms say for example different analysis that needs to be performed on the pattern. Here the clients tell the context what it wants to do that what algorithm (business defined custom algorithm) and then performs that.
https://www.tutorialspoint.com/design_pattern/strategy_pattern.htm
Both implements open close principle so the developer has the capability to add new states to the state pattern and new algorithm.
But the difference is what they are used that is state pattern used to execute different logic based on a state of the object. And in a case of strategy different logic.
The difference is discussed in http://c2.com/cgi/wiki?StrategyPattern. I have used the Strategy pattern for allowing different algorithms to be chosen within an overall framework for analysing data. Through that you can add algorithms without having to change the overall frameworks and its logic.
A typical example is that you amy have a framework for optimising a function. The framework sets up the data and parameters. The strategy pattern allows you to select algorithms such as sttepest descents, conjugate gradients, BFGS, etc. without altering the framework.
As wikipedia says about State pattern:
The state pattern is a behavioral software design pattern that allows an object to alter its behavior when its internal state changes. This pattern is close to the concept of finite-state machines.
Let us talk about real world example, it is a steering wheel in automobile. Steering wheel can be replaced. We can set bigger or smaller steering wheel. It is not a rule, however, let us think that small steering wheel makes bigger angle of automobile front wheels, than bigger steering wheel.
So, we can conclude that our automobile behaves differently dependent on the steering tool we set. For example, if we set smaller steering wheel, our automobile will turn left or right faster.
Thus, automobile responds to the events such as TurnLeft() or TurnRight(). However, the angle of the automobile wheels which can be turned depending on the currently selected steering wheel. Let us try to code:
public interface ISteeringWheel
{
void TurnLeft();
void Straight();
void TurnRight();
}
public class BigSteeringWheel : ISteeringWheel
{
public void Straight()
{
Console.WriteLine("BigSteeringWheel is straight");
}
public void TurnLeft()
{
Console.WriteLine("BigSteeringWheel is turned left 10
degrees");
}
public void TurnRight()
{
Console.WriteLine("BigSteeringWheel is turned right 10
degrees");
}
}
public class SmallSteeringWheel : ISteeringWheel
{
public void Straight()
{
Console.WriteLine("SmallHandleBar is straight");
}
public void TurnLeft()
{
Console.WriteLine("SmallHandleBar is turned left
20 degrees");
}
public void TurnRight()
{
Console.WriteLine("SmallHandleBar is turned right 20
degrees");
}
}
and Automobile class:
public class Automobile
{
public ISteeringWheel SteeringWheel { get; private set; }
public Automobile()
{
SteeringWheel = new BigSteeringWheel();
}
public void TurnLeft()
{
SteeringWheel.TurnLeft();
}
public void TurnRight()
{
SteeringWheel.TurnRight();
}
public void SetSteeringWheel(ISteeringWheel handleBar)
{
SteeringWheel = handleBar;
}
}
Strategy pattern:
Definition from the Wikipedia:
The strategy pattern (also known as the policy pattern) is a behavioral software design pattern that enables selecting an algorithm at runtime. Instead of implementing a single algorithm directly, code receives run-time instructions as to which in a family of algorithms to use.
Pay attention to words such as «family of algorithms to use». So let us imagine we have a real automobile and when a driver turns the steering wheel left, we want that our automobile will do the following actions:
So, the above two actions could be considered as «family algorithms to use». Let us code this example.
The Steering wheel algorithm:
public interface ISteeringWheel
{
void TurnLeft();
void Straight();
void TurnRight();
}
public class BigSteeringWheel : ISteeringWheel
{
public void Straight()
{
Console.WriteLine("BigSteeringWheel is straight");
}
public void TurnLeft()
{
Console.WriteLine("BigSteeringWheel is turned left
10 degrees");
}
public void TurnRight()
{
Console.WriteLine("BigSteeringWheel is turned right
10 degrees");
}
}
The turn signal algorithm:
public interface ITurnSignal
{
void TurnOnLeft();
void TurnOnRight();
}
public class OrangeTurnSignal : ITurnSignal
{
public void TurnOnLeft()
{
Console.WriteLine("Left OrangeTurnSignal is turned on");
}
public void TurnOnRight()
{
Console.WriteLine("Right OrangeTurnSignal is turned on");
}
}
And the automobile class:
public class Automobile
{
public ISteeringWheel SteeringWheel { get; private set; }
public ITurnSignal TurnSignal { get; private set; }
public Automobile()
{
SteeringWheel = new BigSteeringWheel();
TurnSignal = new OrangeTurnSignal();
}
public void TurnLeft()
{
SteeringWheel.TurnLeft();
TurnSignal.TurnOnLeft();
}
public void TurnRight()
{
SteeringWheel.TurnRight();
TurnSignal.TurnOnRight();
}
}
CONCLUSION:
The State pattern and the Strategy pattern look very similar to each other. However, there is a tiny difference that State pattern has a single state and all behaviors such as «TurnLeft» and «TurnRight» are encapsulated in one class. On the other hand, Strategy pattern does not have a single state, but it has many states such as «SteeringWheel» and «TurnSignal». These different behaviors are encapsulated using different strategy objects such as «SteeringWheel» and «TurnSignal» objects. Therefore, this is a main difference between State and Strategy pattern.
In addition, we can think of the Strategy pattern as a nice alternative to subclassing. Inheritance gives to us a very tight coupling between classes and this coupling between classes is defined at compile time. However, Strategy pattern uses composition that allows setting behavior at run time by composing with a different object.
State pattern is also can be considered as an alternative to replace many if — else statements in class.
When you have a project which can be divided into 2 tasks:
task 1: you can use one of two different algorithms to accomplish: alg1, alg2
task 2: you can use one of three different algorithms to accomplish: alg3, alg4, alg5
alg1 and alg2 are interchangeable; alg3, alg4 and alg5 are interchangeable.
Choosing which algorithm to perform in task 1 and task 2 depends on states:
state 1: you need alg1 in task 1 and alg3 in task 2
state 2: you need alg2 in task 1 and alg5 in task 2
You context can change state object from state 1 to state 2. Then your task would be accomplished by alg2 and alg5, instead of alg1 and alg3.
You can add more interchangeable algorithms for task 1 or task 2. This is strategy pattern.
You can have more states with different combination of algorithms in task 1 and task 2. State pattern allows you to switch from one state to another and perform different combination of algorithms.
'Strategy' is only an algorithm that you can change it in different circumstances upon your need, and it processes something for you. Ex. you can choose how compress a file. zip or rar ... in a method.
But 'State' CAN change all your object behaviour, when it changes, Even it can change other fields... that's why it has a reference to its owner. You should notice that changing an object field can change object behaviour at all. Ex. when you change State0 to State1 in obj, you change an integer to 10.. so when we call obj.f0() that do some calculation and use that integer, it affects the result.
Both of the patterns are used for changing the behavior of an object,
By design, the state pattern object has a single state and the behavior of an object is based on the implemented single state (Class) and its subclasses.
In contrast, the strategy doesn't have a single state, and the behavior of an object is determined by the implementation of the different strategy objects.
