State design pattern is a behavioural design pattern which provides an elegant solution for managing the behavior of an object based on its internal state. By encapsulating different states into separate classes and allowing the object to change its behavior dynamically, the State pattern promotes cleaner code, improved scalability, and increased flexibility.

Real World Analogy

A real-world analogy that can help understand the State design pattern is a vending machine.

In a vending machine, the behavior of the machine changes based on its internal state and the user’s actions. The different states of the vending machine can include “Idle,” “Selecting Product,” “Accepting Payment,” “Dispensing Product,” and “Out of Stock.”

When a user interacts with the vending machine by pressing buttons and inserting coins, the machine transitions between these states.

For example, when the user selects a product, the vending machine enters the “Selecting Product” state. Then, when the user inserts the required payment, the machine transitions to the “Accepting Payment” state. Finally, if the product is available and the payment is successful, the machine moves to the “Dispensing Product” state and delivers the selected item.

Applications of State Design Pattern

1. Traffic Control Systems: State machines are often used to model traffic lights. Each state represents a different light color (e.g., “Red,” “Yellow,” “Green”), and the state transitions occur based on timing or external events.
2. Document Editors: State machines can be employed in document editors to manage different editing modes such as “Insert Mode,” “Overwrite Mode,” and “Selection Mode.” Each state defines specific behavior and transitions based on user input.
3. Game Development: State machines are widely used in game development to control the behavior of game characters or entities. Different states such as “Attack,” “Defend,” or “Patrol” determine how the character responds to player input or game events.
4. Workflow Management Systems: State machines are utilized to model and control complex workflows. Each state represents a specific step or stage in the workflow, and the transitions occur based on predefined rules or conditions.
5. E-commerce Order Processing: State machines can be employed to manage the lifecycle of an e-commerce order. States such as “Placed,” “Shipped,” and “Delivered” represent the different stages of the order, and state transitions occur as the order progresses through the fulfillment process.
6. User Interface (UI) Management: State machines can be used to handle UI states and transitions in applications. This includes managing different screens, views, or components based on user actions or system events.
7. Network Protocols: State machines are often used to implement network protocols, where each state represents a different protocol state (e.g., “Handshake,” “Data Transfer,” “Connection Closed”), and transitions occur based on network events and protocol rules.

Applicability of State Design Pattern

The State design pattern is applicable in scenarios where object behavior varies based on internal states, and there is a need to manage state transitions, encapsulate state-specific behavior, and achieve a flexible and extensible design.

Here are a few scenarios:

1. Object behavior depends on internal state: When the behavior of an object needs to vary based on its internal state and when the state changes, the State pattern can be applied. It allows the object to delegate its behavior to different state objects, promoting clean and modular code.
2. Multiple conditional statements: If an object has multiple conditional statements that determine its behavior, using the State pattern can simplify the code by encapsulating each behavior into separate state classes. This improves code readability and maintainability.
3. State-specific operations: When different states require specific operations or algorithms, the State pattern can be used to encapsulate those operations within the corresponding state classes. This helps in achieving better organization and separation of concerns.
4. State transitions and dynamic behavior: If an object needs to transition between different states and its behavior should change dynamically based on the current state, the State pattern provides a structured approach to handle these transitions and manage the associated behavior.
5. Context-based behavior: In situations where the behavior of an object is influenced by the context in which it operates, the State pattern can be employed to represent the different contexts as states and define behavior accordingly. This allows for flexible and context-specific behavior.
6. Extensibility and flexibility: The State pattern facilitates the addition of new states and behaviors without modifying existing code. It allows for easy extension of the system by introducing new state classes, making it suitable for situations where future changes and enhancements are expected.

State Design Pattern Implementation

Key Components of State Design Pattern

• Context: It represents the object whose behavior changes based on different states. The context maintains a reference to the current state object and delegates the behavior-related requests to that state.
• State: It defines the interface for encapsulating the behavior associated with a particular state of the context. The state interface declares methods that handle the requests and state transitions.
• Concrete State: These are the classes that implement the State interface. Each concrete state encapsulates a specific behavior associated with a state of the context. The context delegates requests to the current concrete state object.

implementation steps

• Identifying the Context and States: To apply the State pattern, start by identifying the context object that exhibits varying behavior based on internal states. Determine the distinct states that affect the behavior of the context. Each state should be defined as a separate class implementing the State interface.
• Creating the State Interface: The State interface defines the methods that handle requests and state transitions. This interface provides a contract for all concrete state classes, ensuring they implement the necessary behavior methods.

// State interface
interface State {
    void doAction(Context context);
}

• Implementing Concrete State Classes: Create concrete state classes that implement the State interface. Each concrete state class encapsulates the behavior associated with a specific state. These classes define the actions performed in response to requests and control the state transitions of the context.

// Concrete state classes
class StateA implements State {
    public void doAction(Context context) {
        System.out.println("State A: Performing action...");
        context.setState(new StateB());
    }
}

class StateB implements State {
    public void doAction(Context context) {
        System.out.println("State B: Performing action...");
        context.setState(new StateC());
    }
}

class StateC implements State {
    public void doAction(Context context) {
        System.out.println("State C: Performing action...");
        context.setState(new StateA());
    }
}

• Integrating the State Pattern with the Context: In the context class, maintain a reference to the current state object. Delegate the requests received by the context to the current state object, allowing the behavior to change dynamically based on the current state. The context can change its state by setting a new state object.

// Context class
class Context {
    private State currentState;

    public Context() {
        currentState = new StateA();
    }

    public void setState(State state) {
        currentState = state;
    }

    public void doAction() {
        currentState.doAction(this);
    }
}

// Usage example
public class StatePatternExample {
    public static void main(String[] args) {
        Context context = new Context();

        context.doAction(); // State A
        context.doAction(); // State B
        context.doAction(); // State C
        context.doAction(); // State A
    }
}

Example of State Design Pattern

This example reflects a real-world scenario of package delivery, where the behavior of a package changes based on its delivery state. It illustrates how the State pattern can be used to model and manage the different states and their associated behaviors in a clean and flexible manner.

// State interface
interface PackageState {
    void updateState(DeliveryContext context);
}

// Concrete state classes
class OrderedState implements PackageState {
    public void updateState(DeliveryContext context) {
        System.out.println("Package is in the ordered state.");
        context.setState(new ShippedState());
    }
}

class ShippedState implements PackageState {
    public void updateState(DeliveryContext context) {
        System.out.println("Package is in the shipped state.");
        context.setState(new DeliveredState());
    }
}

class DeliveredState implements PackageState {
    public void updateState(DeliveryContext context) {
        System.out.println("Package is in the delivered state.");
        // Additional logic or actions specific to the delivered state
    }
}

// Context class
class DeliveryContext {
    private PackageState currentState;

    public DeliveryContext() {
        currentState = new OrderedState();
    }

    public void setState(PackageState state) {
        currentState = state;
    }

    public void update() {
        currentState.updateState(this);
    }
}

// Usage example
public class StatePatternExample {
    public static void main(String[] args) {
        DeliveryContext context = new DeliveryContext();

        context.update(); // Transition to ShippedState
        context.update(); // Transition to DeliveredState
    }
}

In this example, we have a PackageState interface that defines the updateState() method, representing the behavior associated with each state of a package during delivery. The concrete state classes (OrderedState, ShippedState, and DeliveredState) implement the PackageState interface and define their specific behavior and state transitions.

The DeliveryContext class represents the context in which the package is being delivered. It maintains the current state and delegates the updateState() method call to the current state object. It also provides a setState() method to change the state dynamically.

In the main() method of the StatePatternExample class, we create an instance of the DeliveryContext and simulate the progression of a package through different states by calling the update() method. The output demonstrates the dynamic behavior change as the package transitions from an ordered state to a shipped state and finally to a delivered state.

Best Practices & Considerations

When implementing the State design pattern, it’s important to keep the following best practices and considerations in mind:

1. Identify the State and Context: Clearly define the states that your object can be in and identify the context or object that will be affected by state changes. This helps in properly structuring the pattern implementation.
2. Separate State-specific Behavior: Encapsulate the behavior associated with each state into separate state classes. This ensures that each state has its own responsibilities and makes the code easier to understand and maintain.
3. Define a State Interface or Abstract Class: Create a common interface or abstract class that all state classes implement. This promotes consistency and allows the context to interact with the states in a uniform manner.
4. Encapsulate State Transitions: Handle state transitions within the context or state classes themselves. This keeps the logic centralized and ensures that state transitions are managed consistently and appropriately.
5. Favor Composition over Inheritance: Use composition to manage the relationship between the context and state objects. This allows for flexibility in switching between different state implementations and avoids tight coupling.
6. Consider Concurrent Access: If your application involves concurrent access to the context object and state changes, ensure thread safety by using appropriate synchronization mechanisms to avoid race conditions.
7. Document State Transitions: Clearly document the allowed state transitions and the conditions under which they occur. This helps in understanding the system behavior and ensures that state changes are properly controlled.
8. Test Different State Scenarios: Create comprehensive unit tests to verify the behavior of the object in different states and state transitions. This ensures that the State pattern is implemented correctly and functions as expected.
9. Keep Code Cohesive and Simple: Aim for clean and concise code by following the Single Responsibility Principle (SRP) and keeping the responsibilities of each class focused. Avoid introducing unnecessary complexity or coupling between classes.
10. Consider Other Design Patterns: The State pattern can often be used in conjunction with other design patterns. Consider the specific requirements of your system and explore if combining the State pattern with other patterns, such as the Strategy or Decorator pattern, can provide a more effective solution.

Pros & Cons of State Design Pattern

Comparing State Pattern with Alternative Approaches

• Conditional statements: Traditional approaches to managing state transitions often involve if/else or switch statements. While they may be suitable for simple cases, they become unwieldy and hard to maintain as the number of states and transitions grows. The State pattern provides a more structured and scalable approach, especially when dealing with complex state behavior.
• State Machine Frameworks: (such as spring StateMachine) State machine frameworks offer a higher-level abstraction for managing state transitions. These frameworks provide tools and features specifically designed for state management. While they can be powerful, they may introduce additional complexity, dependencies, and learning curves.
  The State pattern, on the other hand, is a language-agnostic design pattern that can be implemented using basic object-oriented principles.
• Inheritance: Inheritance can be used to model different states as subclasses of a common base class. Each subclass can override methods to provide state-specific behavior. While this approach allows for state-specific customization, it can lead to class explosion as the number of states grows. Additionally, it may not be flexible if states need to change dynamically during runtime.
• Strategy Pattern: The Strategy pattern allows behavior to be encapsulated in separate strategy objects, which can be dynamically swapped at runtime. This approach can be used as an alternative to the State pattern when behavior needs to vary independently of an object’s internal state. However, if state-specific behavior and state transitions are the primary concern, the State pattern provides a more explicit and focused solution.
• Enumerations: In some cases, where the states are limited and fixed, enumerations can be used to represent states. Each state can have associated methods and properties to define behavior. This approach works well for simple systems but may lack the flexibility and extensibility provided by the State pattern.
• Event-driven Architecture: an event-driven architecture can be employed where object behavior is triggered by events and event handlers. This approach allows for loosely coupled components and dynamic behavior changes based on events. However, it may not be suitable for systems with complex state-dependent behavior.

Relation With Other Patterns

Here are a few design patterns that are commonly used in conjunction with the State pattern:

1. Strategy Pattern: The Strategy pattern is closely related to the State pattern. Both patterns involve encapsulating behavior and allowing it to vary independently. While the State pattern focuses on managing object behavior based on internal states, the Strategy pattern focuses on encapsulating interchangeable algorithms. In some cases, the State pattern can be seen as a specific use of the Strategy pattern, where the context object represents the strategy context and the states represent the different strategies.
2. Decorator Pattern: The Decorator pattern can be used to enhance the behavior of objects based on their current state. By combining the State pattern with the Decorator pattern, you can dynamically add additional responsibilities to an object based on its state. This allows for a flexible and extensible way to modify behavior without changing the core implementation.
3. Observer Pattern: The Observer pattern can be used to notify interested objects about state changes in the State pattern. The context object can act as a subject, and the state objects can act as observers that are notified when the state changes. This allows other objects to react and respond to state transitions, enabling better coordination and interaction between different components.
4. Singleton Pattern: In some cases, the State pattern can be combined with the Singleton pattern to ensure that there is only one instance of each state object. This ensures that state objects are shared among multiple context objects and helps in conserving memory and maintaining consistency across the system.
5. Composite Pattern: The Composite pattern can be used to represent complex states or state hierarchies in the State pattern. By using the Composite pattern, you can create composite state objects that contain substates, allowing for hierarchical state structures and more sophisticated state management.