🧩State Pattern – Overview

🧠 Concept

The State Pattern allows an object to change its behavior when its internal state changes. It appears as if the object’s class changes at runtime.

👉 It removes complex if-else or switch statements for state transitions and encapsulates state-specific behavior into separate classes.

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🧱 Key Roles

Role	Description
Context	The main object whose behavior changes depending on its state.
State (Interface)	Declares methods that represent behavior in a given state.
ConcreteState	Implements behavior specific to a particular state.

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💻 Java Example – Report Generator (States: Draft → Review → Published)

// Context
class ReportContext {
    private ReportState state;

    public ReportContext() {
        this.state = new DraftState(); // initial state
    }

    public void setState(ReportState state) {
        this.state = state;
    }

    public void request() {
        state.handle(this);
    }
}

// State interface
interface ReportState {
    void handle(ReportContext context);
}

// Concrete States
class DraftState implements ReportState {
    public void handle(ReportContext context) {
        System.out.println("Report in Draft → Sending for Review...");
        context.setState(new ReviewState());
    }
}

class ReviewState implements ReportState {
    public void handle(ReportContext context) {
        System.out.println("Report under Review → Publishing...");
        context.setState(new PublishedState());
    }
}

class PublishedState implements ReportState {
    public void handle(ReportContext context) {
        System.out.println("Report already Published. No further actions.");
    }
}

✅ Usage Example:

ReportContext report = new ReportContext();

report.request(); // Draft → Review
report.request(); // Review → Published
report.request(); // Already Published

/*
Output:
Report in Draft → Sending for Review...
Report under Review → Publishing...
Report already Published. No further actions.
*/

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🧠 Flow Summary (Step-by-Step)

Step	Current State	Action	Next State	Output
1️⃣	Draft	request()	Review	"Report in Draft → Sending for Review..."
2️⃣	Review	request()	Published	"Report under Review → Publishing..."
3️⃣	Published	request()	Published	"Report already Published. No further actions."

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🪜 Summary

• Encapsulates state-specific behavior into separate classes.

• Eliminates conditional logic for state transitions.

• Makes adding new states easier without modifying the core logic.

• Promotes Open/Closed and Single Responsibility principles.

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📊 Real-world Analogy

📄 Document Workflow: A report changes its behavior as it moves through stages: Draft → Review → Published — each state has its own rules and actions.