# Purpose

The State pattern allows an object to alter its behavior when its internal state changes. It encapsulates variable states as independent objects.

# Problem

• Variable Behavior: The object must act differently depending on its state at runtime.

• Complex Conditionals: Avoid massive if/switch statements that depend on state variables.

# Solution

Create separate classes for each state. The main object (Context) delegates execution to the current state object.

# Structure

# Participants

• Context: Maintains the reference to the current state and delegates tasks.
• State (Interface): Defines the interface for the states.
• ConcreteState: Specific implementation of each behavior.

# When to Use It

When an object has clearly identifiable states that drastically alter its behavior in response to the same method calls.

# Example: Traffic Light

Management of Red, Yellow and Green states with automatic transition rules.

# Java Code

interface State { void handle(Context c); }

class GreenState implements State {
    public void handle(Context c) {
        System.out.println("Green light");
        c.setState(new YellowState());
    }
}

class Context {
    private State state = new GreenState();
    public void change() { state.handle(this); }
}

# Mapping Participants

• TrafficLightState: State Interface.
• RedState/GreenState: ConcreteStates.
• TrafficLightContext: Context.

# Conclusions

It eliminates the need for tangled conditionals and makes the transition flow explicit and easy to audit.

# Related Patterns