State

Type: Behavioral Purpose: Allows an object to alter its behavior when its internal state changes. The object will appear to change its class.

Problem Statement

Vending Machine. Behavior of "Insert Coin" or "Press Button" depends on current state (Idle, HasCoin, Dispensing, OutOfStock).

Bad Code (If-Else)

public void insertCoin() {
    if (state.equals("Idle")) accept();
    else if (state.equals("HasCoin")) reject();
    else if (state.equals("OutOfStock")) reject();
}

Problem: Adding a new state requires changing all methods.

Implementation (State Pattern)

// 1. State Interface
interface State {
    void insertCoin();
    void pressButton();
    void dispense();
}

// 2. Context (The Machine)
class VendingMachine {
    private State idleState;
    private State hasCoinState;
    private State currentState;
    
    public VendingMachine() {
        this.idleState = new IdleState(this);
        this.hasCoinState = new HasCoinState(this);
        this.currentState = this.idleState;
    }
    
    public void setState(State state) {
        this.currentState = state;
    }
    
    public void insertCoin() {
        currentState.insertCoin();
    }
    
    public void pressButton() {
        currentState.pressButton();
    }
   
    public State getIdleState() { return idleState; }
    public State getHasCoinState() { return hasCoinState; }
}

// 3. Concrete States
class IdleState implements State {
    private VendingMachine machine;
    
    public IdleState(VendingMachine machine) {
        this.machine = machine;
    }
    
    @Override
    public void insertCoin() {
        System.out.println("Coin inserted");
        machine.setState(machine.getHasCoinState());
    }
    
    @Override
    public void pressButton() {
        System.out.println("Insert coin first");
    }
    
    @Override
    public void dispense() {
        System.out.println("Insert coin first");
    }
}

class HasCoinState implements State {
    private VendingMachine machine;
    
    public HasCoinState(VendingMachine machine) {
        this.machine = machine;
    }
    
    @Override
    public void insertCoin() {
        System.out.println("Coin already inserted");
    }
    
    @Override
    public void pressButton() {
        System.out.println("Button pressed...");
        machine. setState(machine.getIdleState()); // Transition logic
    }
    
    @Override
    public void dispense() {
        System.out.println("Dispensing...");
    }
}

// Client
public class Main {
    public static void main(String[] args) {
        VendingMachine vm = new VendingMachine();
        vm.insertCoin();   // State changes to HasCoin
        vm.pressButton();  // Action allowed
    }
}

Class Diagram