Introduction
The State Pattern is a behavioral design pattern that allows an object to change its behavior when its internal state changes. Instead of having a monolithic object that manages its behavior through conditional statements, we transition between objects representing different states.

Motivation
Consider a scenario where a class has multiple states, and its behavior changes based on the state. Managing this using conditional or switch statements can make the code complex and hard to maintain. The State Pattern aims to overcome this issue by representing each state as a separate class, promoting code organization and flexibility.

Implementation
When developing an application that runs on a vending machine, it is important to ensure the customers get the product that was selected. when a customer selects an product, it goes through various states:

  1. No Selection State
  2. Has Selection State
  3. Dispensing State

No Selection state
In this state, the vending machine is waiting for a customer to select for a product.

Has Selection State
In this state, the vending machine has a product selection made by the customer and is ready to dispense the selected product.

Dispensing State
In this state, the vending machine is actively dispensing the selected product to the customer.

  • In a typical scenario, this is how the system transitions between states:
  • Initially, the vending machine is in the "No Selection" state.
  • When a customer selects a product, it transitions to the "Has Selection" state.
  • When the customer confirms the purchase, it transitions to the "Dispensing" state.
  • After dispensing, it goes back to the "No Selection" state.

The method in VendingMachineState outlines what actions can be carried out on the vending machine, different selection state extends this behavior and modify it to suit their own needs.

public interface VendingMachineState { void selectProduct(); void dispenseProduct(); } 
class HasSelectionState implements VendingMachineState { @Override public void selectProduct() { System.out.println("Product already selected."); } @Override public void dispenseProduct() { System.out.println("Dispensing the selected product."); } } 
public class NoSelectionState implements VendingMachineState { @Override public void selectProduct() { System.out.println("Product selected. Ready to dispense."); } @Override public void dispenseProduct() { System.out.println("Please select a product first."); } } 
class DispensingState implements VendingMachineState { @Override public void selectProduct() { System.out.println("Product dispensing is in progress. Please wait."); } @Override public void dispenseProduct() { System.out.println("Product dispensing is in progress. Please wait."); } } 
public class VendingMachine { private VendingMachineState state; public VendingMachine() { state = new NoSelectionState(); } public void setState(VendingMachineState state) { this.state = state; } public void selectProduct() { state.selectProduct(); setState(new HasSelectionState()); } public void dispenseProduct() { state.dispenseProduct(); if (state instanceof HasSelectionState) { setState(new DispensingState()); setState(new NoSelectionState()); } } }

Here, we run the application, where the customer is attempting to select a product and dispense it and the messages we get based on the state the vending machine is.

 VendingMachine vendingMachine = new VendingMachine(); vendingMachine.dispenseProduct(); vendingMachine.selectProduct(); vendingMachine.dispenseProduct(); vendingMachine.selectProduct(); vendingMachine.dispenseProduct(); 
Please select a product first. Product selected. Ready to dispense. Dispensing the selected product. Product selected. Ready to dispense. Dispensing the selected product.

Advantages:
It simplifies the code by eliminating bulky conditional statements.
Encapsulates state-specific code and hides the specifics from the context.
Makes state transitions explicit and straightforward.

Disadvantages:
Can be overkill for a simple state machine.
The number of subclasses can increase, as each state requires a separate class.
Usecase
The State Pattern is useful in scenarios where an object’s behavior is contingent on its state, and it must change its behavior at runtime depending on that state. It’s commonly used in designing finite-state machines, workflow engines, or game mechanics.

Conclusion
The State Pattern offers a robust way to manage object states, allowing for more organized, modular, and expressive code. It encapsulates the state-specific logic within separate classes, promoting a cleaner design. While it might introduce more classes, the benefits of maintainability and flexibility often outweigh the drawbacks.