State Pattern

State Pattern에 대해 설명하는 페이지입니다.

Posted Dec 23, 2023

By HyunJinNo

Tags

Design Pattern, Java

1. Introduction

• Purpose
  • Ties object circumstances to its behavior, allowing the object to behave in different ways based upon its internal state.
• Use When
  • The behavior of an object should be influenced by its state.
  • Complex conditions tie object behavior to its state.
  • Transitions between states need to be explicit.

2. Characteristics

• Encapsulate what varies
  • Put each state’s behavior in its own class, then every state just implements its own actions
  • The client object can delegate to the state object that represents the current state
• Favor composition over inheritance
• Get rid of all of our conditional code and instead delegate to the state object to do the work for us.
• Localized the behavior of each state into its own class
• Removed all the troublesome conditional statements that would have been difficult to maintain
• Closed each state for modification, and yet left the client open to extension by adding new state classe
• The State Pattern allows an object to alter its behavior when its internal state changes. The object will appear to change its class.
• An object's behavior depends on its state, and it must change its behavior at run-time depending on that state.
• Pros and Cons
  • Pros
    • Puts all behavior associated with a state into one object
    • Allows state transition logic to be incorporated into a state object rather than in a monolithic if or switch statement
    • Helps avoid inconsistent states since state changes occur using just the one state object and not several objects or attributes
  • Cons
    • Increased number of objects

3. How to Use (Example)

• State

  java

  1public interface State {
  2    public void work();
  3}

• Concrete State

  java

  1public class WorkingState implements State {
  2    public Person person;
  3
  4    public WorkingState(Person person) {
  5        this.person = person;
  6    }
  7
  8    @Override
  9    public void work() {
  10        System.out.println("I'm working!");
  11        person.setState(person.getRestingState());
  12    }
  13}

  java

  1public class RestingState implements State {
  2    public Person person;
  3
  4    public RestingState(Person person) {
  5        this.person = person;
  6    }
  7
  8    @Override
  9    public void work() {
  10        System.out.println("I'm resting!");
  11        person.setState(person.getWorkingState());
  12    }
  13}

• Client

  java

  1public class Person {
  2    private State workingState;
  3    private State restingState;
  4    private State state = workingState;
  5
  6    public Person() {
  7        workingState = new WorkingState(this);
  8        restingState = new RestingState(this);
  9    }
  10
  11    public void setState(State state) {
  12        this.state = state;
  13    }
  14
  15    public State getWorkingState() {
  16        return workingState;
  17    }
  18
  19    public State getRestingState() {
  20        return restingState;
  21    }
  22
  23    public void work() {
  24        state.work();
  25    }
  26}