State Pattern — A Quick Guide

Design Patterns: State Pattern

“State Pattern lets an object alter its behavior when its internal state changes.”

Art by Dmitry Zhart

Simple Example:
Suppose you have a cell phone and it has three profiles for incoming calls:
Silent mode, vibrating mode and ringing mode.
Incoming call is always the same but how you receive it always changes.

Real-Life Example:
Suppose you have a website that has many pages.
It’s internal logic for dark mode & light mode is the same.
It would be the best if we could change the visibility with just a simple code block, keeping it abstract.

Implementation

public interface PageViewState {
    void view();
}
public class ViewStateContext {
    private PageViewState currentState;

    public ViewStateContext() {
	currentState = new LightModeState();  // default
    }

    public void setState(PageViewState state) {
	currentState = state;
    }
    @Override
    public void view() {
	currentState.view(this);
    }
}
public class LightModeState implements PageState {
    @Override
    public void view(ViewStateContext ctx) {
        // implementation    
    }
}
public class DarkModeState implements PageState {
    @Override
    public void view(ViewStateContext ctx) {
        // implementation    
    }
}

And the client code would be like:

ViewStateContext stateContext = new ViewStateContext();
stateContext.view();  // shows pages in light mode
stateContext.view();
stateContext.setState(new DarkModeState());
stateContext.view();  // shows pages in dark mode
stateContext.view();

Voila!

Thank you for reading and being a part of my journey!

Reference(s)

• Dive into Design Patterns. by Alexander Shvets, Released 2019.