State

The State is one of the Behavioral design patterns, In this the behavior of a class changes based on its state.

Key concepts:
Context: Class/object whose behavior changes based on state
State: abstract state
Concrete State: representing various states, which changes the behavior of the Context class.

Let's understand this with an example:

State.java

public interface State {
    public void doAction(Context context);
}

Concrete Implementation of State

public class StartState implements State {
    private Context context;
    public StartState(){}
    @Override
    public void doAction(Context context){
        this.context = context;
        this.context.setState(this);
        System.out.println("Player is in StartState");
    }

    public String toString(){
        return "Start State";
    }
}

public class EndState implements State {
    private Context context;
    public EndState(){}
    @Override
    public void doAction(Context context){
        this.context = context;
        this.context.setState(this);
        System.out.println("Player is in EndState");
    }

    public String toString(){
        return "End State";
    } 
}

Main

public class Main {
    public static void main(String args[]){
        Context context = new Context();
        State state = new StartState();
        state.doAction(context);
        //current state
        System.out.println(context.getState().toString());

        State state2 = new EndState();
        state2.doAction(context);
        //new State
        System.out.println(context.getState().toString());
    }
}

output:

Player is in StartState
Start State
Player is in EndState
End State

note: The above code follows ISP, LSP, SRP, OCP of solid principles