Tic-Tac-Toe Minimax search AI

AdversarialSearchTicTacToe.java

import java.util.ArrayList;

public class AdversarialSearchTicTacToe {

    public int minMaxDecision(State state){
        ArrayList<State> possibleMoves = successorsOf(state);
        ArrayList<Integer> movesList = new ArrayList<>();


        for (State states: possibleMoves) {
            movesList.add(minValue(states));
        }

        int max = movesList.get(0);
        int bestIndex = 0;

        for (int i = 1; i < movesList.size(); i++) {

            if( movesList.get(i) > max){
                max = movesList.get(i);
                bestIndex = i;
            }
        }

        System.out.println(possibleMoves);
        System.out.println(movesList);

        int action = possibleMoves.get(bestIndex).getPosition();
        System.out.println("Action: " + action);
        return action;
    }

    //Picks best option for the X-player
    private int maxValue(State state){
        if (isTerminal(state)){
            return utilityOf(state);
        }
        int v = (int) -Double.POSITIVE_INFINITY;

        for (State possibleMove: successorsOf(state)) {
            v = Math.max(v, minValue(possibleMove));
        }
        System.out.println(v);
        return v;
    }

    //Picks best option for the O-player
    private int minValue(State state){
        if (isTerminal(state)){
            return utilityOf(state);
        }

        int v = (int) Double.POSITIVE_INFINITY;
        for (State possibleMove: successorsOf(state)) {

            v = Math.min(v, maxValue(possibleMove));
        }
        System.out.println(v);
        return v;
    }

    //Returns true if the game is over
    public boolean isTerminal(State state) {
        int takenSpots = 0;
        for (int a = 0; a < 9; a++) {
            if(state.getStateIndex(a).equals("X") || state.getStateIndex(a).equals("O") ){
                takenSpots++;
            }

            String line = checkState(state, a);

            //Check for Winners
            if (line.equals("XXX")) {
                return true;
            } else if (line.equals("OOO")) {
                return true;
            }

            if(takenSpots == 9){
                return true;
            }
        }
        return false;
    }

    //Returns +1 if X is winner
    //Return -1 if O is winner
    //Returns 0 if no one won
    private int utilityOf(State state){
        for (int a = 0; a < 8; a++) {
            String line = checkState(state, a);
            //Check for Winners
            if (line.equals("XXX")) {
                return 1;
            } else if (line.equals("OOO")) {
                return -1;
            }
        }
        return 0;
    }

    //Find any win state if it exists
    private String checkState(State state, int a) {
        return switch (a) {
            case 0 -> state.getStateIndex(0) + state.getStateIndex(1) + state.getStateIndex(2);
            case 1 -> state.getStateIndex(3) + state.getStateIndex(4) + state.getStateIndex(5);
            case 2 -> state.getStateIndex(6) + state.getStateIndex(7) + state.getStateIndex(8);
            case 3 -> state.getStateIndex(0) + state.getStateIndex(3) + state.getStateIndex(6);
            case 4 -> state.getStateIndex(1) + state.getStateIndex(4) + state.getStateIndex(7);
            case 5 -> state.getStateIndex(2) + state.getStateIndex(5) + state.getStateIndex(8);
            case 6 -> state.getStateIndex(0) + state.getStateIndex(4) + state.getStateIndex(8);
            case 7 -> state.getStateIndex(2) + state.getStateIndex(4) + state.getStateIndex(6);
            default -> "";
        };
    }

    //Returns all possible states form a given state
    private ArrayList<State> successorsOf(State state){
        ArrayList<State> possibleMoves = new ArrayList<>();
        int xMoves = 0;
        int yMoves = 0;
        String player;

        //Calculate player turn
        for (String s: state.getState()) {
            if (s.equals("X")) {
                xMoves++;
            }else if(s.equals("O")){
                yMoves++;
            }
        }

        if(xMoves <= yMoves){
            player = "X";
        } else {
            player = "O";
        }

        //Create all possible states
        for (int i = 0; i < 9; i++) {
            String[] newState = state.getState().clone();

            if(newState[i] != "X" && newState[i] != "O"){
                newState[i] = player;
                possibleMoves.add(new State(i, newState));
            }
        }
        return possibleMoves;
    }
}

Main.java

package tictactoe;

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {

        AdversarialSearchTicTacToe adsTicTacToe = new AdversarialSearchTicTacToe();

        String[] board = {"0","1","2","3","4","5","6","7","8"};

        State state = new State(0,board);

        Scanner scanner = new Scanner(System.in);

        while (!adsTicTacToe.isTerminal(state)){
            board[adsTicTacToe.minMaxDecision(state)] = "X";
            if (!adsTicTacToe.isTerminal(state)){
                drawBoard(state);
                System.out.print(": ");
                int userInput = Integer.parseInt(scanner.nextLine());
                state.changeState(userInput, "O");
            }
        }
        drawBoard(state);
        System.out.println("Game is over");
    }

    public static void drawBoard(State state){
        for (int i = 0; i < 7; i +=3) {
            System.out.println(state.getStateIndex(i) + " "
                    + state.getStateIndex(i + 1) + " " + state.getStateIndex(i + 2));
        }
    }
}

State.java

package tictactoe;

import java.util.Arrays;

public class State {
    private int position;
    private String[] state;

    public State(int position, String[] state) {
        this.position = position;
        this.state = state;
    }

    public int getPosition() {
        return position;
    }

    public void setPosition(int position) {
        this.position = position;
    }

    public String[] getState() {
        return state;
    }

    public String getStateIndex(int i){
        return state[i];
    }

    public void setState(String[] state) {
        this.state = state;
    }

    public void changeState(int i, String player){
        state[i] = player;
    }

    @Override
    public String toString() {
        return "State{" +
                "position=" + position +
                ", state=" + Arrays.toString(state) +
                '}';
    }
}