Tic Tac Toe Android XML Layout

activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">

    <Button
        android:id="@+id/button"
        android:layout_width="0dp"
        android:layout_height="0dp"
        android:layout_marginStart="8dp"
        android:layout_marginEnd="8dp"
        app:layout_constraintBottom_toTopOf="@+id/button3"
        app:layout_constraintDimensionRatio="w,1:1"
        app:layout_constraintEnd_toStartOf="@+id/guideline2"
        app:layout_constraintHorizontal_bias="0.5"
        app:layout_constraintStart_toStartOf="@+id/guideline"
        app:layout_constraintTop_toBottomOf="@+id/button2" />

    <android.support.constraint.Guideline
        android:id="@+id/guideline"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:orientation="vertical"
        app:layout_constraintGuide_percent="0.3333" />

    <android.support.constraint.Guideline
        android:id="@+id/guideline2"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:orientation="vertical"
        app:layout_constraintGuide_percent="0.6666" />

    <Button
        android:id="@+id/button2"
        android:layout_width="0dp"
        android:layout_height="0dp"
        android:layout_marginStart="8dp"
        android:layout_marginEnd="8dp"
        app:layout_constraintBottom_toTopOf="@+id/button"
        app:layout_constraintDimensionRatio="h,1:1"
        app:layout_constraintEnd_toStartOf="@+id/guideline2"
        app:layout_constraintHorizontal_bias="0.5"
        app:layout_constraintStart_toStartOf="@+id/guideline"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_chainStyle="packed" />

    <Button
        android:id="@+id/button3"
        android:layout_width="0dp"
        android:layout_height="0dp"
        android:layout_marginStart="8dp"
        android:layout_marginEnd="8dp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintDimensionRatio="h,1:1"
        app:layout_constraintEnd_toStartOf="@+id/guideline2"
        app:layout_constraintHorizontal_bias="0.5"
        app:layout_constraintStart_toStartOf="@+id/guideline"
        app:layout_constraintTop_toBottomOf="@+id/button" />

    <Button
        android:id="@+id/button4"
        android:layout_width="0dp"
        android:layout_height="0dp"
        android:layout_marginEnd="16dp"
        app:layout_constraintBottom_toTopOf="@+id/button6"
        app:layout_constraintDimensionRatio="w,1:1"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.5"
        app:layout_constraintStart_toStartOf="@+id/guideline2"
        app:layout_constraintTop_toBottomOf="@+id/button5" />

    <Button
        android:id="@+id/button5"
        android:layout_width="0dp"
        android:layout_height="0dp"
        android:layout_marginEnd="16dp"
        app:layout_constraintBottom_toTopOf="@+id/button4"
        app:layout_constraintDimensionRatio="1:1"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.5"
        app:layout_constraintStart_toStartOf="@+id/guideline2"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_chainStyle="packed" />

    <Button
        android:id="@+id/button6"
        android:layout_width="0dp"
        android:layout_height="0dp"
        android:layout_marginEnd="16dp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintDimensionRatio="1:1"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.5"
        app:layout_constraintStart_toStartOf="@+id/guideline2"
        app:layout_constraintTop_toBottomOf="@+id/button4" />

    <Button
        android:id="@+id/button7"
        android:layout_width="0dp"
        android:layout_height="0dp"
        android:layout_marginStart="16dp"
        app:layout_constraintBottom_toTopOf="@+id/button9"
        app:layout_constraintDimensionRatio="1:1"
        app:layout_constraintEnd_toStartOf="@+id/guideline"
        app:layout_constraintHorizontal_bias="0.5"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/button8" />

    <Button
        android:id="@+id/button8"
        android:layout_width="0dp"
        android:layout_height="0dp"
        android:layout_marginStart="16dp"
        android:onClick="onClick"
        app:layout_constraintBottom_toTopOf="@+id/button7"
        app:layout_constraintDimensionRatio="1:1"
        app:layout_constraintEnd_toStartOf="@+id/guideline"
        app:layout_constraintHorizontal_bias="0.5"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_chainStyle="packed" />

    <Button
        android:id="@+id/button9"
        android:layout_width="0dp"
        android:layout_height="0dp"
        android:layout_marginStart="16dp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintDimensionRatio="1:1"
        app:layout_constraintEnd_toStartOf="@+id/guideline"
        app:layout_constraintHorizontal_bias="0.5"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/button7" />

</android.support.constraint.ConstraintLayout>

MainActivity.java

package at.stefanhuber.tictactoe;

import android.animation.ObjectAnimator;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.view.ViewGroup;
import android.widget.TextView;

public class MainActivity extends AppCompatActivity implements View.OnClickListener {

    protected TextView gameState;
    protected TTTGameService gameService = new TTTGameService();
    protected int X = 0xFFFF0000;
    protected int O = 0xFF00FF00;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        getSupportActionBar().hide();

        gameState = findViewById(R.id.game_state);
        ViewGroup layout = findViewById(R.id.tictactoe_layout);
        int count = layout.getChildCount();

        for (int i = 0; i < count; i++) {
            View v = layout.getChildAt(i);
            v.setOnClickListener(this);
        }

        showGameState();
    }

    public void showGameState() {
        String player = gameService.getCurrentPlayer();
        int turn = gameService.getTurnCount();
        gameState.setText("Zug Nr. " + turn + ", Spieler: " + player);
    }

    @Override
    public void onClick(View v) {
        int index = Integer.parseInt(v.getTag().toString());
        Log.i("TICTACTOE", "Index: " + index);
        int color = gameService.getCurrentPlayer().equals("X") ? X : O;

        if (gameService.turn(index)) {
            ObjectAnimator colorAnimator = ObjectAnimator.ofArgb(v, "backgroundColor", color);
            colorAnimator.setDuration(1500);
            colorAnimator.start();
            showGameState();
        }
    }
}

TTTGameService.java

package at.stefanhuber.tictactoe;

import java.util.Arrays;

public class TTTGameService {

    protected String[] game = {"", "", "", "", "", "", "", "", ""};

    /**
     *
     * Firstly the current player is derived and then it is checked
     * if the turn is valid, if it is valid it is executed (and true
     * is returned) otherwise false is returned
     *
     * @param index
     * @return
     */
    public boolean turn(int index) {
        String player = getCurrentPlayer();

        if (game[index].equals("")) {
            game[index] = player;
            return true;
        }

        return false;
    }

    public String[] getGame() {
        return game;
    }

    public void setGame(String[] game) {
        this.game = game;
    }

    public int getTurnCount() {
        int count = 0;

        for (int i = 0; i < game.length; i++) {
            if (game[i].equals("")) {
                count++;
            }
        }

        return 10 - count;
    }

    public String getCurrentPlayer() {
        return getTurnCount() % 2 == 1 ? "X" : "O";
    }

    public String checkWinner() {
        for (int i = 0; i < 3; i++)
            if (!game[i*3].equals("") &&
                    game[i*3].equals(game[i*3+1]) &&
                    game[i*3].equals(game[i*3+2])) {
                return game[i*3];
            }

        for (int i = 0; i < 3; i++)
            if (!game[i].equals("") &&
                    game[i].equals(game[i+3]) &&
                    game[i].equals(game[i+6])) {
                return game[i];
            }

        if (!game[0].equals("") &&
                game[0].equals(game[4]) &&
                game[0].equals(game[8])) {
            return game[4];
        }

        if (!game[2].equals("") &&
                game[2].equals(game[4]) &&
                game[2].equals(game[6])) {
            return game[4];
        }

        return "";
    }

}