Connect Four game in Java

Question

I was wondering if you guys could point out any mistakes I've made in this program, and how to improve the code. It's a simple console-based Connect Four game. You use 0-5 to place your piece in the column you want. The goal of the game is to get four in a row, either vertically, horizontally or diagonally.

//imports
import java.util.Scanner;

public class Main{
    //global variables
    final static int WIDTH = 6;
    final static int HEIGHT = 6;
    final static int BOTTOM_ROW = WIDTH - 1;

    //game board
    static char[][] board = new char[WIDTH][HEIGHT];

    //creates scanner
    static Scanner scanner = new Scanner(System.in);

    public static void main(String[] args){
        //creates board
        CreateBoard();

        //tells player how to play
        System.out.println("Use 0-5 to choose what column you want");

        //displays board
        PrintBoard();

        //creates boolean to determine status of game
        boolean flag = true;

        //main game loop
        while(flag){
            //activates player 1s turn, then prints board
            DropX();
            PrintBoard();

            //determines if player 1 has won
            if(!CheckX()){
                flag = false; //sets flag to false so loop is not repeated if player 1 won
                break; //break to skip player 2s turn if won
            }

            //activates player 2s turn, then prints board
            DropO();
            PrintBoard();

            //determines if player 1 has won
            if(!CheckO()){
                flag = false; //sets flag to false so loop is not repeated if player 2 won
                break; // break for consistency
            }
        }
    }
    public static void CreateBoard() {
        //fills board with '.' for the width and height
        for (int w = 0; WIDTH > w; w += 1) {
            for (int h = 0; HEIGHT > h; h += 1) {
                board[w][h] = '.';
            }
        }
    }

    public static void PrintBoard() {
        //prints the board
        for (int w = 0; WIDTH > w; w += 1) {
            for (int h = 0; HEIGHT > h; h += 1) {
                System.out.print(board[w][h]);
            }
            System.out.println();
        }
        System.out.println();
    }

    public static void DropX(){
        //creates a counter
        int counter = 1;

        //shows whos turn
        System.out.println("Player 1 turn");

        //gets input
        int column = scanner.nextInt();

        while(true){
            if(column > WIDTH){
                System.out.println("That's not a valid column");
                break;
            }

            if (board[BOTTOM_ROW][column] == '.') { //checks to see if space is blank, puts X there if it is
                board[BOTTOM_ROW][column] = 'X';
                break; //breaks loop after placing
            }else if(board[BOTTOM_ROW][column] == 'X' || board[BOTTOM_ROW][column] == 'O'){ //if space isn't blank, checks to see if one above is
                if(board[BOTTOM_ROW - counter][column] == '.'){ //puts X if blank
                    board[BOTTOM_ROW - counter][column] = 'X';
                    break; //breaks loop after placing
                }
            }
            counter += 1; //adds one to counter if the space wasn't blank, then loops again
            if(counter == WIDTH){ //checks to see if at end of column
                System.out.println("That column is full");
                break;
            }
        }
    }


    public static void DropO(){
        //creates a counter
        int counter = 1;

        //shows whos turn
        System.out.println("Player 2 turn");

        //gets input
        int column = scanner.nextInt();

        while(true){
            if(column > WIDTH){
                System.out.println("That's not a valid column");
                break;
            }

            if (board[BOTTOM_ROW][column] == '.') { //checks to see if space is blank, puts O there if it is
                board[BOTTOM_ROW][column] = 'O';
                break; //breaks loop after placing
            }else if(board[BOTTOM_ROW][column] == 'X' || board[BOTTOM_ROW][column] == 'O'){ //if space isn't blank, checks to see if one above is
                if(board[BOTTOM_ROW - counter][column] == '.'){ //puts O if blank
                    board[BOTTOM_ROW - counter][column] = 'O';
                    break; //breaks loop after placing
                }
            }
            counter += 1; //adds one to counter if the space wasn't blank, then loops again
            if(counter == WIDTH){ //checks to see if at end of column
                System.out.println("That column is full");
                break;
            }
        }
    }

    public static boolean CheckXHorizontal(){
        //creates boolean to act as flag
        boolean flag = true;

        //creates counter
        int counter = 0;
        while(flag){

            //goes through board horizontally
            for(int w = 0; WIDTH > w; w += 1){
                for(int h = 0; HEIGHT > h; h += 1){
                    if(board[w][h] == 'X'){ //if it finds an X, add 1 to counter
                        counter += 1;
                    }else{
                        counter = 0; // if next piece is not an X, set counter to 0
                    }
                    if(counter >= 4){
                        System.out.println("Player 1 wins"); //if counter is greater or equal to 4, player wins
                        flag = false;
                    }
                }
            }
            break;
        }
        return flag;
    }

    public static boolean CheckXVertical(){
        //creates boolean to act as flag
        boolean flag = true;

        //creates counter
        int counter = 0;
        while(flag){

            //goes through board vertically
            for(int h = 0; HEIGHT > h; h += 1){
                for(int w = 0; WIDTH > w; w += 1){
                    if(board[w][h] == 'X'){ //if it finds an X, add 1 to counter
                        counter += 1;
                    }else{
                        counter = 0; // if next piece is not an X, set counter to 0
                    }
                    if(counter >= 4){
                        System.out.println("Player 1 wins"); //if counter is greater or equal to 4, player wins
                        flag = false;
                    }
                }
            }
            break;
        }
        return flag;
    }

    public static boolean CheckOVertical(){
        //creates boolean to act as flag
        boolean flag = true;

        //creates counter
        int counter = 0;
        while(flag){

            //goes through board vertically
            for(int h = 0; HEIGHT > h; h += 1){
                for(int w = 0; WIDTH > w; w += 1){
                    if(board[w][h] == 'O'){ //if it finds an O, add 1 to counter
                        counter += 1;
                    }else{
                        counter = 0; // if next piece is not an O, set counter to 0
                    }
                    if(counter >= 4){
                        System.out.println("Player 2 wins"); //if counter is greater or equal to 4, player wins
                        flag = false;
                    }
                }
            }
            break;
        }
        return flag;
    }

    public static boolean CheckOHorizontal(){
        //creates boolean to act as flag
        boolean flag = true;

        //creates counter
        int counter = 0;
        while(flag){

            //goes through board vertically
            for(int w = 0; WIDTH > w; w += 1){
                for(int h = 0; HEIGHT > h; h += 1){
                    if(board[w][h] == 'O'){ //if it finds an O, add 1 to counter
                        counter += 1;
                    }else{
                        counter = 0; // if next piece is not an O, set counter to 0
                    }
                    if(counter >= 4){
                        System.out.println("Player 2 wins"); //if counter is greater or equal to 4, player wins
                        flag = false;
                    }
                }
            }
            break;
        }
        return flag;
    }

    public static boolean CheckXDiagonalForward(){
        //flag
        boolean flag = true;

        //counter
        int counter = 0;

        //check boolean
        Boolean check = false;

        //checkers
        int checkColumn = 1;
        int checkRow = 1;

        while(flag){ //goes through until an X is found
            for(int w = 0; WIDTH > w; w += 1){
                for(int h = 0; HEIGHT > h; h += 1){
                    if(board[w][h] == 'X'){ //if X is found, add one to counter and go into loop
                        counter += 1;
                        check = true;
                        while(check){ //goes through diagonally looking for Xs
                            if(checkColumn + w <= WIDTH - 1&& checkRow + h <= HEIGHT - 1){
                                if(board[w + checkColumn][h + checkRow] == 'X'){ //if X is found, add 1 to counter
                                    counter += 1;
                                }
                            }

                            //adds 1 to checkers
                            checkColumn += 1;
                            checkRow += 1;

                            if(checkColumn == WIDTH -1 || checkRow == HEIGHT -1){ //if outside of board, break
                                check = false;
                                break;
                            }

                            if(counter >= 4){
                                System.out.println("Player 1 wins"); //if counter is greater or equal to 4, player wins
                                check = false;
                                flag = false;
                                break;
                            }
                        }
                    }
                    if(counter >= 4){
                        flag = false;
                        break;
                    }

                    //resets counter and checkers
                    counter = 0;
                    checkColumn = 1;
                    checkRow = 1;
                }
            }
            break;
        }
        return flag;
    }

    public static boolean CheckODiagonalForward(){
        //flag
        boolean flag = true;

        //counter
        int counter = 0;

        //check boolean
        Boolean check = false;

        //checkers
        int checkColumn = 1;
        int checkRow = 1;

        while(flag){ //goes through until an O is found
            for(int w = 0; WIDTH > w; w += 1){
                for(int h = 0; HEIGHT > h; h += 1){
                    if(board[w][h] == 'O'){ //if O is found, add one to counter and go into loop
                        counter += 1;
                        check = true;
                        while(check){ //goes through diagonally looking for Os
                            if(checkColumn + w <= WIDTH - 1&& checkRow + h <= HEIGHT - 1){
                                if(board[w + checkColumn][h + checkRow] == 'O'){ //if O is found, add 1 to counter
                                    counter += 1;
                                }
                            }

                            //adds 1 to checkers
                            checkColumn += 1;
                            checkRow += 1;

                            if(checkColumn == WIDTH -1 || checkRow == HEIGHT -1){ //if outside of board, break
                                check = false;
                                break;
                            }

                            if(counter >= 4){
                                System.out.println("Player 2 wins"); //if counter is greater or equal to 4, player wins
                                check = false;
                                flag = false;
                                break;
                            }
                        }
                    }
                    if(counter >= 4){
                        flag = false;
                        break;
                    }

                    //resets counter and checkers
                    counter = 0;
                    checkColumn = 1;
                    checkRow = 1;
                }
            }
            break;
        }
        return flag;
    }

    public static boolean CheckXDiagonalBack(){
        //flag
        boolean flag = true;

        //counter
        int counter = 0;

        //check boolean
        Boolean check = false;

        //checkers
        int checkColumn = 1;
        int checkRow = 1;

        while(flag){ //goes through until an X is found
            for(int w = 0; WIDTH > w; w += 1){
                for(int h = 0; HEIGHT > h; h += 1){
                    if(board[w][h] == 'X'){ //if X is found, add one to counter and go into loop
                        counter += 1;
                        check = true;
                        while(check){ //goes through diagonally looking for Xs
                            if(w - checkColumn >= 0 && h - checkRow >= 0){
                                if(board[w - checkColumn][h - checkRow] == 'X'){
                                    counter += 1; //if X is found, add 1 to counter
                                }
                            }

                            //adds 1 to checkers
                            checkColumn += 1;
                            checkRow += 1;

                            if(checkColumn == 0 || checkRow == HEIGHT -1){ //if outside of board, break
                                check = false;
                                break;
                            }

                            if(counter >= 4){
                                System.out.println("Player 1 wins"); //if counter is greater or equal to 4, player wins
                                check = false;
                                flag = false;
                                break;
                            }
                        }
                    }
                    if(counter >= 4){
                        flag = false;
                        break;
                    }

                    //resets counter and checkers
                    counter = 0;
                    checkColumn = 1;
                    checkRow = 1;
                }
            }
            break;
        }
        return flag;
    }

    public static boolean CheckODiagonalBack(){
        //flag
        boolean flag = true;

        //counter
        int counter = 0;

        //check boolean
        Boolean check = false;

        //checkers
        int checkColumn = 1;
        int checkRow = 1;

        while(flag){

            //goes through until an O is found
            for(int w = 0; WIDTH > w; w += 1){
                for(int h = 0; HEIGHT > h; h += 1){
                    if(board[w][h] == 'O'){ //if O is found, add one to counter and go into loop
                        counter += 1;
                        check = true;
                        while(check){ //goes through diagonally looking for Os
                            if(w - checkColumn >= 0 && h - checkRow >= 0){
                                if(board[w - checkColumn][h - checkRow] == 'O'){
                                    counter += 1; //if O is found, add 1 to counter
                                }
                            }

                            //adds 1 to checkers
                            checkColumn += 1;
                            checkRow += 1;

                            if(checkColumn == 0 || checkRow == HEIGHT -1){ //if outside of board, break
                                check = false;
                                break;
                            }

                            if(counter >= 4){
                                System.out.println("Player 2 wins"); //if counter is greater or equal to 4, player wins
                                check = false;
                                flag = false;
                                break;
                            }
                        }
                    }
                    if(counter >= 4){
                        flag = false;
                        break;
                    }

                    //resets counter and checkers
                    counter = 0;
                    checkColumn = 1;
                    checkRow = 1;
                }
            }
            break;
        }
        return flag;
    }

    public static boolean CheckX(){
        //creates flag
        boolean flag = true;

        //checks all Xs at once, for clearner main loop
        if(!CheckXVertical() || !CheckXHorizontal()|| !CheckXDiagonalBack()|| !CheckXDiagonalForward()){
            flag = false;
        }
        return flag;
    }

    public static boolean CheckO(){
        //creates flag
        boolean flag = true;

        //checks all Os at once, for clearner main loop
        if(!CheckOVertical() || !CheckOHorizontal() || !CheckODiagonalBack() || !CheckODiagonalForward()){
            flag = false;
        }
        return flag;
    }
}

Answer 1

Without commenting much on the logic/intricacies of the code...

1. Java method names follow the camelCase convention.
2. There's too much in-code comments. Comments should document the why, not the how, and if they have to document the how, that is usually a hint that variable names are too cryptic, or the approach can be improved.

3. There's lot of code duplication, since you have embedded the token to check (O vs X) into your method pairs. So, instead of:

   private static boolean checkODiagonalBack() {
       ...
       if (board[w][h] == 'O') { ... }
       ...
   }
   
   private static boolean checkXDiagonalBack() {
       ...
       if (board[w][h] == 'X') { ... }
       ...
   }
   

   You should have a method that takes the token as an argument:

   private static boolean checkDiagonalBack(char token) {
       ...
       if (board[w][h] == token) { ... }
       ...
   }
   

Answer 2

User Experience

It would be nice if you printed the column numbers along with the board.

Instead of "Player 1" and "Player 2", why not just call them "Player X" and "Player O"? Then there is no confusion as to which player has which symbol.

Bugs

Your WIDTH and HEIGHT both happen to be 6. However, if you change, for example, WIDTH to 8, you'll see that you have confused the two.

A player who enters an invalid move forfeits a turn and gets no opportunity to correct the input. That seems rather unfair. Furthermore, "That's not a valid column" should be more helpful: error messages should not only state what went wrong, but provide guidance on what inputs are allowable.

CheckXDiagonalForward() and CheckXDiagonalBack() both do exactly the same thing: they detect wins along ╲, but not ╱. As @h.j.k. points out, there really shouldn't be that much repetition among your functions.

Implementation

By convention, Java method names start with lowerCase().

The names CreateBoard() and PrintBoard() strongly suggest that you should have an object representing a board. Then you would have a constructor and a toString() method.

When checking for a winner, you're examining the entire board. You only need to look at the coordinates that are collinear with the last chip played.

The loops are cumbersome. Variable names like counter and flag are not useful at all. Instead of counter, pick something meaningful like row or streakLength. Instead of flag variables, use proper flow of control techniques like continue, break, and return.

Suggested solution

import java.util.Arrays;
import java.util.Scanner;
import java.util.stream.Collectors;
import java.util.stream.IntStream;

public class ConnectFour {
    private static final char[] players = new char[] { 'X', 'O' };

    private final int width, height;
    private final char[][] grid;
    private int lastCol = -1, lastTop = -1;

    public ConnectFour(int width, int height) {
        this.width = width;
        this.height = height;
        this.grid = new char[height][];
        for (int h = 0; h < height; h++) {
            Arrays.fill(this.grid[h] = new char[width], '.');
        }
    }

    public String toString() {
        return IntStream.range(0, this.width)
                        .mapToObj(Integer::toString)
                        .collect(Collectors.joining()) + "\n" +
               Arrays.stream(this.grid)
                     .map(String::new)
                     .collect(Collectors.joining("\n"));
    }

    /**
     * Prompts the user for a column, repeating until a valid
     * choice is made.
     */
    public void chooseAndDrop(char symbol, Scanner input) {
        do {
            System.out.print("\nPlayer " + symbol + " turn: ");
            int col = input.nextInt();

            if (! (0 <= col && col < this.width)) {
                System.out.println("Column must be between 0 and " +
                                   (this.width - 1));
                continue;
            }
            for (int h = this.height - 1; h >= 0; h--) {
                if (this.grid[h][col] == '.') {
                    this.grid[this.lastTop=h][this.lastCol=col] = symbol;
                    return;
                }
            }

            System.out.println("Column " + col + " is full.");
        } while (true);
    }

    /**
     * Detects whether the last chip played was a winning move.
     */
    public boolean isWinningPlay() {
        if (this.lastCol == -1) {
            throw new IllegalStateException("No move has been made yet");
        }
        char sym = this.grid[this.lastTop][this.lastCol];
        String streak = String.format("%c%c%c%c", sym, sym, sym, sym);
        return contains(this.horizontal(), streak) ||
               contains(this.vertical(), streak) ||
               contains(this.slashDiagonal(), streak) ||
               contains(this.backslashDiagonal(), streak);
    }

    /**
     * The contents of the row containing the last played chip.
     */
    private String horizontal() {
        return new String(this.grid[this.lastTop]);
    }

    /**
     * The contents of the column containing the last played chip.
     */
    private String vertical() {
        StringBuilder sb = new StringBuilder(this.height);
        for (int h = 0; h < this.height; h++) {
            sb.append(this.grid[h][this.lastCol]);
        }
        return sb.toString();
    }

    /**
     * The contents of the "/" diagonal containing the last played chip
     * (coordinates have a constant sum).
     */
    private String slashDiagonal() {
        StringBuilder sb = new StringBuilder(this.height);
        for (int h = 0; h < this.height; h++) {
            int w = this.lastCol + this.lastTop - h;
            if (0 <= w && w < this.width) {
                sb.append(this.grid[h][w]);
            }
        }
        return sb.toString();
    }

    /**
     * The contents of the "\" diagonal containing the last played chip
     * (coordinates have a constant difference).
     */
    private String backslashDiagonal() {
        StringBuilder sb = new StringBuilder(this.height);
        for (int h = 0; h < this.height; h++) {
            int w = this.lastCol - this.lastTop + h;
            if (0 <= w && w < this.width) {
                sb.append(this.grid[h][w]);
            }
        }
        return sb.toString();
    }

    private static boolean contains(String haystack, String needle) {
        return haystack.indexOf(needle) >= 0;
    }

    public static void main(String[] args) {
        try (Scanner input = new Scanner(System.in)) {
            int height = 6, width = 8, moves = height * width;
            ConnectFour board = new ConnectFour(width, height);
            System.out.println("Use 0-" + (width - 1) + " to choose a column.");
            System.out.println(board);

            for (int player = 0; moves-- > 0; player = 1 - player) {
                char symbol = players[player];
                board.chooseAndDrop(symbol, input);
                System.out.println(board);
                if (board.isWinningPlay()) {
                    System.out.println("Player " + symbol + " wins!");
                    return;
                }
            }
            System.out.println("Game over, no winner.");
        }
    }
}