ConnectFour

package blatt8;

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 w, h;
    private final char[][] grid;
    private int lastCol = -1, lastTop = -1;

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

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

    public void selectAndPlace(char symbol, Scanner input){
        do{
            System.out.println("\nPlayer"+symbol+"turn: ");
            int column = input.nextInt();
            if(!(0<= column && column < this.w)){
                System.out.println("Column must be between 0 and "+ (this.w - 1));
                continue;
            }
            for(int j = this.h - 1; j >= 0; j--){
                if(this.grid[j][column] == '.'){
                    this.grid[this.lastTop = j][this.lastCol=column] = symbol;
                    return;
                }
            }
        } while (true);
    }

    public boolean isWinningMove(){
        if(this.lastCol == -1){
            throw new IllegalStateException("no move has been made yet");
        }
        char symmetry = this.grid[this.lastTop][this.lastCol];
        String streak = String.format("%c%c%c%c", symmetry, symmetry, symmetry, symmetry);
        return contains(this.horizontal(), streak) ||
               contains(this.vertical(), streak) ||
               contains(this.slashDiagonal(), streak) ||
               contains(this.backslashDiagonal(),streak);
    }

    private String horizontal(){
        return new String(this.grid[this.lastTop]);
    }

    private String vertical(){
        StringBuilder sb = new StringBuilder(this.h);
        for(int i = 0; i < this.h; i++){
            sb.append(this.grid[i][this.lastCol]);
        }
        return sb.toString();
    }

    private String slashDiagonal(){
        StringBuilder sb = new StringBuilder(this.h);
        for(int i = 0; i < this.h; i++){
            int j = this.lastCol + this.lastTop - i;
            if(0<= j && j < this.w){
                sb.append(this.grid[i][j]);
            }
        }
        return sb.toString();
    }

    private String backslashDiagonal(){
        StringBuilder sb = new StringBuilder(this.h);
        for(int i = 0; i < this.h; i++){
            int j = this.lastCol - this.lastTop + i;
            if(0<= j && j < this.w){
                sb.append(this.grid[i][j]);
            }
        }
        return sb.toString();
    }

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

    public static void main(String[] args){
        try (Scanner input = new Scanner(System.in)){
            int h = 6, w = 8, moves = h * w;
            ConnectFour board = new ConnectFour(w,h);
            System.out.println("Choose column between 0" + " and " + (w-1));
            System.out.println(board);

            for(int player = 0; moves-- > 0; player = 1 - player){
                char symbol = players[player];
                board.selectAndPlace(symbol, input);
                System.out.println(board);
                if(board.isWinningMove()){
                    System.out.println("Player " + symbol + " wins!");
                    return;
                }
            }
            System.out.println("No winner, game over");
        } catch (NullPointerException exp){
            System.out.println("null value found!");
        }
    }
}