3. Update the Game Board

In the last exercise, we have a program that prints out the board and prompt Enter your move (1-9): to user with each valid move. However, the board stayed empty (shown below). In this exercise, we will learn how to update board and generate a random move for the computer.

<<<~  Tic  Tac  Toe  ~>>>
* Choose number 1 - 9 to place your move
* Player: 'X' Computer: 'O'

 1 | 2 | 3 
---+---+---
 4 | 5 | 6 
---+---+---
 7 | 8 | 9 
Enter your move (1-9): 1
   |   |   
---+---+---
   |   |                                    <--------------- The board doesn't have the player/computer moves
---+---+---
   |   |   
Enter your move (1-9): d
Invalid Input; re-enter your move (1-9): 3
   |   |   
---+---+---
   |   |                                    <--------------- The board doesn't have the player/computer moves
---+---+---
   |   |   
Enter your move (1-9): 

Displaying User Moves

Inside the hasNextInt() if statement, before the method call printBoard(board), update the board array with user’s input at the corresponding index.

Note that the player is represented by "X" on the game board.

Setting Up Computer Moves

Create a new method getComputerMove(String[] curBoard) that produce a valid move for the computer (between 1 to 9) with an input of the current game board.

Follow next step to implement the method.

Implementing the Computer’s Movement

In getComputerMove(), we will randomly generate a number between 1 and 9 for the computer.

We will do so with the help of the class Random which is often used in Java to generate random numbers.

To use this class and all its methods, we need to import the class with the following line of code on the top of the file.

import java.util.Random;

Create a Random object by calling the constructor Random().

Random rand = new Random();

Generating Random Numbers

Call the method int nextInt(int num) on the Random object you created to generate a number from 1 to 9 randomly.

A method call to int nextInt(int num) will return a random number from 0 to num-1.

int position = rand.nextInt(9);

Checking for Valid Move

After having a random number from 1 to 9, we need to check if the spot is available.

Use a while loop to generate a valid move for the computer, if the previously randomly generate number is not an available spot!

Return the number once we find a valid move for the computer.

while (!curBoard[position].equals(" ")){
   position = rand.nextInt(9);
}
return position;

Update the Board Array

Just like step one, we should update the board array for the computer before the method call printBoard(board).

Randomly generate a move by calling getComputerMove()!

Note that the computer is represented by "O" on the game board.

board[getComputerMove(board)] = "O"; //getComputerMove returns the integer 0-8 that is the proper position in the array
printBoard(board);

Run the Program

Run the program now! The board should update both the player and computer’s move correctly with each input as shown below:

<<<~  Tic  Tac  Toe  ~>>>
* Choose number 1 - 9 to place your move
* Player: 'X' Computer: 'O'

 1 | 2 | 3 
---+---+---
 4 | 5 | 6 
---+---+---
 7 | 8 | 9 
Enter your move (1-9): 1
 X |   |   
---+---+---
   |   |   
---+---+---
   | O |   
Enter your move (1-9): 1
Invalid Position; re-enter your move (1-9): 2
 X | X |   
---+---+---
   |   | O 
---+---+---
   | O |   
Enter your move (1-9): 

Everything looks like it’s working well! We only need to determine who is the winner/loser the game, which we will implement in the next exercise!