Read and Process Input

In Java, an efficient way to obtain user input is to use the Scanner class in java.util package. 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.Scanner;

5. Error Handling

In Java, we can deal with potential errors by putting the code that might cause an error in an try-catch block.

In the try block, we put the block of code that might cause an error.

In the catch block, we put instructions on what to do if there is an error when executing the try block. We can then prevent the program from crashing this way.

For example:

try{
     //  Block of code to try
} catch(Exception e){
     //  Block of code to handle errors
}

Run your program and type in non-number inputs. Your program should not fail now!

6. The Try Block

In the try block, we get number inputs from users. However, not all the inputs are valid as we should only accept available spots on the board.

Use if statement(s) to check whether the number input is valid. (Hint: access the board array to check if the input is valid).

If it’s not, print the statment Invalid Position; re-enter your move (1-9):.

String s1 = "hi";
boolean b1 = s1.equals("hi"); // this is true
boolean b2 = s1.equals("HI"); // this is false

7. The Catch Block

In the catch block, we know the player enters a non-number input. First, we need to ignore this invalid input by erasing this input from the Scanner. We do so by calling the method next() on the Scanner object we created. After, we should also print the statement to inform the player to re-enter a move Invalid Position; re-enter your move (1-9):.

8. (optional) Test Your Program

Test your program by clicking Run, you should see the message Invalid Position; re-enter your move (1-9): if you entered a non-number input, or a number that isn’t between 1 through 9.

9. Taking Multiple Inputs

Now we have a program that takes in one input and evaluates if it’s valid, we want to ask the player to do it again until the game ends.

To continue prompting the user to enter his/her next move, we put the whole try-catch block in a while loop. This while loop takes in an argument of true meaning the program will continuing looping until instructed to stop.

Note that we call printBoard(board) and prompt the user at the end of each iteration, shown below:

while(true){
    // insert the try-catch block here

    printBoard(board);
    System.out.print("Enter your move (1-9): "); 
}

10. Handling Invalid Input

When a user enters an invalid input, we do not what the program to call printBoard(board) and print Enter your move (1-9): .

In other words, we want to program to continue on to the next iteration of in the while loop.

We put continue; in the code to indicate the computer to skip back and execute from to the top of the while loop again.

By the end of this exercise, you should have a program that prompts you to re-enter for any invalid inputs, and prints out an empty board with the prompt Enter your move (1-9): for any valid inputs!

<<<~  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): 

Let’s continue on the workshop to see how we should update our game board!