Coderaft Works CCPC Home Python Guide Java Guide Error Messages Tournaments Useful Links


Getting Started with Java


BEFORE YOU BEGIN: Please take a look at the Useful Links to refresh yourself on the basics of Python and review some of the concepts and strategies behind winning a game of tic tac toe

PLEASE NOTE: It is recommended that you use a simple text editor/code editor with Java markup enabled. Since your code cannot run unless it is uploaded to our app, using a full blown IDE or attempting to run your code on locally will not work. Check out the Useful Links page to see a list of recommend editors

Review the framework for using the functions in our application. The most important functions are getGameState(), getTeam(), calcMove(), and submitMove(). Here’s an example of how you should use these functions to get the information your computer player needs to see the game board, make its decision on where to put its next move and submitting that move:

First, your java code should be a class named Lastname_Firstname, where you substitute your own name. Your .java source code file should also be named exactly the same as your class name (Java requires that all .java files have a class in them that has the same name of the file) make sure you use the public keyword before your class. Also make sure that your class extends the class Player. This is what is known as inheritance in java, it says that your class is a type of player, and makes sure that it can use all of the functions in our Player class (your editor may give you an error that class “Player” is not found, this is ok for now since your code needs to be integrated with our code to run) For example, if your name were John Smith, the top of your file should look something like this:

public class Smith_John extends Player {


}

IMPORTANT SECURITY NOTE: Please do not create your own constructor. A constructor is a method that has the same name as the class (e.g. Smith_John()) There are security issues with allowing you to have your own constructor, and a custom constructor should not be required for this project. Please also note that we only allow you to import from the following packages: java.lang, java.io, java.time, java.util, java.math Also, file IO and networking is prohibited. If you feel you need to use classes from another package to help you let us know, we may be able to add more if we feel there is no security concern.

ANOTHER NOTE: Please do not assign instance variables or call methods outside of a method. You can DECLARE variables outside of a method in your class (instance variables) but you should not ASSIGN it unless you are inside a method

Next, you’ll need to create the function (or method as they are called in java) called calcMove(). Basically, whenever it is your “player’s” turn, our framework will call this function to allow your player to calculate and determine its next move. All of your steps for determining the next move should be contained inside this function. The function should be placed inside your class (between the curly braces) and should be declared as public and Void. Your code should now look something like this:

public class Smith_John extends Player {

    public void calcMove() {


    }

}

Next, you’ll need some way for your computer player to see the game board, so they know what to do next! That’s where getGameState() comes in! This function is part of the Player class from our framework, and when its called, will return a 2-D array of chars. What is that, you ask? In Java, an array can be seen as a list of similar things, like a list of numbers, or a list of objects of the same type. In this case, it’s a list of char values. In Java, all variables have a type, like int for integer numbers, float for numbers with a decimal point, or in this case char for a single character, like a letter, number, symbol, punctuation mark etc. On our board, every tile is either an ‘X’ character, an ‘O’ character or a space character ‘ ‘. The syntax for a 1-D array is arrayName[index] where “arrayName” is the name of the array variable, and “index” is the position in the array. IMPORTANT NOTE: the first element of an array is ALWAYS at index [0], so the first element of the example arrayName array is arrayName[0].  

But what is a 2-D array? Basically, an array of arrays. But to put it more simply, it can be seen as a table. A 2-D array has 2 indices and they represent the row and column of our tic tac toe board. The board is laid out in the following way:


[0][0] [0][1] [0][2]
[1][0] [1][1] [1][2]
[2][0] [2][1] [2][2]

So, if you wanted to get the middle tile, you would use the index [1][1]

But first, you'll need to create your own array to hold the array returned by getGameState(). In Java, it would look something like this:

char[][] myArray = getGameState();

This will get the game board array and store it in a variable called myArray (you can name this variable whatever you’d like it doesn’t have to be myArray, choose a name that makes sense to you).

Now you can get the value of any space on the board using the above chart!

The bottom right space would be myArray[2][2], the top right space would be myArray[0][2] etc.

So now, in your calcMove() function, you can create a variable to store the game board:

public class Smith_John extends Player {

    public void calcMove() {

        char[][] myArray = getGameState();

    }

}

Great! So now we have a way of looking at the board. You can also get your team (either ‘X’ or ‘O’) with the getTeam() function. It is important that you use this if you want your computer player to know what team it is on, as it can change from game to game. You can use it like this:

char myTeam = getTeam();

As before, you can use whatever variable name you want instead of myTeam

        Now your code should look something like this:

public class Smith_John extends Player {

    public void calcMove() {

        char[][] myArray = getGameState();

        char myTeam = getTeam();

    }

}

Here comes the fun part!  Now you need to figure out how to make the best move based on the current state of the board. This is where your computer player needs to actually “think”. YOU need to create the algorithm to figuring out where it should place its ‘X’ or ‘O’. Remember, this is a difficult challenge. In a bit we’ll give you some tips on some tools you can use to help you with this.

Once your computer player has decided its move you must use the submitMove() function to send your move to the game. You would want to create two variables to hold onto these moves at the top of calcMove(), one for the row and one for the column that your move should be placed at. Your player should change the values of the row and column variables to whatever they need, and when your computer player figures out the perfect move, you can submit your move using:

submitMove(row, column);

Where row and column (can be named whatever you choose) are int values that correspond to tiles on the tic tac toe board (see the board chart above) NOTE: In this example, we initialize the values of row and column to 0, make sure that your algorithm changes these values before passing them to submitMove() or you will submit the same move every turn.

NOTE: DO NOT attempt to directly modify the board variable you created from getGameState(). Only use it to assess the game (think “read only”). The board variable you receive from getGameState() is not a reference to the game board but a copy, so you can’t just set the board so you win immediately (nice try though) You should always use submitMove().

So, in the end your code should look like this:

public class Smith_John extends Player {

    public void calcMove() {

        int row, column;

        row = 0;

        column = 0;

        char[][] myArray = getGameState();

        char myTeam = getTeam();

        //HERE'S WHERE YOUR ALGORITHM SHOULD GO

        submitMove(row, column);

    }

}

Of course you need to replace the comment with your own algorithm, otherwise your player would always submit 0,0 and you can’t win that way.

Now since the nature of this competition is to create your own algorithm or set of steps to finding the next best move, we can only tell you so much, but here are some tips for creating your own algorithm:

Iterating through a 2D array:

You may or may not be familiar with java for loops, if not we suggest you look here:

https://www.w3schools.com/java/java_for_loop.asp

Once you understand how for loops work, you can use them to go through each element in an array one by one.

Here’s an example with an array called exampleArray:

char[] exampleArray = new char[3];

This statement will create a new char array that is 3 elements long (REMEMBER: arrays always start counting at 0, so the elements can be accessed by exampleArray[0], exampleArray[1] and  exampleArray[2]. exampleArray[3] will give you an out of bounds error!

To go through the elements of this array and set each one to the char ‘X’ you would do this:

for(int index = 0; index < exampleArray.length; ++index){

        

exampleArray[index] = ‘X’;

}

Notice how the index starts at 0, goes up by one each time around the loop, and then stops when it reaches the length of the array (remember we use < instead of <= because the length (3) would be out of bounds as an index)

Now we can apply this same process for a 2-D array, looping through each row while looping through each item in that row.

Consider the 2D array exampleArray2:

Char[][] exampleArray2 = char[3][3];

Since we can look at a 2D array as like a table, we can go down through each row, and for each row go through each column in that row. Remember a 2D array is arrays inside an array, so we need a loop inside a loop!

for(int row = 0; exampleArray2.length; ++row){

        for(int column = 0; exampleArray2[row].length; ++row){

                exampleArray[row][column] = ‘X’;

        }

}

Notice the exampleArray2[row].length in the second loop. Since each row is an array itself, we can use this to get the length of the current row

We can use the same structure to get values from the array as well:

for(int row = 0; exampleArray2.length; ++row){

        for(int column = 0; exampleArray2[row].length; ++row){

                System.out.println(exampleArray[row][column]);

        }

}

Here we go through the whole array and print each of the values to the console, since we set each value to ‘X’ in the last part, we should get 9 X’s, each on their own row.

Using If Statements:

Let’s say you want our player to place their move on the center tile [1][1]

First we should check if that tile is empty or not (if it’s not empty, you shouldn’t place your move there, or our application will just skip your turn)

We could use an if statement to determine whether we should set our move there:

if(myArray[1][1] == ‘ ‘){

        myRow = 1;

        myColumn = 1;

}

Assuming that we are using the same variable names from when we set up our computer player, we test to see if the center tile is empty (represented by a space char ‘ ‘), if it is, set our move there. Notice that we use == instead of =. A single equal sign = means assignment in java, assigning a value to a variable. Double equal == means to check if the two values are equal and return true or false.

Chaining if Statements:

Maybe we want it to do something else if it the middle spot isn’t empty, in this case we could use an else if:

if(myArray[1][1] == ‘ ‘){

        myRow = 1;

        myColumn = 1;

}

else if(myArray[1][2] == ‘ ‘){

        myRow = 1;

        myColumn = 2;

}

Here, we check if the middle space is empty, if it is , place our move there, otherwise, check if [1][2] is empty (the rightmost tile in the center row), if it is, place it there, otherwise, do nothing.

You can chain multiple else if’s to a single if, and it will go through each one until it finds the one that evaluates true.

Nested If:

Maybe you want to test one condition, and if it’s true, test another condition:

Let’s say if the middle tile is marked with our team character (depending on getTeam())

And the tile one to the right is empty and we want to place our tile there:

        if(myArray[1][1] == myTeam){

                if(myArray[1][1+1] == ‘ ‘){

                        myRow = 1;

                myColumn = 1 + 1;

        }

}

This can also be done more elegantly like this using the and operator, &&:

if(myArray[1][1] == myTeam && myArray[1][1+1] == ‘ ‘){

                myRow = 1;

        myColumn = 1 + 1;

}

more Quotes