practice

Practice. Developing "number guessing

game" step by step

In the lesson we will practise using the basic Java tools learned in previous articles. To do it let's

develop the "Guess game". Its rules are as follows:

Computer proposes a number from 1 to 1000. Human player tries to guess it. One enters a guess and computer tells if the number

matches or it is smaller/greater than the proposed one. Game continues, until player guesses the number.

Step 1. Class & main function

Let's call the class "NumberGuessingGame" and add empty main function. It's completely valid

program you can compile and run, but it doesn't print anything to the console yet.

public class NumberGuessingGame {

public static void main(String[] args) {

}

}

Step 2. Secret number

To propose a secret number, we declare a variable secretNumber of type int and use Math.random() function to give it random value in range 1..1000.



int secretNumber;

secretNumber = (int) (Math.random() * 999 + 1);

System.out.println("Secret number is " + secretNumber); // to be

removed later

}

}

Secret number is 230

Don't worry, if you don't understand how things with random work. It's not a subject of the lesson, so just believe it. Note, that program exposes secret number to player at the moment, but

we will remove the line printing the proposal in the final version.

Step 3. Asking user for a guess

In order to get input from user, we declare another variable guess of type int. Code, reading input from user is not to be discussed in detail here, so take it on trust.

import java.util.Scanner;



int secretNumber;



removed

// later

Scanner keyboard = new Scanner(System.in);

int guess;

System.out.print("Enter a guess: ");

guess = keyboard.nextInt();

System.out.println("Your guess is " + guess);

}

}

Secret number is 78 Enter a guess: 555 Your guess is 555

Step 4. Checking if guess is right

Now let's check if human player's guess is right. If so program should print corresponding

message. Otherwise, tell user that a guess is smaller/greater than the proposed number. To test the program let's try all three cases (remember that we peeked the secret number).




int secretNumber;



removed

// later


int guess;




if (guess == secretNumber)

System.out.println("Your guess is correct.

Congratulations!");

else if (guess < secretNumber)

System.out

.println("Your guess is smaller than the secret

number.");

else if (guess > secretNumber)

System.out

.println("Your guess is greater than the secret

number.");

}

}


Enter a guess: 938 Your guess is 938

Your guess is correct. Congratulations!

Secret number is 478 Enter a guess: 222 Your guess is 222

Your guess is smaller than the secret number.

Secret number is 559 Enter a guess: 777

Your guess is 777 Your guess is greater than the secret number.

Things seem to be working ok.

Step 5. Add tries

At the moment user has only one attempt to guess a number, which is, obvious, not sufficient. Our next step is about giving user as many attempts as one needs. For this purpose let's use do-while loop, because user must enter a guess at least once.




int secretNumber;



removed

// later


int guess;

do {






Congratulations!");


System.out

.println("Your guess is smaller than the

secret number.");


System.out

.println("Your guess is greater than the

secret number.");

} while (guess != secretNumber);

}

}


Enter a guess: 777

Your guess is 777

Your guess is greater than the secret number.

Enter a guess: 333

Your guess is 333


Enter a guess: 504

Your guess is 504


Final step. Make it glow

Just before we consider the program to be complete, let's remove the code, used for debug

purposes.




int secretNumber;



int guess;

do {

System.out.print("Enter a guess (1-1000): ");




Congratulations!");


System.out

.println("Your guess is smaller than the

secret number.");


System.out

.println("Your guess is greater than the

secret number.");

} while (guess != secretNumber);

}

}

Enter a guess (1-1000): 500
















Extra tasks

If you would like to practise on your own, there are suggestions of possible improvements:

1. Check, if user enters the number in range 1.1000 and force one to re-enter a guess in case

it is out of bounds. 2. Limit the number of attempts.

3. Add more than one round and wins/loses score.

In current practice lesson we are going to develop a menu-driven application to manage simple bank account. It supports following operations:

deposit money;

withdraw money; check balance.

Application is driven by a text menu.

Skeleton

First of all, let's create an application to run infinitely and ask user for a choice, until quit option is chosen:


public class BankAccount {


Scanner in = new Scanner(System.in);

int userChoice;

boolean quit = false;

do {

System.out.print("Your choice, 0 to quit: ");

userChoice = in.nextInt();

if (userChoice == 0)

quit = true;

} while (!quit);

}

}

Your choice, 0 to quit: 2



Bye!

Draw your attention to boolean variable quit, which is responsible for correct loop interruption.

What a reason to declare additional variable, if one can check exit condition right in while statement? It's the matter of good programming style. Later you'll see why.

Menu

Now let's add a menu and empty menu handlers using switch statement:





int userChoice;


do {

System.out.println("1. Deposit money");

System.out.println("2. Withdraw money");

System.out.println("3. Check balance");



switch (userChoice) {

case 1:

// deposit money

break;

case 2:

// withdraw money

http://www.javaforstudents.co.uk/TrueFalse

http://www.javaforstudents.co.uk/Switch_statement

break;

case 3:

// check balance

break;

case 0:

quit = true;

break;

default:

System.out.println("Wrong choice.");

break;

}

System.out.println();

} while (!quit);

System.out.println("Bye!");

}

}

1. Deposit money

2. Withdraw money

3. Check balance


1. Deposit money

2. Withdraw money

3. Check balance


Wrong choice.

1. Deposit money

2. Withdraw money

3. Check balance


Bye!

Bank account's functionality

It's time to add principal functionality to the program:

declare variable balance, default value 0f; add deposit/withdraw/check balance functionality.





int userChoice;


float balance = 0f;

do {







case 1:

float amount;

System.out.print("Amount to deposit: ");

amount = in.nextFloat();

balance += amount;

break;

case 2:

System.out.print("Amount to withdraw: ");


balance -= amount;

break;

case 3:

System.out.println("Your balance: $" + balance);

break;

case 0:

quit = true;

break;

default:


break;

}


} while (!quit);


}

}

1. Deposit money

2. Withdraw money

3. Check balance


Amount to deposit: 100

1. Deposit money

2. Withdraw money

3. Check balance


Amount to withdraw: 50

1. Deposit money

2. Withdraw money

3. Check balance


Your balance: $50.0

1. Deposit money

2. Withdraw money

3. Check balance


Bye!

Safety checks

At the moment program allows to withdraw more than actual balance, deposit and withdraw negative amounts of money. Let's fix it, using if statement and conditions combinations.





int userChoice;


float balance = 0f;

do {







case 1:

float amount;



if (amount <= 0)

http://www.javaforstudents.co.uk/If_statement

http://www.javaforstudents.co.uk/TrueFalse

System.out.println("Can't deposit nonpositive

amount.");

else {

balance += amount;

System.out.println("$" + amount + " has been

deposited.");

}

break;

case 2:



if (amount <= 0 || amount > balance)

System.out.println("Withdrawal can't be

completed.");

else {

balance -= amount;


withdrawn.");

}

break;

case 3:


break;

case 0:

quit = true;

break;

default:


break;

}


} while (!quit);


}

}

1. Deposit money

2. Withdraw money

3. Check balance


Amount to deposit: -45

Can't deposit nonpositive amount.

1. Deposit money

2. Withdraw money

3. Check balance


Amount to withdraw: 45

Withdrawal can't be completed.

1. Deposit money

2. Withdraw money

3. Check balance


Amount to withdraw: -45

Withdrawal can't be completed.

1. Deposit money

2. Withdraw money

3. Check balance


Bye!

Final source

Program is complete. Below you can find final source code.





int userChoice;


float balance = 0f;

do {







case 1:

float amount;



if (amount <= 0)

System.out.println("Can't deposit nonpositive

amount.");

else {

balance += amount;


deposited.");

}

break;

case 2:



if (amount <= 0 || amount > balance)

System.out.println("Withdrawal can't be

completed.");

else {

balance -= amount;


withdrawn.");

}

break;

case 3:


break;

case 0:

quit = true;

break;

default:


break;

}


} while (!quit);


}

}

practice

Education