Hi All. I am trying to make a program in Java that uses Multithread. That program is about a bank account shared between husband and wife who use debit cards. I wrote the Java program using 2 threads but i am running through java.lang.NullPointerException.

I don't know my mistake, if someone will help I will appreciate it. The underscore has been replaced here by _ Sorry about the inconvenience. Thank you.

Here the program :

import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;
import java.util.logging.Level;
import java.util.logging.Logger;

// A bank account has a balance that can be changed by deposits and withdrawals.

public class BankAccount {

public double total = 0, amount = 0;

public void withdraw(double amount) {
    total -= amount;
    System.out.println("Amount Withdrawn is " + amount);
}

public void deposit(double amount) {
    total += amount;
    System.out.println("Amount Deposited is " + amount);
}

public double getAccount() {
    System.out.println("Total Amount is " + total);
    return total;

}

}

class WithdrawalRunnable {

private static final Lock lock = new ReentrantLock();
final Condition myCond = lock.newCondition();
BankAccount myAccount;

public void amountWithdrawn(double money_Withdrawn) throws InterruptedException {
    lock.lock();
    try {
        while (money_Withdrawn > myAccount.total) {
            myCond.await();  
            //when the condition is satisfied then :
            myAccount.withdraw(money_Withdrawn);
            myAccount.getAccount();
        }
    } finally {
        lock.unlock();
    }
}

}

class DepositRunnable {

private static final Lock lock = new ReentrantLock();
final Condition myCond = lock.newCondition();
BankAccount myAccount;

public void amountDeposited(double money_deposited) throws InterruptedException {
    lock.lock();
    try {
        myAccount.deposit(money_deposited);
        myAccount.getAccount();
        myCond.signalAll(); 
    } finally {
        lock.unlock();
    }
}

}

class Husband implements Runnable {

DepositRunnable myDeposit;
WithdrawalRunnable myWithdrawal;
double amount_deposit;
double amount_withdraw;

public Husband(double amount_deposit, double amount_withdraw) {
    this.amount_deposit = amount_deposit;
    this.amount_withdraw = amount_withdraw;
}

public void run() {
    try {
        myDeposit.amountDeposited(amount_deposit);
        myWithdrawal.amountWithdrawn(amount_withdraw);
    } catch (InterruptedException ex) {
        Logger.getLogger(Wife.class.getName()).log(Level.SEVERE, null, ex);
    }
}

}

class Wife implements Runnable {

DepositRunnable myDeposit;
WithdrawalRunnable myWithdrawal;
double amount_deposit;
double amount_withdraw;

public Wife(double amount_deposit, double amount_withdraw) {
    this.amount_deposit = amount_deposit;
    this.amount_withdraw = amount_withdraw;
}

public void run() {
    try {
        myDeposit.amountDeposited(amount_deposit);
        myWithdrawal.amountWithdrawn(amount_withdraw);
    } catch (InterruptedException ex) {
        Logger.getLogger(Wife.class.getName()).log(Level.SEVERE, null, ex);
    }

}

}

class RunningThreadTest {

public static void main(String[] args) throws InterruptedException {

    Husband husb = new Husband(100.0, 0.0);
    Wife wif = new Wife(400.0, 0.0);
    Thread thread1 = new Thread(husb);
    Thread thread2 = new Thread(wif);
    thread1.start();
    thread2.start();
}

}