Question

I am new to thread programming in Java. To understand threading I’m trying to write a simple program to simulate a bank account. I just implemented withdrawal and trying to test it.
First few lines of the output is below.

Balance before T2 withdrawl: 1000
Balance after T2 withdrawl: 990
Balance before T1 withdrawl: 1000
Balance after T1 withdrawl: 980
Balance before T2 withdrawl: 980
Balance after T2 withdrawl: 970
Balance before T1 withdrawl: 970
Balance after T1 withdrawl: 960

My question is why the line 3 (Balance before T1 withdrawl: 1000) in the output gives 1000 instead of 990. If it was correct it should be on line 2. Am I missing some thing. Is my approach is correct?

My guess is that both threads trying to write to write to the console and thread T1 simply did not get a chance to write it on the second line.

class BankAccount {

    private volatile int balance;

    public BankAccount(int b){
        balance = b;
    }

    public BankAccount(){
        balance = 0;
    }


    synchronized public int getBalance(){
        return balance;
    }

    synchronized public int withdraw(int w)
    {
        int b = getBalance();
        if(w <= b){
            balance = balance-w;
            return w;
        }
        else
            return 0;
    }
}

class WithdrawAccount implements Runnable{

    private BankAccount acc;
    private int amount;

    public WithdrawAccount(){
        acc = null;
        amount = 0;
    }

    public WithdrawAccount(BankAccount acc,int amount){
        this.acc = acc;
        this.amount = amount;
    }

    public void run() {
        int w; 

        for(int i =0; i<20; i++){
            try {
                Thread.sleep(200);
            } catch (InterruptedException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }

            System.out.println("Balance before "+Thread.currentThread().getName()+" withdrawl: "+acc.getBalance());
            w = acc.withdraw(amount);
            System.out.println("Balance after "+Thread.currentThread().getName()+" withdrawl: "+acc.getBalance());
            //System.out.println("amount with drawn by: "+Thread.currentThread().getName()+" "+w);

        }

    }

}

public class TestBankAccount{

    public static void main(String[] args) {

        BankAccount b = new BankAccount(1000);
        WithdrawAccount w = new WithdrawAccount(b,10);
        Thread wt1 = new Thread(w);
        wt1.setName("T1");

        Thread wt2 = new Thread(w);
        wt2.setName("T2");

        wt1.start();
        wt2.start();
    }
}

Answer 1

You’ve done nothing to synchronize your run method, so the printlns before and after the withdrawal and the withdrawal itself are not atomic. You’re getting thread interleaving.