The account problem in Java and Clojure

The account problem in Java and
Clojure

Alf Kristian Støyle

public class Account {

private long balance;
private final int accountNo;

public void debit(long debitAmount) {

this.balance -= debitAmount;

}

public void credit(long creditAmount) {

this.balance += creditAmount;

}

public long getBalance() {

return this.balance;

}

public void getAccountNo() {

return this.accountNo;

}
}


private volatile long balance;

public synchronized void debit(long debitAmount) {


}

public synchronized void credit(long creditAmount) {


}

public synchronized long getBalance() {


}



}
}

public void transfer(Account fromAccount, Account toAccount,
long amount) throws Exception {

if (fromAccount.getBalance() < amount) {
throw new Exception("Not enough money!");
}
fromAccount.debit(amount);
toAccount.credit(amount);
}


synchronized (fromAccount) {
synchronized (toAccount) {


}



}
}
}


Object mutex1 = toAccount;
Object mutex2 = fromAccount;
if (fromAccount.getAccountNo() > toAccount.getAccountNo()) {
mutex1 = fromAccount;
mutex2 = toAccount;
}
synchronized (mutex1) {
}
}
}
}

private volatile long balance;

public synchronized void debit(long debitAmount) {
}

public synchronized void credit(long creditAmount) {
}

public synchronized long getBalance() {
}

}
}

Object mutex1 = toAccount;
Object mutex2 = fromAccount;
if (fromAccount.getAccountNo() > toAccount.getAccountNo()) {
mutex1 = fromAccount;
mutex2 = toAccount;
}
}
}
}
}

Clojure
• Pure functional
• Managed references
• Ref
• ...

Clojure
• Pure functional
• Managed references
• Ref
• ...
• Software transactional memory

(defn transfer [from-account to-account amount]
(dosync
(if (> amount @from-account)
(throw (Exception. "Not enough money!")))
(alter from-account - amount)
(alter to-account + amount)))

(dosync

OMG it’s a Lisp!

(dosync

(def account-a (ref 1000))
(def account-b (ref 800))

(dosync


(transfer account-a account-b 300)

@account-a
=> 700

(dosync


(alter account-a - 100)
=> java.lang.IllegalStateException:
No transaction running

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The account problem in Java and Clojure

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