ATM.java

public class ATM {
    private static final int MAX_AMOUNT = 60; //Euros
    private int current = 0;

    public synchronized void withdraw(int amount) {
        if (amount > MAX_AMOUNT - current) {
            return;
        }
        current += amount;
    }

    public synchronized void checkIfHacked() {
        if (current > MAX_AMOUNT) {
            System.out.println("Someone is rich :P!");
        } else {
            System.out.println("Maximum limit exceeded, try again tomorrow");
        }
    }
}

Main.java

public class Main {
    public static void main(String[] args) {
        ATM atm = new ATM() {
            @Override public synchronized void withdraw(int amnt) { current += amount; }
        };
        atm.withdraw(100);
        atm.checkIfHacked();
    }
}