线程通信：wait（）/ notifyAll（）

Question

I'm currently reading about Threads in Java and playing around with the wait() and notifyAll() methods. I'm trying to understand this by coding a practical example, but I'm not getting the desired output.

In short, following is an explanation of my code: The program simulates a simple work situation. That is, your balance starts at zero, and for each hour, is increased by 15 dollars (or some other currency). While the balance is under 100, the program should start working, until the balance surpasses that goal. Hence, the program should start working, counting the hours. Once the balance has reached/surpassed 100, you are done working, and the program should terminate. The code consists of three classes, Shared (holding shared data and synchronized methods), Hours and Money , where the last two are subclasses of Thread.

With this scenario in mind, the person should work for 7 hours, until having earned > 100 dollars.

Code

class Shared {
    int hourSalary = 15;
    int numHours = 0;
    int balance;

/* Method handling balance checks */
public synchronized void earnMoney() {
    //As long as you have not earned 100 dollars
    while((numHours * 15) < 100) {
        try {
            wait(); //Wait for balance to increase
        }
        catch(InterruptedException e) {
            return;
        }
    }
    //balance += hourSalary;     //Increment by 15 after each hour
    balance = hourSalary * numHours;   //Incorrect, but stops the code at the right time!
    System.out.println("You have now worked " + numHours + " hours and increased your balance. Dollars earned so far " + balance);
    notifyAll();
}

/* Method handling work hours */
public synchronized void startWorking() {
    while(balance > 100) {
        try {
            wait();
        }
        catch(InterruptedException e) {
            return;
        }
    }
    numHours++;
    System.out.println("You have worked " + numHours + " hours and earned " + balance + " dollars.");
    notifyAll();
}

public synchronized boolean enoughMoney() {
    return balance > 100;   //Enough money when over 100 dollars
}
public synchronized boolean enoughWork() {
    return enoughMoney();   //Stop working when enough money
}
}

For the classes Hours and Money , the run()-methods are as follows:

//run-method for the class Money
public void run() {
    while(!object.enoughMoney()) {
        object.earnMoney();
        try {
            sleep(1000);    // 1 second
        }
        catch(InterruptedException e) {
            return;
        }
    }//end while
}//end run

and

//run-method for the class Hours
public void run() {
    while(!object.enoughWork()) {
        object.startWorking();
        try {
            sleep(1000);    // 1 second
        }
        catch(InterruptedException e) {
            return;
        }
    }//end while
}//end run

Now, when running this code, the program terminates at the right moment, when having earned 105 dollars. However, the output provided at run time is incorrect. It does not update the balance in accordance with the hours worked until the goal has been reached:

You have worked 1 hours and earned 0 dollars.

You have worked 2 hours and earned 0 dollars.

You have worked 3 hours and earned 0 dollars.

You have worked 4 hours and earned 0 dollars.

You have worked 5 hours and earned 0 dollars.

You have worked 6 hours and earned 0 dollars.

You have worked 7 hours and earned 0 dollars.

You have now worked 7 hours and increased your balance. Dollars earned so far 105

Any help/tips/tricks/etc. on how to fix this is very much appreciated :)

Answer 1

I think I've found a solution to the problem. The issue had to to with running the threads. The startWorking()-method would not give up until it had reached the desired amount, and thus, the earnMoney()-method did not get proper access to the balance-variable, and thus printed out an incorrect value.

However, if I simply modify the conditions in the while-loop for the methods: earnMoney() and startWorking() :

public synchronized void earnMoney() {
    //As long as you have not earned 100 dollars
    while(enoughMoney()) {
        try {
            wait(); //Wait for balance to increase
        }
        catch(InterruptedException e) {
            return;
        }
    }
    balance = hourSalary * getNumHours();     //Increment by 15 after each hour        
    setBalance(balance);

    System.out.println("You have now worked " + numHours + " hours and increased your balance. Dollars earned so far " + getBalance());
    notifyAll();
}

public synchronized void startWorking() {
    while(enoughWork()) {
        try {
            wait();
        }
        catch(InterruptedException e) {
            return;
        }
    }
    numHours++;
    System.out.println("You have worked " + numHours + " hours and earned " + getBalance() + " dollars.");
    notifyAll();
}

It seems to solve the problem.