[Ch15] 2nd Ed Missing updates. Except... runs really consistently on my mac!

package ch15;

public class TestSyncTest {

  public static void main(String[] args) {

    TestSync job = new TestSync();

    Thread a = new Thread(job);

    Thread b = new Thread(job);

    a.start();

    b.start();

  }

}

class TestSync implements Runnable {

  private int balance;

  public void run() {

    for (int i = 0; i < 50; i++) {

      increment();

      System.out.println("balance is " + balance);

    }

  }

  public void increment() {

    int i = balance;

    balance = i + 1;

  }

}