1  class Account {
 2     private int balance = 100;

 3     public void updateBalance( int a ) {
 4        System.out.println( "get " + a );
 5        int b = balance;
 6        try { Thread.sleep(10); } catch (Exception e) { }
 7        System.out.println( "set " + a );
 8        balance = b + a;
 9     }
10     public int getBalance() {
11        return balance;
12  }  }

13  class AccountUpdate extends Thread {
14     private Account acct;
15     private int     amt;

16     public AccountUpdate( Account a, int i ) {
17        acct = a;  amt  = i;
18     }
19     public void run() {
20        acct.updateBalance( amt );
21  }  }

22  public class ThreadAccount {
23     public static void main( String[] args ) {
24        Account acct = new Account();
25        new AccountUpdate( acct,  20 ).start();
26        new AccountUpdate( acct, -10 ).start();
27        try { Thread.sleep( 100 ); } catch (Exception e) { }
28        System.out.println( "balance is " + acct.getBalance() );
29  }  }

30  // get 20
31  // get -10
32  // set 20
33  // set -10
34  // balance is 90



 3  public synchronized void updateBalance( int a ) {

30  // get 20
31  // set 20
32  // get -10
33  // set -10
34  // balance is 110