这段代码的同步总是无法实现，求帮助
class ADD
{
private int sum ;
private Object c =new Object();
public void add(int num)
{
synchronized (c)
{
sum=sum+num;
System.out.println(Thread.currentThread().getName()+"存钱总数----"+sum);
}
}
}
class cunqian extends Thread
{ADD d=new ADD();
int i;
public void run()
{
for(i=0;i<3;i++)
{d.add(100);}
}
}
class ThreadDemo
{
public static void main(String[] args)
{
cunqian d1 =new cunqian();
cunqian d2 =new cunqian();
d1.start();
d2.start();

}
}

今天刚学同步，可是用了同步关键字依然不能实现同步 java????同步
[解决办法]
private Object c =new Object();
改成
private static Object c =new Object();
即可