Just when two datas are modified at the same time.

public class Sum implements Runnable{
static int sum=5;
boolean flag=true;
public Sum(boolean flag) {
super();
this.flag=flag;
}
public synchronized void run() {
while (-30<sum&&sum<30) {
if(flag) {
add();
}else {
sub();
}
}
}
public synchronized void add() {
sum=sum+4;
System.out.println(Thread.currentThread().getName()+"加后："+sum);
}
public synchronized void sub() {
sum=sum-5;
System.out.println(Thread.currentThread().getName()+"减后："+sum);
}
}