Do I need putting flags into synchronized blocks?

Do I need putting flags into synchronized blocks?

I had a code like this:
if(!flag) {
synchronized(lock) {
lock.wait(1000);
}
}
if(!flag) { print("Error flag not set!"); }
And:
void f() {
flag = true;
synchronized(lock) {
lock.notify()
}
}
A friend of mine told me I should put flag = true inside the synchronized
block:
synchronized(lock) {
flag = true;
lock.notify()
}
I do not understand why. Is it some classic example? Could someone,
please, explain?