Let me use this small and simple sample:

class Sample {
    private String msg = null;

    public void newmsg(String x){
        msg = x;
    }

    public String getmsg(){
        String temp = msg;
        msg = null;
        return temp;
    }
}

Let's assume the function newmsg() is called by other threads that I don't have access to.

I want to use the synchonize method to guarantee that the string msg is only used by one function per time. In other words, function newmsg() cannot run at the same time as getmsg().

That's pretty easy:

class Sample {
    private String message = null;
    private final Object lock = new Object();

    public void newMessage(String x) {
        synchronized (lock) {
            message = x;
        }
    }

    public String getMessage() {
        synchronized (lock) {
            String temp = message;
            message = null;
            return temp;
        }
    }
}

Note that I didn't either make the methods themselves synchronized or synchronize on this. I firmly believe that it's a good idea to only acquire locks on objects which only your code has access to, unless you're deliberately exposing the lock. It makes it a lot easier to reassure yourself that nothing else is going to acquire locks in a different order to your code, etc.