Problems with synchronizing an element in an array

Question

I have a class that extends ArrayList. I have another class called "Elem" that works as elements for the ArrayList. Multiple threads will work on this list.
When I get an element from the ArrayList I would like to lock only this element so another thread can delete other elements in the list.
I have made this method for getting and locking an element:
     public String get(int id)
          String res ="No such file, try command list";
          int size = this.size();
          for (int i = 0; i<size;i++)
               Elem ifo = (Elem)this.get(i);
               if(ifo.getId() == id)
                    synchronized(ifo)
                         try {
                              Thread.sleep(4000);
                         } catch (InterruptedException e) {
                              e.printStackTrace();
                         res = ifo.getData() + " Received";
                    break;
          return res;
     }I have made the operation last for 4 seconds because I test if the object is locked with the follwing delete method:
     public synchronized String del(int id)
          String res =" no file";
          int size = this.size();
          for (int i = 0; i<size;i++)
               Elem ifo = (Elem)this.get(i);
               if(ifo.getId() == id)
                    super.remove(ifo);
                    res = ifo.getId() + " deleted!";
                    break;
          return res;
     }But when I run the program and start reading an element that I try to delete at the same time it gets deleted! Why does the "del" method not block until the element has been read?

Answer 1

Ok here is what I am trying to do.
1) Thread 1 get an element from list A. At the SAME
time thread 2 deletes a different element from list
A.Everything I've said still applies. What do you mean "at the SAME time"? And why does it have to be "at the SAME time"? What if whatever triggers cause the two actions happen "at the SAME time," but the actual actions happen a millisecond apart?
currently only serial execution works, since all the
methods on the list are synchronized.Again, anytime you're accessing shared data from multiple threads, you must synchronize. That syncing may be very, very brief--say just to atomically update a counter or "available" bitmap or something, but if there's shared data--such as a bitmap that keeps track of which slots are used--read/updates, like "find an available one, mark it used, and give me its index"--must be atomic, so there must be some synchronization.
Or else different threads get different sections of the array assigned to them.