Java Programming

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?