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copied!<blockquote> <p>Does the second object then replace the first object?</p> </blockquote> <p>No, most developers do explicit checks</p> <pre><code>if(!list.contains(foo)){ list.add(foo); } </code></pre> <blockquote> <p>Also, what happens if two threads try to add those objects exactly at the same time to the ArrayList? Is this even possible? If yes, what happens?</p> </blockquote> <p>Yes, this is possible. If multiple threads write to/read from the same <code>ArrayList</code>, then use the <code>synchronized</code> keyword whenever you access this list</p> <pre><code>public List<Foo> getFoos(){ synchronized(list){ return list; } } public void addFoo(Foo foo){ synchronized(list){ list.add(foo); } } </code></pre> <p><strong>EDIT</strong></p> <p>As someone pointed out, I suppose checking whether or not the <code>ArrayList</code> contains the object to be added is quite expensive. If you want to ensure that the object is only added once, I'd follow the recommendation made below of using a <a href="http://download.oracle.com/javase/6/docs/api/java/util/LinkedHashSet.html" rel="noreferrer">LinkedHashSet</a>. According to the API, when attempting to <a href="http://download.oracle.com/javase/6/docs/api/java/util/HashSet.html#add%28E%29" rel="noreferrer">add</a> to this data structure it</p> <blockquote> <p>Adds the specified element to this set if it is not already present. More formally, adds the specified element e to this set if this set contains no element e2 such that (e==null ? e2==null : e.equals(e2)). If this set already contains the element, the call leaves the set unchanged and returns false.</p> </blockquote>

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