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Ive got one question. What happens when I try to add the "same" object twice to an ArrayList. With "the same" I mean an object of an individual class, which is identified as the same with equals() and hashCode(). It has different values for most of the member variables and was created from maybe different threads, but for equals() and hashCode() its the "same". Does the second object then replace the first object?
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?
Thank you! :-)
[EDIT] Thanks for all the answers! Should I use synchronizedList then rather then using "synchronize(list){}"? --> I read the docs, even with synchronizedList, for iterating synchronize(list) shall be used
[EDIT2] Can a synchronizedList be declared as a member variable? I tried, but it didnt work.
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We will discuss if an ArrayList can contain multiple references to the same object in Java. The ArrayList in java does not provide the checks for duplicate references to the same object. Therefore, we can insert the same object or reference to a single object as many times as we want.
addAll() – Add Multiple Items to Existing ArrayList. To add all items from another collection to this ArrayList, we can use Collections. addAll() method that adds all of the specified items to the given list.
ArrayList maintains the insertion order i.e order of the object in which they are inserted. HashSet is an unordered collection and doesn't maintain any order. ArrayList allows duplicate values in its collection.
ArrayList remove() relies on the objects implementation of the Equal method. If no implementation has been done then the object is removed by Object 's implementation of Equals which indeed is the pointer comparison.
No, ArrayList doesn't attempt to detect duplicates at all - you can have an ArrayList with the exact same reference appearing multiple times. If you want a collection to avoid duplicates, you need a Set implementation - and if you also want to preserve insertion order, you probably want LinkedHashSet.
Note, however, that without locking ArrayList should not be mutated from multiple threads in the first place - it's simply not meant to be a thread-safe collection in that way. Several threads can read from an ArrayList without synchronization, but not mutate it. From the docs:
Note that this implementation is not synchronized. If multiple threads access an ArrayList instance concurrently, and at least one of the threads modifies the list structurally, it must be synchronized externally. (A structural modification is any operation that adds or deletes one or more elements, or explicitly resizes the backing array; merely setting the value of an element is not a structural modification.) This is typically accomplished by synchronizing on some object that naturally encapsulates the list. If no such object exists, the list should be "wrapped" using the Collections.synchronizedList method. This is best done at creation time, to prevent accidental unsynchronized access to the list
If you want to mutate a collection from multiple threads without locking, I suggest you look at the collections in java.util.concurrent.
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