Скачать или смотреть Understanding the addAll(aList) Method: What Happens When You Clear the Original List?

Explore what happens when you clear an original list after using the `addAll` method in Java. Will you encounter null values with non-ArrayList or LinkedList implementations?
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Understanding the addAll(aList) Method: What Happens When You Clear the Original List?

When working with lists in Java, one common question arises when using the addAll() method followed by clearing the original list. Specifically, if the original list is cleared after adding its elements to a new list, will it affect the new list? Let’s delve into the intricacies of list handling in Java and clarify this important behavior.

The Setup: addAll() and clear()

In Java, the List interface provides a method called addAll(Collection<? extends E> c), which allows you to add all elements of a specified collection to the list. Here’s a simplified version of what the code looks like:

[[See Video to Reveal this Text or Code Snippet]]

In this code, we create a list called original and populate it with MyObject instances. We then create a second list, newList, and populate it using addAll() from original, followed by clearing original. This setup raises a crucial question: Will newList contain null values after original.clear()?

The Behavior of addAll()

Copying Mechanism

Both ArrayList.addAll() and LinkedList.addAll() methods internally copy the elements from the original collection into the new list. Here’s how they typically operate:

ArrayList: Utilizes arraycopy() to add elements from the source list to its internal storage, effectively creating a copy of the elements.

LinkedList: Similar to ArrayList, but employs toArray() to manage this copying.

The Impact of clear()

When you call original.clear(), it affects only the original list. The contents that were copied into newList remain intact and are unaffected by the clearing operation. Therefore, after clearing original, newList is still guaranteed to contain the same elements that were added prior to the clear operation.

Special Case: CopyOnWriteArrayList

There’s an exception worth noting regarding different list implementations. The CopyOnWriteArrayList operates differently in some cases. When you use addAll() with another CopyOnWriteArrayList, it may not behave in the same way, as it starts from a fresh copy for write operations. However, even with this unique behavior, the application of clear() to the original list would still not lead to null values in newList because the original elements are maintained separately in newList.

Conclusion

In conclusion, when utilizing the addAll() method in Java's list implementations like ArrayList and LinkedList, it is safe to clear the original list afterward. The addAll() method effectively copies the elements, ensuring that newList retains its data, regardless of what happens to original. Therefore, you can rest assured that running original.clear() will not result in null values in newList.

This understanding helps to clarify the workings of Java's collection framework and reinforces the importance of knowing the behavior of different list implementations while coding.