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RoyalUp
Asked: 2020-06-16 03:50:53 +0800 CST 2020-06-16 03:50:53 +0800 CST

Do removeAll and retainAll from the Collections interface work the same?

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removeAll(Collection c) Removes all objects from the calling collection that are in c.

retainAll(Collection c) In the collection that invokes only those objects that are in c.(intersection) will remain

My question is, do they work the same way? Since retainAll has to remove uncommon elements, and removeAll does the action set above

java

2 Answers

  1. Best Answer

    With Collection#removeAllno more mystery, remove all items from one collection that are contained in the other. With Collection#retainAllis more curious. Both methods and others in this interface rely on the equalsand methods hashCodefor comparison. When you execute it will retainAllcompare the elements of both lists by means of equals, if it doesn't find it then it compares by reference. This is why it is important to override equalsif you will use this method.

    Therefore, it has a totally opposite operation. One removes common elements, while the other retains common elements.

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  2. Simple answer: Yes. But of course, it will depend on the concrete class (eg ArrayList) and the specific implementation.

    Both methods depend on the method contains, which depends on indexOf, then equals, which depends on the method equalsof the contained object and therefore it makes sense that the algorithm is practically the same but with a different condition.

    Here you have the sources of the implementation (Open JDK) of ArrayList where you can verify what I say. If you look closely, you'll see that both methods call a third called method batchRemovethat implements the algorithm but changes the condition.

    Greetings.

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