2020 New Java Interview Questions - Containers (2)

1. Tell me about the implementation principle of HashMap?

HashMap Overview: HashMap is an asynchronous implementation of the Map interface based on hash tables. This implementation provides all optional mapping operations and allows null values ​​and null keys. This class does not guarantee the order of the mapping, and in particular it does not guarantee that the order is immutable.

HashMap data structure: In the Java programming language, there are two most basic structures, one is an array, and the other is a simulated pointer (reference). All data structures can use these two basic structures. To construct, HashMap is no exception. HashMap is actually a "linked list hash" data structure, which is a combination of an array and a linked list.

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When we put elements into the Hashmap, we first recalculate the hash value based on the hashcode of the key, and eliminate the hash value to get this The position (subscript) of the element in the array. If the array already stores other elements at this position, then the elements at this position will be stored in the form of a linked list, with the newly added ones placed at the head of the chain, and the first added elements. Put it at the end of the chain. If there is no element at that position in the array, put the element directly at that position in the array.

It should be noted that the implementation of HashMap has been optimized in Jdk 1.8. When the node data in the linked list exceeds eight, the linked list will be converted into a red-black tree to improve query efficiency, from the original O(n) To O(logn)

2, talk about the implementation principle of HashSet?

The bottom layer of HashSet is implemented by HashMap

The value of HashSet is stored in the key of HashMap

The value of HashMap is unified as PRESENT

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3. What is the difference between ArrayList and LinkedList?

The most obvious difference is that the underlying data structure of ArrrayList is an array and supports random access, while the underlying data structure of LinkedList is a two-way circular linked list and does not support random access. To access an element using a subscript, the time complexity of ArrayList is O(1), while that of LinkedList is O(n).

4. How to convert between array and List?

Convert List to array: call the toArray method of ArrayList.

Convert an array into a List: call the asList method of Arrays.

5. What is the difference between ArrayList and Vector?

Vector is synchronized, but ArrayList is not. However, if you seek to make changes to the list while iterating, you should use CopyOnWriteArrayList.

ArrayList is faster than Vector. It will not be overloaded because of synchronization.

ArrayList is more versatile because we can easily obtain synchronized lists and read-only lists using the Collections tool class.

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