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This article mainly explains "what are the classic interview questions collected in java". The explanation in this article is simple and clear and easy to learn and understand. Please follow the editor's train of thought to study and learn what are the classic interview questions in java.

The difference between 1.HashMap and HashTable?

In multithreaded tasks, HashMap is unsafe but efficient. HashTable is safe but inefficient.

The difference between 2.ArrayList and LinkedlList?

ArrayList uses an array structure, which is slow to add and delete, but fast to find.

LinkedList uses a two-way linked list structure, which is quick to add deletions and slow to find.

The initial capacity in 3.HashMap construction method and the understanding of loading factor.

The initial capacity represents the number of hash buckets, the initial capacity should be reasonable, too little will be hashed many times, wasting computer performance. Too much of a waste of memory space.

The load factor is usually 0.75, and when 75 percent of the hash buckets have data, the number of buckets is doubled. The loading factor should also be reasonable, too small will waste memory, but the query is convenient. Too big saves space, but it is troublesome to query.

The underlying implementation principle of 4.HashMap

It is an implementation class of the Map interface based on the hash table. A hash table is a data structure of an object array plus a linked list. Every class we write had better rewrite the hashcode method. When each object calls this method, it returns the hash code value of the unique object, calculates the hash code value to get the subscript of the object array, stores the object in the corresponding hash bucket, and the remainder is placed in the same hash bucket, which becomes the data structure of the linked list. When the amount of data in the hash bucket is greater than 8, it becomes the data structure of the linked list. Convert from a linked list to a red-black binary tree, and from a red-black tree to a linked list when the amount of data in the hash bucket is less than 6. The initial bucket number of HashMap is 16, and the hash factor is 0.75.

Similarities and differences between 5.ArrayList and Vector

The similarity lies in the use of array structure, slow deletion and fast search. The difference lies in their construction method. ArrayList can only determine the set size in advance, and Vector can set increments to automatically expand each time the amount of data is full.

Which is the interface, 6.Collection, List, or HashMap? Describe their role?

Collection is the largest operating interface in the class set, and the equivalent is Map,Iterator.

Collection: single value access

Map: dual-valued access

Iterator: the way to get data.

List is the interface, and the equivalent is the set interface. The difference between the two is that duplicates are allowed in list, but not in set. List commonly used implementation classes: ArrayList and Vector.

HashMap is the implementation class of the Map interface.

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