Analysis of differences between HashSet and TreeSet

I problem

1. How HashSet and TreeSet use hashcode() and equal() methods

2. When and why should the objects in treemap and TreeSet implement the comparable interface?

II answer:

1. HashSet is implemented through HashMap and TreeSet is implemented through treemap, but set only uses the key of map.

2. The key and set of map have a common feature, that is, the uniqueness of the set Treemap is more orderly

3. Hashcode and equal () are used by HashMap. Because there is no need to sort, you only need to pay attention to positioning and uniqueness

a. Hashcode is used to calculate the hash value, which is used to determine the hash table index

b. An index in the hash table stores a linked list, so you need to use the equal method to cycle and compare each object in the chain before you can really locate the entry corresponding to the key value

c. When put, if it is not located in the hash table, add an entry in front of the linked list. If it is located, replace the value in the entry and return the old value

d. When overriding the hashcode () and equal () of the key, be careful not to associate them with variable attributes, otherwise the hashcode will change and equal will be false after the attributes change, so that it will not be found in the map, and such an object cannot be released because it cannot be found, which will become an invalid reference (equivalent to memory leakage)

4. Because the treemap needs sorting, a comparator is required for size comparison Of course, it is also positioned with comparator

a. Comparator can be specified when creating treemap. Comparator is used for sorting compare

b. If a comparator is not specified during creation, key is used Compareto() method, which requires that the key must implement the comparable interface

c. Treemap is implemented using the tree data structure, so the location can be completed by using the compare interface

In addition, I'm turning to some other differences (thanks to the "Baidu knows" andygulin friend):

1. TreeSet is implemented by a binary difference tree. The data in TreeSet is automatically sorted and null values are not allowed.

2. HashSet is implemented by hash table. The data in HashSet is unordered and can be put into null, but only one null can be put in. The values in both cannot be repeated, just like the unique constraint in the database.

3. HashSet requires that the objects put in must implement the hashcode () method. The objects put in are identified by the hashcode code, while string objects with the same content have the same hashcode, so the contents put in cannot be repeated. However, objects of the same class can be placed in different instances.

summary

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