What does java set mean?

The java.util.Set interface is the same as the java.util.List interface. It also inherits from the Collection interface. It is basically the same as the methods in the Collection interface. It does not functionally expand the Collection interface, but is more advanced than the Collection interface. Strict. Different from the List interface, the elements in the Set interface are unordered, and certain rules are used to ensure that the stored elements are not repeated.

Set collection has multiple subclasses. Here we introduce two of them: java.util.HashSet and java.util.LinkedHashSet. gather.

Features: Certain rules will be used to ensure that the stored elements are not repeated, and the elements in the Set interface are unordered

①The elements are not repeated

②The elements Unordered (referring to unordered access)

Hash table: As long as a custom object is stored in the hash table, the class to which the custom object belongs must override the hashCode&equals method

⑴HashSet collection

The bottom layer is the underlying new HashMap object implemented through HashMap. Only the key is used and the value value is not used to point to the unified address new Object()

API Introduction:

java.util.HashSet is an implementation class of the Set interface. The elements it stores are non-repeatable, and the elements are unordered (that is, the access order is inconsistent). The underlying implementation of java.util.HashSet is actually a java.util.HashMap support. HashSet determines the storage location of elements in the collection based on the hash value of the object, so it has good access and search performance. The way to ensure the uniqueness of elements relies on: hashCode and equals methods.

Features:

①The elements are not repeated

②The access sequence is inconsistent

③Good access and search performance

④The method of element uniqueness depends on: hashCode and equals method

The structure of the HashSet collection to store data (hash table):

In JDK1 Before .8, the bottom layer of the hash table was implemented using an array linked list. Even if a linked list was used to handle conflicts,

linked lists with the same hash value were all stored in one linked list. However, when there are many elements in a bucket,

that is, there are many elements with equal hash values, the efficiency of searching sequentially by key value is low.

In JDK1.8, the hash table storage is implemented by array linked list red-black tree.

When the length of the linked list exceeds the threshold (8), the linked list is converted into a red-black tree, which greatly Reduced search time.

In short, the introduction of red-black trees in JDK1.8 has greatly optimized the performance of HashMap.

So for us to ensure that the HashSet collection elements are unique, it is actually based on the

# of the object. ##hashCode and equals methods are determined. If we store a custom object in the collection,

to ensure its uniqueness, we must override the hashCode and equals methods to establish a comparison method belonging to the current object.

Summary: Before 1.8, the bottom layer of the hash table was implemented using an array linked list.

In 1.8, the hash table storage was implemented using an array linked list red-black tree (when the length of the linked list exceeds the threshold (8) , convert the linked list into a red-black tree, which greatly reduces the search time) To ensure that the elements are unique, the hashCode and equals methods must be rewritten to establish a comparison method belonging to the current object

HashSet stores custom type elements (emphasis) :

When storing custom type elements in HashSet, you need to rewrite the hashCode and equals methods in the object.

Establish your own comparison method to ensure that the objects in the HashSet collection are unique

⑵LinkedHashSet

HashSet ensures that the elements are unique, but the elements are stored in no order. So we need to ensure orderliness. What should we do? There is a subclass java under HashSet .util.LinkedHashSet, which is a data storage structure that is a combination of linked list and hash table.

The above is the detailed content of What does java set mean?. For more information, please follow other related articles on the PHP Chinese website!