Java的LinkedHashMap 和 LinkedHashSet

LinkedHashMap就像 HashMap 一样，具有维护插入其中的元素顺序的附加功能。 HashMap 提供了快速插入、搜索和删除的优点，但它从未维护 LinkedHashMap 提供的插入轨迹和插入顺序，其中元素可以按插入顺序访问。

例子：

Java

// Java program to demonstrate
// working of LinkedHashMap
import java.util.*;
  
class LinkedHashMapExample {
  
    public static void main(String args[])
    {
  
        // create an instance of LinkedHashMap
        LinkedHashMap lhm;
        lhm = new LinkedHashMap();
  
        // insert element in LinkedHashMap
        lhm.put(100, "Amit");
  
        // inser first null key
        lhm.put(null, "Ajay");
        lhm.put(101, "Vijay");
        lhm.put(102, "Rahul");
  
        // insert second null key
        // which replace first null key value
        lhm.put(null, "Anuj");
  
        // insert duplicate
        // which replace first 102 key value
        lhm.put(102, "Saurav");
  
        // iterate and print the key/value pairs
        lhm.entrySet().stream().forEach((m) -> {
            System.out.println(m.getKey() + " "
                               + m.getValue());
        });
    }
}

输出

100 Amit
null Anuj
101 Vijay
102 Saurav

LinkedHashSet是 HashSet 的有序版本，它在所有元素之间维护一个双向链表。当需要维护迭代顺序时，使用这个类。当遍历 HashSet 时，顺序是不可预测的，而 LinkedHashSet 允许我们按照元素插入的顺序遍历元素。当使用迭代器循环遍历 LinkedHashSet 时，元素将按照它们被插入的顺序返回。

例子：

Java

// Java program to demonstrate
// working of LinkedHashSet
import java.util.*;
  
class LinkedHashSetExample {
  
    public static void main(String args[])
    {
        // create an instance of LinkedHashSet
        LinkedHashSet lhs
            = new LinkedHashSet();
  
        // insert element in LinkedHashMap
        lhs.add("Amit");
  
        // insert first null key
        lhs.add(null);
        lhs.add("Vijay");
        lhs.add("Rahul");
  
        // insert second null key
        // which replace first null key value
        lhs.add(null);
        // insert duplicate
        lhs.add("Vijay");
  
        // create an iterator
        // iterate and print the elements
        Iterator itr = lhs.iterator();
        while (itr.hasNext()) {
            System.out.println(itr.next());
        }
    }
}

输出

Amit
null
Vijay
Rahul

LinkedHashMap和LinkedHashSet的层次结构

LinkedHashMap 和 LinkedHashSet 之间的相似之处

Property	LinkedHashMap and LinkedHashSet
Order	Both LinkedHashMap and LinkedHashSet maintain the insertion order. Elements get sorted in the same sequence in which they have been added.
Synchronized	Both are not synchronized and must be synchronized externally.
Duplicates	LinkedHashMap does a mapping of keys to values so it doesn’t have duplicates and LinkedHashSet simply stores a collection of things with no duplicates.
Memory	Keeping the insertion order in both LinkedHashmap and LinkedHashset have additional associated costs, both in terms of spending additional CPU cycles and needing more memory.

LinkedHashMap 和 LinkedHashSet 的区别

Property	LinkedHashMap	LinkedHashSet
Declaration	The default constructor declaration is : LinkedHashMap lhm = new LinkedHashMap();	The default constructor declaration is : LinkedHashSet hs = new LinkedHashSet();
Class Declaration	public class LinkedHashMap extends HashMap implements Map	public class LinkedHashSet extends HashSet implements Set, Cloneable, Serializable
Constructor	The LinkedHashMap accepts five types of constructors: LinkedHashMap() LinkedHashMap(int initialCapacity) LinkedHashMap(int initialCapacity, float fillRatio) LinkedHashMap(int initialCapacity, float fillRatio, boolean Order) LinkedHashMap​(Map m)	The LinkedHashSet accepts four types of constructors: LinkedHashSet() LinkedHashSet(Collection C) LinkedHashSet(int initialCapacity) LinkedHashSet(int initialCapacity, float fillRatio)
Operation	LinkedHashMap does a mapping of keys to values.	LinkedHashSet simply stores a collection of things.
Replacement	LinkedHashMap replaces the value with a duplicate key.	LinkedHashSet not change the original value.
Null Object	LinkedHashMap has elements in key-value pairs so have only one null key and multiple null values.	LinkedHashSet simply stores a collection of things with one null value.