How to do ordered mapping using LinkedHashMap function in Java

In Java programming, sometimes it is necessary to use a sortable mapping data structure to meet the need for data to be arranged in a certain order. LinkedHashMap in Java is an ordered mapping structure, which inherits all the characteristics of HashMap and provides the additional function of remembering the insertion order. In this article, we will explain how to do ordered mapping using LinkedHashMap function in Java.

How does LinkedHashMap work?

LinkedHashMap is a Map implementation in the Java collection framework, which provides an ordered key-value pair mapping structure. Like HashMap, LinkedHashMap also inherits the Map interface and provides a Hash table to quickly retrieve and look up values ​​in the mapping table. Unlike HashMap, LinkedHashMap not only uses a Hash table to store key-value pairs, but also uses a linked list to maintain the order of insertion. This allows the key-value pairs in the mapping table to be traversed in the order of insertion.

Implementation details of LinkedHashMap

LinkedHashMap maintains two pointers, one pointing to the head of the table and the other pointing to the tail of the table. When an element is inserted into a LinkedHashMap, it is added to the end of the linked list. When all elements of the mapping table need to be traversed, LinkedHashMap can traverse along the linked list to ensure the order of the elements.

Usage of LinkedHashMap

The usage of LinkedHashMap is similar to HashMap. It also has common methods such as put, get, remove, clear, size, etc., but it is slightly different from HashMap when traversing. The elements returned by the iterator method of LinkedHashMap are ordered, so it can be guaranteed that when traversing a LinkedHashMap, iteration is always performed in a fixed order.

First, we need to import the LinkedHashMap class:

import java.util.LinkedHashMap;

Then, we can create an empty LinkedHashMap instance to store key-value pairs:

LinkedHashMap<String, Integer> map = new LinkedHashMap<String, Integer>();

Next, we can use put Method to add key-value pairs to the map:

map.put("apple", 10); // 插入键 "apple"，值为 10
map.put("banana", 20); // 插入键 "banana"，值为 20
map.put("orange", 30); // 插入键 "orange"，值为 30

We can use the get method to query by key:

Integer value = map.get("apple"); // 查询键 "apple" 的值，返回10

By using the keys() and values() methods, we can get the values ​​in the map respectively The collection of all keys and all values.

Set<String> keys = map.keySet(); // 获取所有键的集合
Collection<Integer> values = map.values(); // 获取所有值的集合

When traversing LinkedHashMap, we can use the entrySet() method. This method returns a Set containing all key-value pairs of the map, where the order of the key-value pairs is in the order of insertion.

for (Map.Entry<String, Integer> entry : map.entrySet()) {
    System.out.println(entry.getKey() + "=>" + entry.getValue());
}

Summary:

In Java programming, LinkedHashMap is a sortable key-value pair mapping structure, which can easily meet the needs of traversal in insertion order. When using LinkedHashMap, we can use put, get, remove and clear methods to manage key-value pairs, and at the same time traverse all key-value pairs through the entrySet() method. By mastering the basic usage of LinkedHashMap, we can perform more flexible programming.

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