How to use the EnumMap class in Java to implement a collection of key-value pairs based on enumerations

How Java uses the EnumMap class to implement an enumeration-based key-value pair collection

Overview:
EnumMap is a special implementation class in the Java collection framework, used to implement enumeration-based key-value pairs gather. Its characteristic is that keys can only be enumeration types, and its performance is better than HashMap. This article will introduce how to use the EnumMap class and provide code examples.

1. Basic introduction to the EnumMap class
   EnumMap is a class in the Java collection framework. It inherits from the AbstractMap class and implements the NavigableMap interface. The key set of EnumMap can only be of enumeration type, and an enumeration class is required to be passed in as a parameter for construction. The EnumMap class provides various methods to manipulate and traverse a collection of key-value pairs of an enumeration type.
2. Construction method of EnumMap
   The EnumMap class provides multiple construction methods, the commonly used ones are as follows:
3. EnumMap(Class enumType): Create an empty object using the specified enumeration type EnumMap instance.
4. EnumMap(EnumMap map): Initialize a new EnumMap instance using the specified EnumMap instance.
5. EnumMap(Map map): Initialize a new EnumMap instance using the specified Map instance.
6. Basic usage of EnumMap
   Next, let’s take a look at how to use the EnumMap class to implement a collection of key-value pairs based on enumeration.

Code example:

import java.util.EnumMap;

public class EnumMapExample {
    
    // 定义一个枚举类型
    enum Day {
        MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY, SATURDAY, SUNDAY
    }
    
    public static void main(String[] args) {
        
        // 创建一个EnumMap实例
        EnumMap<Day, String> enumMap = new EnumMap<>(Day.class);
        
        // 添加键值对
        enumMap.put(Day.MONDAY, "星期一");
        enumMap.put(Day.TUESDAY, "星期二");
        enumMap.put(Day.WEDNESDAY, "星期三");
        
        // 获取值
        System.out.println(enumMap.get(Day.MONDAY));
        
        // 遍历EnumMap
        for (Day day : enumMap.keySet()) {
            System.out.println(day + "：" + enumMap.get(day));
        }
        
        // 判断键是否存在
        System.out.println(enumMap.containsKey(Day.SUNDAY));
    }
}

Output result:

星期一
MONDAY：星期一
TUESDAY：星期二
WEDNESDAY：星期三
false

In the above code example, we first define an enumeration type Day, Contains seven enumeration constants. Then we created an EnumMap instance and added several key-value pairs. The value of the specified key can be obtained through the enumMap.get(key) method. We also showed how to iterate over the EnumMap and use the containsKey(key) method to determine whether a key exists in the EnumMap.

4. Performance advantages of EnumMap
   EnumMap has several performance advantages over HashMap:
5. The internal implementation is based on arrays. Compared with HashMap's linked list or Red-black tree, EnumMap query and insert operations are faster.
6. EnumMap takes up less storage space because no additional storage space is required to save the hash value of the key.

Summary:
EnumMap is a special implementation class in the Java collection framework, used to implement enumeration-based key-value pair collections. Has efficient performance and small storage space footprint. Through the introduction of this article, we have learned about the construction method and basic usage of the EnumMap class, and provided corresponding code examples. In actual development, when we need to implement a collection of key-value pairs based on an enumeration class, EnumMap is a very good choice.

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