Java中的ConcurentHashMap类的keySet()方法用于从哈希映射中包含的关键元素中创建一个集合。它本质上返回键的集合视图，或者我们可以创建一个新集合并将键元素存储在其中。

用法:

ConcurrentHashMap.keySet()

参数：该方法不带任何参数。

返回值：该方法返回一个具有哈希映射键的集合。

以下程序用于说明keySet()方法的用法：

示例1:将字符串值映射到整数键。

// Java code to illustrate the keys() method

import java.util.*;

import java.util.concurrent.*;

public class ConcurrentHashMapDemo {

public static void main(String[] args)

{

// Creating an empty ConcurrentHashMap

ConcurrentHashMap hash_map

= new ConcurrentHashMap();

// Mapping string values to int keys

hash_map.put(10, "Geeks");

hash_map.put(15, "4");

hash_map.put(20, "Geeks");

hash_map.put(25, "Welcomes");

hash_map.put(30, "You");

// Displaying the HashMap

System.out.println("Initial Mappings are: "

+ hash_map);

// Using keySet() to get the set view of keys

System.out.println("The set is: "

+ hash_map.keySet());

}

}

输出：

Initial Mappings are: {20=Geeks, 25=Welcomes, 10=Geeks, 30=You, 15=4}

The set is: [20, 25, 10, 30, 15]

示例2:将整数值映射到字符串键。

// Java code to illustrate the keys() method

import java.util.*;

import java.util.concurrent.*;

public class ConcurrentHashMapDemo {

public static void main(String[] args)

{

// Creating an empty ConcurrentHashMap

ConcurrentHashMap

hash_map = new ConcurrentHashMap

Integer>();

// Inserting elements into the table

hash_map.put("Geeks", 10);

hash_map.put("4", 15);

hash_map.put("Geeks", 20);

hash_map.put("Welcomes", 25);

hash_map.put("You", 30);

// Displaying the HashMap

System.out.println("Initial Mappings are: "

+ hash_map);

// Using keySet() to get the set view of keys

System.out.println("The set is: "

+ hash_map.keySet());

}

}

输出：

Initial Mappings are: {4=15, Geeks=20, You=30, Welcomes=25}

The set is: [4, Geeks, You, Welcomes]

注意：可以对具有不同数据类型的变化和组合的任何类型的映射执行相同的操作。