java.util.Map.containsKey()方法用于检查特定键是否已映射到Map中。它使用key元素作为参数，如果该元素在映射中映射，则返回True。
用法:
boolean containsKey(key_element)
参数：该方法仅采用一个参数key_element，该参数表示应该在映射内检查其映射的键。
返回值：如果检测到键的存在，则该方法返回boolean true，否则返回false。
以下程序用于说明java.util.Map.containsKey()方法的用法：
示例1:将字符串值映射到整数键。
// Java code to illustrate the containsKey() method
import java.util.*;
public class Map_Demo {
public static void main(String[] args)
{
// Creating an empty Map
Map map = new HashMap();
// Mapping string values to int keys
map.put(10, “Geeks”);
map.put(15, “4”);
map.put(20, “Geeks”);
map.put(25, “Welcomes”);
map.put(30, “You”);
// Displaying the Map
System.out.println(“Initial Mappings are: ” + map);
// Checking for the key_element ’20’
System.out.println(“Is the key ’20’ present? “
+ map.containsKey(20));
// Checking for the key_element ‘5’
System.out.println(“Is the key ‘5’ present? “
+ map.containsKey(5));
}
}
输出：
Initial Mappings are: {20=Geeks, 25=Welcomes, 10=Geeks, 30=You, 15=4}
Is the key ’20’ present? true
Is the key ‘5’ present? false
示例2:将整数值映射到字符串键。
// Java code to illustrate the containsKey() method
import java.util.*;
public class Map_Demo {
public static void main(String[] args)
{
// Creating an empty Map
Map map = new HashMap();
// Mapping int values to string keys
map.put(“Geeks”, 10);
map.put(“4”, 15);
map.put(“Geeks”, 20);
map.put(“Welcomes”, 25);
map.put(“You”, 30);
// Displaying the Map
System.out.println(“Initial Mappings are: ” + map);
// Checking for the key_element ‘Welcomes’
System.out.println(“Is the key ‘Welcomes’ present? “
+ map.containsKey(“Welcomes”));
// Checking for the key_element ‘World’
System.out.println(“Is the key ‘World’ present? “
+ map.containsKey(“World”));
}
}
输出：
Initial Mappings are: {4=15, Geeks=20, You=30, Welcomes=25}
Is the key ‘Welcomes’ present? true
Is the key ‘World’ present? false
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。