Java中的 ConcurrentHashMap containsValue() 方法

Java .util.concurrent.ConcurrentHashMap.containsValue()方法是Java中的一个内置函数，它接受一个值并在一个或多个键映射到指定值时返回 true。该方法遍历整个哈希表。因此，它是一个比 containsKey() 方法慢得多的函数。

句法：

chm.containsValue(Object val_element)

参数：该方法接受对象类型的单个参数val_element ，以检查它是否映射到映射中的任何键。

返回值：如果指定的val_element映射到此映射中的任何键，则该方法返回 true，否则返回 false。

异常：当指定的value_element为 null 时，函数会抛出 NullPointerException。

下面的程序说明了Java.util.concurrent.ConcurrentHashMap.containsValue()方法的使用：

程序 1：该程序涉及将整数值映射到字符串键。

/* Java Program to demonstrate containsValue()
   method of ConcurrentHashMap */
  
import java.util.concurrent.*;
class ConcurrentHashMapDemo {
    public static void main(String[] args)
    {
        ConcurrentHashMap chm = 
                    new ConcurrentHashMap();
        chm.put("Geeks", 120);
        chm.put("for", 11);
        chm.put("GeeksforGeeks", 15);
        chm.put("Gfg", 50);
        chm.put("GFG", 25);
  
        // Check whether a key is mapped to 100
        if (chm.containsValue(100)) {
            System.out.println("100 is mapped.");
        }
        else {
            System.out.println("100 is not mapped.");
        }
  
        // Check whether a key is mapped to 120
        if (chm.containsValue(120)) {
            System.out.println("120 is mapped.");
        }
        else {
            System.out.println("120 is not mapped.");
        }
    }
}

输出：

100 is not mapped.
120 is mapped.

程序 2：该程序涉及将字符串值映射到整数键。

/* Java Program to demonstrate containsValue()
   method of ConcurrentHashMap */
  
import java.util.concurrent.*;
class ConcurrentHashMapDemo {
    public static void main(String[] args)
    {
        ConcurrentHashMap chm = 
                 new ConcurrentHashMap();
        chm.put(100, "Geeks");
        chm.put(101, "for");
        chm.put(102, "Geeks");
        chm.put(103, "Gfg");
        chm.put(104, "GFG");
  
        // Check whether a key is mapped to Geeks
        if (chm.containsValue("Geeks")) {
            System.out.println("Geeks is mapped.");
        }
        else {
            System.out.println("Geeks is not mapped.");
        }
  
        // Check whether a key is mapped to GfG
        if (chm.containsValue("GfG")) {
            System.out.println("GfG is mapped.");
        }
        else {
            System.out.println("GfG is not mapped.");
        }
    }
}

输出：

Geeks is mapped.
GfG is not mapped.

参考： https: Java/util/concurrent/ConcurrentHashMap.html#containsValue()