Java 中的 ConcurrentHashMap containsKey()方法

原文:https://www . geesforgeks . org/concurrenthashmap-contains key-method-in-Java/

java . util . concurrenthashmap . contains KeY()方法是 Java 中的一个内置函数，它接受一个参数并检查它是否是这个映射中的一个键。

语法:

chm.containsKey*(Object key_element)*

参数:该方法接受对象类型的单个参数 key_element ，检查其是否为键。

返回值:如果指定的 key_element 是该地图的的一个键，则该方法返回 true，否则返回 false。

异常:当指定的 key_element 为空时，函数抛出 NullPointerException 。

下面的程序举例说明了Java . util . concurrenthashmap . contains key()方法的使用:

程序 1: 该程序涉及将字符串值映射到整数键。

/* Java Program Demonstrate containsKey()
   method of ConcurrentHashMap */

import java.util.concurrent.*;
class ConcurrentHashMapDemo {
    public static void main(String[] args)
    {
        ConcurrentHashMap<Integer, String> chm =
                 new ConcurrentHashMap<Integer, String>();
        chm.put(100, "Geeks");
        chm.put(101, "for");
        chm.put(102, "Geeks");

        // Checking whether 105 is a key of the map
        if (chm.containsKey(105)) {
            System.out.println("105 is a key.");
        }
        else {
            System.out.println("105 is not a key.");
        }

        // Checking whether 100 is a key of the map
        if (chm.containsKey(100)) {
            System.out.println("100 is a key.");
        }
        else {
            System.out.println("100 is not a key.");
        }
    }
}

输出:

105 is not a key.
100 is a key.

程序 2: 该程序涉及将整数值映射到字符串键。

/* Java Program Demonstrate containsKey()
   method of ConcurrentHashMap */

import java.util.concurrent.*;
class ConcurrentHashMapDemo {
    public static void main(String[] args)
    {
        ConcurrentHashMap<String, Integer> chm = 
                  new ConcurrentHashMap<String, Integer>();
        chm.put("Geeks", 120);
        chm.put("for", 11);
        chm.put("GeeksforGeeks", 15);
        chm.put("Gfg", 50);
        chm.put("GFG", 25);

        // Checking whether GFG is a key of the map
        if (chm.containsKey("GFG")) {
            System.out.println("GFG is a key.");
        }
        else {
            System.out.println("GFG is not a key.");
        }

        // Checking whether Geek is a key of the map
        if (chm.containsKey("Geek")) {
            System.out.println("Geek is a key.");
        }
        else {
            System.out.println("Geek is not a key.");
        }
    }
}

输出:

GFG is a key.
Geek is not a key.

参考:https://docs . Oracle . com/javase/7/docs/API/Java/util/concurrenthashmap . html # contains key()

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