Java ConcurrentHashMap get()方法

java.util.concurrent.ConcurrentHashMap的 get() 方法是Java中的一个内置函数，它接受一个键作为参数并返回映射到它的值。如果不存在作为参数的键的映射，则返回空。

语法

Concurrent.get(Object key_element)

参数： 该方法接受一个对象类型的参数 key_element ，指的是要返回其相关值的键。

返回值： 该方法返回与参数中 key_element 相关的值。

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

下面的程序说明了 java.util.concurrent.ConcurrentHashMap.get() 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 get()// method of ConcurrentHashMap  import java.util.concurrent.*;  class GFG {    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");        chm.put(103, "Contribute");          // Displaying the HashMap        System.out.println("The Mappings are: ");        System.out.println(chm);          // Display the value of 100        System.out.println("The Value associated to "                           + "100 is : " + chm.get(100));          // Getting the value of 103        System.out.println("The Value associated to "                           + "103 is : " + chm.get(103));    }}

输出:

The Mappings are: {100=Geeks, 101=for, 102=Geeks, 103=Contribute}The Value associated to 100 is : GeeksThe Value associated to 103 is : Contribute

程序2： 该程序涉及将整数值映射到字符串键。

// Java Program Demonstrate get()// method of ConcurrentHashMap  import java.util.concurrent.*;  class GFG {    public static void main(String[] args)    {          ConcurrentHashMap<String, Integer>            chm = new ConcurrentHashMap<String, Integer>();          chm.put("Geeks", 100);        chm.put("GFG", 10);        chm.put("GeeksforGeeks", 25);        chm.put("Contribute", 102);          // Displaying the HashMap        System.out.println("The Mappings are: ");        System.out.println(chm);          // Display the value of Geeks        System.out.println("The Value associated to "                           + "Geeks is : " + chm.get("Geeks"));          // Getting the value of Contribute        System.out.println("The Value associated to "                           + "Contribute is : " + chm.get("Contribute"));    }}

输出:

The Mappings are: {GeeksforGeeks=25, Geeks=100, GFG=10, Contribute=102}The Value associated to Geeks is : 100The Value associated to Contribute is : 102

参考资料 : https://docs.oracle.com/javase/7/docs/api/java/util/concurrent/ConcurrentHashMap.html#get()