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 : Geeks
The 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 : 100
The Value associated to Contribute is : 102

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