Java ConcurrentHashMap putIfAbsent()方法

java.util.concurrent.ConcurrentHashMap.putIfAbsent() 是Java中的一个内置函数，它接受一个键和一个值作为参数，如果指定的键没有映射到任何值，就会映射它们。

语法

chm.putIfAbsent(key_elem, val_elem)

参数： 该函数接受两个参数，描述如下。

key_elem: 如果key_elem没有与任何值相关联，这个参数指定指定的val_elem所要映射的键。
val_elem: 这个参数指定要映射到指定key_elem的值。

返回值： 该函数返回映射到键的现有值，如果之前没有值映射到键，则返回null。

异常： 当指定的参数为空时，该函数抛出NullPointerException。

下面的程序说明了ConcurrentHashMap.putIfAbsent()方法。

程序1： 一个现有的键被作为参数传递给函数。

// Java Program Demonstrate putIfAbsent()
// 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");
        chm.put(103, "Gfg");
        chm.put(104, "GFG");
  
        // Displaying the HashMap
        System.out.println("Initial Mappings are: "
                           + chm);
  
        // Inserting non-existing key along with value
        String returned_value = (String)chm.putIfAbsent(108, "All");
  
        // Verifying the returned value
        System.out.println("Returned value is: "
                           + returned_value);
  
        // Displayin the new map
        System.out.println("New mappings are: "
                           + chm);
    }
}

输出。

Initial Mappings are: {100=Geeks, 101=for, 102=Geeks, 103=Gfg, 104=GFG}
Returned value is: null
New mappings are: {100=Geeks, 101=for, 102=Geeks, 103=Gfg, 104=GFG, 108=All}

程序2： 一个不存在的键被作为参数传给函数。

// Java Program Demonstrate putIfAbsent()
// 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");
        chm.put(103, "Gfg");
        chm.put(104, "GFG");
  
        // Displaying the HashMap
        System.out.println("Initial Mappings are: "
                           + chm);
  
        // Inserting existing key along with value
        String returned_value = (String)chm.putIfAbsent(100, "All");
  
        // Verifying the returned value
        System.out.println("Returned value is: "
                           + returned_value);
  
        // Displayin the new map
        System.out.println("New mappings are: "
                           + chm);
    }
}

输出。

Initial Mappings are: {100=Geeks, 101=for, 102=Geeks, 103=Gfg, 104=GFG}
Returned value is: Geeks
New mappings are: {100=Geeks, 101=for, 102=Geeks, 103=Gfg, 104=GFG}

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