Java ConcurrentSkipListSet pollFirst()方法

java.util.concurrent.ConcurrentSkipListSet 的 pollFirst() 方法是Java中的一个内置函数，它返回检索和删除第一个（最低）元素，如果这个集合为空，则返回null。

语法

public E pollFirst()

返回值： 该函数返回检索并删除第一个（最低）元素，如果这个集合为空，则返回null。

下面的程序说明了ConcurrentSkipListSet.pollFirst（）方法。

程序1 :

// Java program to demonstrate pollFirst()
// method of ConcurrentSkipListSet
  
import java.util.concurrent.*;
  
class ConcurrentSkipListSetpollFirstExample1 {
    public static void main(String[] args)
    {
        // Creating a set object
        ConcurrentSkipListSet<Integer> 
Lset = new ConcurrentSkipListSet<Integer>();
  
        // Adding elements to this set
        for (int i = 10; i <= 50; i += 10)
            Lset.add(i);
  
        // Printing the content of the set
        System.out.println("Contents of the set: " + Lset);
  
        // Retrieving and removing first element of the set
        System.out.println("The first element of the set: " 
+ Lset.pollFirst());
  
        // Printing the content of the set after pollFirst()
        System.out.println("Contents of the set after pollFirst: " 
+ Lset);
    }
}

输出:

Contents of the set: [10, 20, 30, 40, 50]
The first element of the set: 10
Contents of the set after pollFirst: [20, 30, 40, 50]

程序2

// Java program to demonstrate pollFirst()
// method of ConcurrentSkipListSet
  
import java.util.concurrent.*;
  
class ConcurrentSkipListSetpollFirstExample2 {
    public static void main(String[] args)
    {
        // Creating a set object
        ConcurrentSkipListSet<Integer> 
Lset = new ConcurrentSkipListSet<Integer>();
  
        // Printing the content of the set
        System.out.println("Contents of the set: " + Lset);
  
        // Retrieving and removing first element of the set
        System.out.println("The first element of the set: " 
+ Lset.pollFirst());
    }
}

输出:

Contents of the set: []
The first element of the set: null

参考资料:
https://docs.oracle.com/javase/8/docs/api/java/util/concurrent/ConcurrentSkipListSet.html#pollFirst-