Java中的 TreeSet pollFirst() 方法

Java中 TreeSet 的 pollFirst() 方法用于检索和删除第一个（最低）元素，如果此 TreeSet 为空，则返回 null。
句法：

E pollFirst()

其中，E 是此 TreeSet 容器维护的元素的类型。
参数：此函数不接受任何参数。
返回类型：如果 TreeSet 不为空，则返回 Treeset 的第一个元素，否则返回 null。
下面的程序说明了Java中的 pollFirst() 方法：

示例 1

// A Java program to demonstrate
// pollFirst() method of TreeSet
  
import java.util.TreeSet;
  
public class GFG {
    public static void main(String[] args)
    {
        // creating TreeSet
        TreeSet ts = new TreeSet();
  
        // adding element to the TreeSet
        ts.add(0);
        ts.add(1);
        ts.add(2);
        ts.add(3);
        ts.add(4);
        ts.add(5);
        ts.add(6);
  
        // before removing element
        System.out.println("Before removing " + 
                   "element from TreeSet: " + ts);
  
        // first element is removed
        System.out.println("First lowest element " + 
                 "removed is : " + ts.pollFirst());
        System.out.println("After removing element" + 
                             " from TreeSet: " + ts);
    }
}

输出：

Before removing element from TreeSet: [0, 1, 2, 3, 4, 5, 6]
First lowest element removed is : 0
After removing element from TreeSet: [1, 2, 3, 4, 5, 6]

示例 2

// A Java program to demonstrate
// pollFirst() method of TreeSet
  
import java.util.TreeSet;
  
public class GFG {
    public static void main(String[] args)
    {
        // creating TreeSet
        TreeSet ts = new TreeSet();
  
        // removing element when TreeSet
        // is empty
        System.out.println("pollFirst() method when " + 
             "TreeSet is empty  :  " + ts.pollFirst());
  
        // adding element to the TreeSet
        ts.add("Geeks");
        ts.add("for");
        ts.add("Geek");
  
        // before removing element
        System.out.println("Before removing element from" + 
                                      " TreeSet: " + ts);
  
        // first element is removed
        System.out.println("First lowest element removed is : "
                           + ts.pollFirst());
        System.out.println("After removing element " + 
                               "from TreeSet: " + ts);
    }
}

输出：

pollFirst() method when TreeSet is empty  :  null
Before removing element from TreeSet: [Geek, Geeks, for]
First lowest element removed is : Geek
After removing element from TreeSet: [Geeks, for]

如果 TreeSet 为空，则此方法返回 null。
示例 3

// A Java program to demonstrate
// pollFirst() method of TreeSet
  
import java.util.TreeSet;
  
public class GFG {
    public static void main(String[] args)
    {
        // creating TreeSet
        TreeSet ts = new TreeSet();
  
        // before removing element
        System.out.println("Before removing " + 
                   "element from TreeSet: " + ts);
  
        // first element is removed
        System.out.println("First lowest element " + 
                 "removed is : " + ts.pollFirst());
        System.out.println("After removing element" + 
                             " from TreeSet: " + ts);
    }
}

输出：

Before removing element from TreeSet: []
First lowest element removed is : null
After removing element from TreeSet: []