Java中的 ConcurrentSkipListSet headSet() 方法

headSet(E toElement)

Java .util.concurrent.ConcurrentSkipListSet.headSet()方法是Java中的一个内置函数，它返回该集合中元素严格小于 toElement 的部分的视图。返回的集合由该集合支持，因此返回集合中的更改会反映在该集合中，反之亦然。返回的集合支持该集合支持的所有可选集合操作。

句法：

public NavigableSet headSet(E toElement)

参数：该函数接受单个参数toElement ，即返回集的高端点。

返回值：该函数返回一个NavigableSet ，它是该集合中元素严格小于 toElement 的部分的视图。

下面的程序说明了 ConcurrentSkipListSet.headSet(E toElement) 方法：
方案一：

// Java Program Demonstrate headSet()
// method of ConcurrentSkipListSet */
import java.util.NavigableSet;
import java.util.concurrent.ConcurrentSkipListSet;
  
class ConcurrentSkipListSetHeadSetExample1 {
    public static void main(String[] args)
    {
  
        // Initializing the set
        ConcurrentSkipListSet
            set = new ConcurrentSkipListSet();
  
        // Adding elements to this set
        for (int i = 10; i <= 50; i += 10)
            set.add(i);
  
        // Creating a headSet object with upper limit 30
        NavigableSet hd_set = set.headSet(30);
  
        // Printing the elements of the set
        System.out.println("Contents of the set: " + set);
  
        // Printing the elements of the headSet set
        System.out.println("Contents of the headset"
                           + " with upper limit 30: "
                           + hd_set);
    }
}

输出：

Contents of the set: [10, 20, 30, 40, 50]
Contents of the headset with upper limit 30: [10, 20]

程序 2：显示 NullPointerException 的程序。

// Java Program Demonstrate headSet()
// method of ConcurrentSkipListSet */
import java.util.NavigableSet;
import java.util.concurrent.ConcurrentSkipListSet;
  
class ConcurrentSkipListSetHeadSetExample2 {
    public static void main(String[] args)
    {
  
        // Initializing the set
        ConcurrentSkipListSet
            set = new ConcurrentSkipListSet();
  
        // Adding elements to this set
        for (int i = 10; i <= 50; i += 10)
            set.add(i);
  
        // Printing the elements of the set
        System.out.println("Contents of the set: " + set);
  
        try {
            // Trying to creating a headSet object with upper limit null
            NavigableSet hd_set = set.headSet(null);
        }
  
        catch (Exception e) {
            System.out.println("Exception: " + e);
        }
        // Printing the elements of the headSet set
        // System.out.println("Contents of the headset"
+" with upper limit 30: "+ hd_set);
    }
}

输出：

Contents of the set: [10, 20, 30, 40, 50]
Exception: java.lang.NullPointerException

headSet(E toElement, boolean inclusive)

Java .util.concurrent.ConcurrentSkipListSet.headSet()方法是Java中的一个内置函数，它返回该集合中元素小于（或等于，如果 inclusive 为真）toElement 的部分的视图。返回的集合由该集合支持，因此返回集合中的更改会反映在该集合中，反之亦然。返回的集合支持该集合支持的所有可选集合操作。

句法：

public NavigableSet headSet(E toElement,
                      boolean inclusive)

参数：该函数接受以下参数

toElement即返回集合的高端点。

inclusive – 如果要在返回的视图中包含高端端点，则为 true。

返回值：该函数返回一个 NavigableSet，它是该集合中元素小于（或等于，如果 inclusive 为真）toElement 的部分的视图。

下面的程序说明了 ConcurrentSkipListSet.headSet(E toElement, boolean inclusive) 方法：

方案 3：

// Java Program Demonstrate headSet()
// method of ConcurrentSkipListSet */
  
import java.util.NavigableSet;
import java.util.concurrent.ConcurrentSkipListSet;
  
class ConcurrentSkipListSetHeadSetExample3 {
    public static void main(String[] args)
    {
  
        // Initializing the set
        ConcurrentSkipListSet
            set = new ConcurrentSkipListSet();
  
        // Adding elements to this set
        for (int i = 10; i <= 50; i += 10)
            set.add(i);
  
        // Creating a headSet object with upper limit 30 inclusive
        NavigableSet hd_set = set.headSet(30, true);
  
        // Printing the elements of the set
        System.out.println("Contents of the set: " + set);
  
        // Printing the elements of the headSet set
        System.out.println("Contents of the headset"
                           + " with upper limit 30 inclusive: "
                           + hd_set);
    }
}

输出：

Contents of the set: [10, 20, 30, 40, 50]
Contents of the headset with upper limit 30 inclusive: [10, 20, 30]

参考： https: Java/util/concurrent/ConcurrentSkipListSet.html#headSet-E-