Java TreeSet descendingSet()方法及示例

java.util.TreeSet <E>类的 descendingSet() 方法用于返回这个集合中所包含的元素的反向顺序视图。降序集合是由这个集合支持的，所以对这个集合的修改会反映在降序集合中，反之亦然。如果在任何一个集合的迭代过程中，任何一个集合被修改了（除了通过迭代器自身的移除操作），那么迭代的结果是未定义的。

返回的集合的排序等同于Collections.reverseOrder(comparator())。表达式s.descendingSet().descendingSet()返回s的视图，本质上等同于s。

语法：

public NavigableSet descendingSet()

返回值： 该方法返回该集合的 倒序视图 。

以下是说明descendingSet()方法的例子

例子1：

// Java program to demonstrate
// descendingSet() method
// for String value
 
import java.util.*;
 
public class GFG1 {
    public static void main(String[] argv)
        throws Exception
    {
        try {
 
            // create tree set object
            TreeSet<String> treeadd = new TreeSet<String>();
 
            // populate the TreeSet using add() method
            treeadd.add("A");
            treeadd.add("B");
            treeadd.add("C");
            treeadd.add("D");
 
            // Print the TreeSet
            System.out.println("TreeSet: " + treeadd);
 
            // getting the reverse order view of element
            // using descendingSet() method
            NavigableSet<String>
                treereverse = treeadd.descendingSet();
 
            // getting iterated view of NavigableSet
            Iterator<String> iterator = treereverse.iterator();
 
            System.out.println("\nValues using DescendingSet:");
 
            // printing the integrated value
            while (iterator.hasNext()) {
                System.out.println("Value : "
                                   + iterator.next());
            }
        }
 
        catch (NullPointerException e) {
 
            System.out.println("Exception thrown : " + e);
        }
    }
}

输出

TreeSet: [A, B, C, D]

Values using DescendingSet:
Value : D
Value : C
Value : B
Value : A

例2：

// Java program to demonstrate
// descendingSet() method
// for Integer value
 
import java.util.*;
 
public class GFG1 {
    public static void main(String[] argv) throws Exception
    {
        try {
 
            // create tree set object
            TreeSet<Integer> treeadd = new TreeSet<Integer>();
 
            // populate the TreeSet using add() method
            treeadd.add(10);
            treeadd.add(20);
            treeadd.add(30);
            treeadd.add(40);
 
            // getting the reverse order view of element
            // using descendingSet() method
            NavigableSet<Integer> treereverse = treeadd.descendingSet();
 
            // getting iterated view of NavigableSet
            Iterator<Integer> iterator = treereverse.iterator();
 
            System.out.println("\nValues using DescendingSet:");
 
            // printing the integrated value
            while (iterator.hasNext()) {
                System.out.println("Value : " + iterator.next());
            }
        }
 
        catch (NullPointerException e) {
 
            System.out.println("Exception thrown : " + e);
        }
    }
}

输出

Values using DescendingSet:
Value : 40
Value : 30
Value : 20
Value : 10