Java TreeMap comparator()方法及示例

java.util.TreeMap 类的comparator () 方法用于返回用于对该地图中的键进行排序的比较器，如果该地图使用其键的自然排序，则为空。

--> java.util Package
    --> TreeMap Class
        --> comparator() Method 

语法

public Comparator comparator()

返回类型： 该方法返回用于对该地图中的键进行排序的 比较器 ，如果该地图使用其键的自然排序，则返回空。

注意： descendingIterator()方法默认为自然排序。

例1： 对于自然排序

// Java program to Illustrate comparator() Method
// for Natural Ordering (Descending Order)
 
// Importing required classes
import java.util.*;
 
// Main class
public class GFG {
 
    // Main driver method
    public static void main(String[] argv) throws Exception
    {
 
        // Try block to check for exceptions
        try {
 
            // Creating an empty TreeMap by
            // creating object of NavigableMap
            NavigableMap<Integer, String> treemap
                = new TreeMap<Integer, String>();
 
            // Populating TreeMap
            // using put() method
            treemap.put(1, "one");
            treemap.put(2, "two");
            treemap.put(3, "three");
            treemap.put(4, "four");
            treemap.put(5, "five");
 
            // Printing the TreeMap
            System.out.println("TreeMap: " + treemap);
 
            // Getting used Comparator in the map
            // using comparator() method
            Comparator comp = treemap.comparator();
 
            // Printing the comparator value
            System.out.println("Comparator value: " + comp);
        }
 
        // Catch block to handle the exception
        catch (NullPointerException e) {
 
            // Display message if exception occurs
            System.out.println("Exception thrown : " + e);
        }
    }
}

输出

TreeMap: {1=one, 2=two, 3=three, 4=four, 5=five}
Comparator value: null

例2： 用于反向排序

// Java program to demonstrate comparator() Method
// for Reverse Ordering
 
// Importing required classes
import java.util.*;
 
// Main class
public class GFG {
 
    // Main driver method
    public static void main(String[] argv) throws Exception
    {
 
        // Try block to check for exceptions
        try {
 
            // Creating an empty TreeMap
            NavigableMap<Integer, String> treemap
                = new TreeMap<Integer, String>(
                    Collections.reverseOrder());
 
            // Populating TreeMap
            // using put() method
            treemap.put(1, "one");
            treemap.put(2, "two");
            treemap.put(3, "three");
            treemap.put(4, "four");
            treemap.put(5, "five");
 
            // Printing the TreeMap
            System.out.println("TreeMap: " + treemap);
 
            // Getting used Comparator in the map
            // using comparator() method
            Comparator comp = treemap.comparator();
 
            // Printing the comparator value
            System.out.println("Comparator value: " + comp);
        }
 
        // Catch block to handle the exceptions
        catch (NullPointerException e) {
 
            // Display message if exception occurs
            System.out.println("Exception thrown : " + e);
        }
    }
}

输出

TreeMap: {5=five, 4=four, 3=three, 2=two, 1=one}
Comparator value: java.util.Collections$ReverseComparator@232204a1