Java TreeSet比较器()方法及实例

TreeSet是Java中SortedSet接口的最重要的实现之一，它使用Tree进行存储。无论是否提供显式比较器，元素的排序都是由一个集合使用其自然排序来维护的。如果要正确实现Set接口，这必须与equals一致。

java.util.TreeSet中的comparator()方法有一个重要的功能，就是设置和返回比较器，可以用来对TreeSet中的元素排序。如果集合遵循元素的自然排序模式，该方法返回一个Null值。

语法

comp_set = (TreeSet)tree_set.comparator()

参数： 该方法不接受任何参数。

返回值： 比较器集合用于将集合中的元素按特定顺序排序。如果集合遵循默认或自然的排序模式，它将返回一个Null值。

在这里我们将提出两个例子，首先我们将使用元素的自然排序，然后使用一个特定的比较器来更好地理解它。

例子1： 使用元素的自然排序

// Java Program to illustrate the use of comparator() method
// While using the natural ordering of the elements
  
// Importing utility classes
import java.util.*;
  
// Main class
// TreeSet Demo class
public class GFG {
  
    // Main driver method
    public static void main(String[] args)
    {
  
        // Creating an empty TreeSet of integer type
        TreeSet<Integer> tree_set = new TreeSet<Integer>();
  
        // Adding elements to the set
        // using add() method
        tree_set.add(20);
        tree_set.add(24);
        tree_set.add(30);
        tree_set.add(35);
        tree_set.add(45);
        tree_set.add(50);
  
        // Printing elements inside TreeSet object
        System.out.println("Tree Set values are: "
                           + tree_set);
  
        // Creating a comparator
        Comparator comp = tree_set.comparator();
  
        // Print and display the comparator values
        System.out.println("Since the Comparator value is: "
                           + comp);
        
        // Display message only 
        System.out.println("it follows natural ordering");
    }
}

输出

Tree Set values are: [20, 24, 30, 35, 45, 50]
Since the Comparator value is: null
it follows natural ordering

例2： 使用一个特定的比较器

// Java code to illustrate the use of comparator()
// While using a specific comparator
  
// Importing Comparator and TreeSet classes
// from java.util package
import java.util.Comparator;
import java.util.TreeSet;
  
// Class 1
// Helper class
class Helper implements Comparator<String> {
  
    // Method
    // To compare two strings
    public int compare(String str1, String str2)
    {
  
        String first_Str;
        String second_Str;
  
        first_Str = str1;
        second_Str = str2;
  
        // using compareTo() to ensure
        return second_Str.compareTo(first_Str);
    }
}
  
// Main class
// TreeSetDemo class
public class GFG {
  
    // Main driver method
    public static void main(String[] args)
    {
  
        // Creating an empty TreeSet of string type
        TreeSet<String> tree_set = new TreeSet<String>();
  
        // Adding elements to our TreeSet object
        // using add() method
        tree_set.add("G");
        tree_set.add("E");
        tree_set.add("E");
        tree_set.add("K");
        tree_set.add("S");
        tree_set.add("4");
  
        // Printing elements in set before using comparator
        System.out.println("Set before using the comparator: " + tree_set);
  
        // Again creating an empty TreeSet of string type
        // with reference to Helper class
        TreeSet<String> tree_set1 = new TreeSet<String>(new Helper());
  
        // Adding elements to our TreeSet object
        // using add() method
        tree_set1.add("G");
        tree_set1.add("E");
        tree_set1.add("E");
        tree_set1.add("K");
        tree_set1.add("S");
        tree_set1.add("4");
  
        // Printing elements in set before using comparator
        System.out.println("The elements sorted in descending order:" + tree_set1);
    }
}

输出

Set before using the comparator: [4, E, G, K, S]
The elements sorted in descending order:[S, K, G, E, 4]