Java TreeMap keySet()方法及实例

在Java中，TreeMap类的keySet()方法 存在于Java的java.util包中，用于创建一个包含在treemap中的关键元素的集合。它基本上返回一个键的集合视图，或者我们可以创建一个新的集合并将键元素以升序存储在其中。因为这个集合是由地图支持的，所以对地图的任何改变都会反映在集合中，反之亦然。

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

语法

tree_map.keySet()

返回类型： 一个具有升序排列的treemap键的集合。

例1： 将字符串值映射到整数键。

// Java Program to illustrate the keySet() method
// of TreeMap class where we are
// Mapping String Values to Integer Keys
 
// Importing required classes
import java.util.*;
 
// Main class
public class GFG {
 
    // MAin driver method
    public static void main(String[] args)
    {
        // Creating an empty TreeMap by
        // declaring object of integer, string pairs
        TreeMap<Integer, String> tree_map
            = new TreeMap<Integer, String>();
 
        // Mapping string values to int keys
        // using put() method
        tree_map.put(10, "Geeks");
        tree_map.put(15, "4");
        tree_map.put(20, "Geeks");
        tree_map.put(25, "Welcomes");
        tree_map.put(30, "You");
 
        // Printing the elements of above TreeMap
        System.out.println("Initial Mappings are: "
                           + tree_map);
 
        // Getting the set view of keys
        // using keySet() method
        System.out.println("The set is: "
                           + tree_map.keySet());
    }
}

输出

Initial Mappings are: {10=Geeks, 15=4, 20=Geeks, 25=Welcomes, 30=You}
The set is: [10, 15, 20, 25, 30]

例2： 将整数值映射到字符串键上

// Java Program to Illustrate keySet() Method
// of TreeMap class where we are
// Mapping Integer Values to String Keys
 
// Importing required classes
import java.util.*;
 
// Main class
public class GFG {
 
    // Main driver method
    public static void main(String[] args)
    {
        // Creating an empty TreeMap by
        // declaring object of string, integer pairs
        TreeMap<String, Integer> tree_map
            = new TreeMap<String, Integer>();
 
        // Mapping int values to string keys
        // using put() method
        tree_map.put("Geeks", 10);
        tree_map.put("4", 15);
        tree_map.put("Geeks", 20);
        tree_map.put("Welcomes", 25);
        tree_map.put("You", 30);
 
        // Printing the elements of TreeMap
        System.out.println("Initial Mappings are: "
                           + tree_map);
 
        // Getting the set view of keys
        // using keySet() method
        System.out.println("The set is: "
                           + tree_map.keySet());
    }
}

输出

Initial Mappings are: {4=15, Geeks=20, Welcomes=25, You=30}
The set is: [4, Geeks, Welcomes, You]

注意： 同样的操作也可以通过不同数据类型的变化和组合对任何类型的映射进行操作。