TreeMap的keySet()方法在Java中的使用及示例

在Java中，TreeMap类的 keySet() 方法位于java.util包中，用于将treemap中包含的键元素创建为一个集合。它基本上返回了键的一个集合视图，或者我们可以创建一个新的集合并按升序将键元素存储在其中。由于集合由映射支持，对映射所做的任何更改都会反映在集合中，反之亦然。
–> java.util Package
–> TreeMap Class
–> keySet() Method

语法:

tree_map.keySet()

返回值: 一个以升序包含treemap键的集合。

示例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]

示例2： 将整数值映射到字符串键。

// 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： 类似的操作可使用不同的数据类型进行组合进行映射。