Java Map 中 keySet() 与 value() 方法的区别

Map 接口存在于Java.util 包中，主要提供 KeySet()、entrySet() 和 values() 三个方法。这些方法分别用于检索地图的键、地图的键值对和地图的值。由于这些方法是 Map 接口的一部分，所以我们可以将这些方法用于实现 Map 接口的所有类，如 TreeMap、HashMap 和 LinkedHashMap。

为了找出差异，让我们首先从概念上逐一分析它们，然后通过实现来找出它们之间的主要差异。

方法一： keySet()方法

此方法用于返回此映射中包含的键的 Set 视图。该集合由地图支持，因此对地图的更改会反映在该集合中，反之亦然。

句法：

Set keySet()

参数：此方法没有参数。

返回：此方法返回一个包含指定映射键的集合。

执行：

例子

Java

// Java program demonstrating use of keySet() method
 
// Importing HashMap, Iterator, MAp and Stream classes
// from java.util package
import java.util.HashMap;
import java.util.Iterator;
import java.util.Map;
import java.util.stream.Stream;
 
// Class
class GFG {
 
    // Main driver method
    public static void main(String[] args)
    {
        // Creating object of Map Class
        // Declaring object of Interfere and string type
        Map map = new HashMap<>();
 
        // Adding the elements to the objects
        // Elements here are key-value pairs in the map
 
        // Custom input entries
        map.put(1, "Geeks");
        map.put(2, "For");
        map.put(3, "Geeks");
 
        // Now, different ways of iteration are illustrated
        // to showcase keySet() method
 
        // Way 1
        // Iterating the keySet() using iterator
 
        // Creating an object of Integer type
        Iterator itr = map.keySet().iterator();
 
        // Condition check where hasNext() method holds true
        // till there is single element remaining in th List
        while (itr.hasNext()) {
 
            // Print all the elements(key-value pairs)
            System.out.print(itr.next() + " ");
        }
 
        // New line
        System.out.println();
 
        // Way 2
        // Iterating the keySet()
        // using for loop
        for (Integer key : map.keySet()) {
 
            // Print all the key-value pairs
            System.out.print(key + " ");
        }
 
        // New line
        System.out.println();
 
        // Way 3
        // Iterating over the keySet() by
        // converting the map to the string
        // using the toString() method
        System.out.println(map.keySet().toString());
    }
}

输出

1 2 3 
1 2 3 
[1, 2, 3]

方法 2： values() 方法

在Java中HashMap类中的Java .util.HashMap.values()方法被用来创建一个集合出来的图中的值的。它基本上返回 HashMap 中值的集合视图。

句法：

Hash_Map.values()

参数：该方法不接受任何参数。

返回值：该方法用于返回包含地图所有值的集合视图。

实现：下面是使用values()方法的Java程序

例子

Java

// Java program demonstrating use of values() method
 
// Importing several classes from
// java.util package
import java.util.HashMap;
import java.util.Iterator;
import java.util.Map;
import java.util.stream.Stream;
 
// Class
// Main class
public class GFG {
 
    // Main driver method
    public static void main(String[] args)
    {
        // Making map of Integer keys and String values
        Map map = new HashMap<>();
 
        // Adding the elements to the above object
        // Declaring object of Integer and String type
 
        // Elements here are key-value pairs
        // Custom input entries
        map.put(1, "Geeks");
        map.put(2, "For");
        map.put(3, "Geeks");
 
        // values() method implemented by
        // demonstrating different ways of traversal
 
        // Way 1
        // Iterating the values() method
        // using iterator
        Iterator itr = map.values().iterator();
 
        // Condition check using hasNet() method which
        // holds true till there is single element remaining
        while (itr.hasNext()) {
            System.out.print(itr.next() + " ");
        }
        System.out.println();
 
        // Way 2
        // Iterating the values() method
        // using for each loop
        for (String key : map.values()) {
            System.out.print(key + " ");
        }
        System.out.println();
 
        // Way 3
        // iterating over the values() method
        // by converting the map to the string
        // using the toString() method
        System.out.println(map.values().toString());
    }
}

输出

Geeks For Geeks 
Geeks For Geeks 
[Geeks, For, Geeks]

现在终于到结论了，让我们看看 keySet() 方法和 values() 方法的区别如下：

keySet() method	values() method
This method returns the Set view of all the keys present in the map, ie it returns a set of keys.	This method returns the collection view of all the values contained in the map.
If any changes happen to the map, then they can be observed in the set also, as a set is backed up by the map.	If any changes happen to the map, then they can be observed in the collection also, as the collection is backed up by the map.
This method is used only when we need to deal with all the keys present in the map.	This method is used when we only need to deal with all the values present in the map.