原文： https://beginnersbook.com/2014/08/java-get-set-view-of-keys-from-hashmap/

描述

程序从HashMap获取键集。

示例

  1. import java.util.Iterator;
  2. import java.util.HashMap;
  3. import java.util.Set;
  4. class HashMapExample{
  6.   public static void main(String args[]) {
  8.     // Create a HashMap
  9.     HashMap<String, String> hmap = new HashMap<String, String>(); 
  11.     // Adding few elements
  12.     hmap.put("Key1", "Jack");
  13.     hmap.put("Key2", "Rock");
  14.     hmap.put("Key3", "Rick");
  15.     hmap.put("Key4", "Smith");
  16.     hmap.put("Key5", "Will");
  18.     // Getting Set of HashMap keys
  19.     /* public Set<K> keySet(): Returns a Set view of the keys contained
  20.      * in this map. The set is backed by the map, so changes to the map 
  21.      * are reflected in the set, and vice-versa. 
  22.      */
  23.     Set<String> keys = hmap.keySet();
  25.     System.out.println("Set of Keys contains: ");
  27.     /* If your HashMap has integer keys then specify the iterator like
  28.      * this: Iterator<Integer> it = keys.iterator();
  29.      */
  30.     Iterator<String> it = keys.iterator();
  31.     // Displaying keys. Output will not be in any particular order
  32.     while(it.hasNext()){
  33.        System.out.println(it.next());
  34.     } 
  35.   }
  36. }

输出：

  1. Set of Keys contains: 
  2. Key2
  3. Key1
  4. Key4
  5. Key3
  6. Key5

注意：这组键由原始HashMap备份，因此如果从Set中删除任何键，它将自动从HashMap中删除。