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Get Set view of Keys from Java HashMap example

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  1. /*
  2.   Get Set view of Keys from Java HashMap example
  3.   This Java Example shows how to get a Set of keys contained in HashMap
  4.   using keySet method of Java HashMap class.
  5. */
  6.  
  7. import java.util.Iterator;
  8. import java.util.HashMap;
  9. import java.util.Set;
  10.  
  11. public class GetSetViewOfKeysFromHashMapExample {
  12.  
  13.   public static void main(String[] args) {
  14.    
  15.     //create HashMap object
  16.     HashMap hMap = new HashMap();
  17.    
  18.     //add key value pairs to HashMap
  19.     hMap.put("1","One");
  20.     hMap.put("2","Two");
  21.     hMap.put("3","Three");
  22.    
  23.     /*
  24.       get Set of keys contained in HashMap using
  25.       Set keySet() method of HashMap class
  26.     */
  27.    
  28.     Set st = hMap.keySet();
  29.    
  30.     System.out.println("Set created from HashMap Keys contains :");
  31.     //iterate through the Set of keys
  32.     Iterator itr = st.iterator();
  33.     while(itr.hasNext())
  34.       System.out.println(itr.next());
  35.      
  36.     /*
  37.        Please note that resultant Set object is backed by the HashMap.
  38.        Any key that is removed from Set will also be removed from
  39.        original HashMap object. The same is not the case with the element
  40.        addition.
  41.     */
  42.    
  43.     //remove 2 from Set
  44.     st.remove("2");
  45.    
  46.     //check if original HashMap still contains 2
  47.     boolean blnExists = hMap.containsKey("2");
  48.     System.out.println("Does HashMap contain 2 ? " + blnExists);
  49.   }
  50. }
  51.  
  52. /*
  53. Output would be
  54. Set created from HashMap Keys contains :
  55. 3
  56. 2
  57. 1
  58. Does HashMap contain 2 ? false
  59. */