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About Byron Kiourtzoglou

Byron Kiourtzoglou
Byron is a master software engineer working in the IT and Telecom domains. He is an applications developer in a wide variety of applications/services. He is currently acting as the team leader and technical architect for a proprietary service creation and integration platform for both the IT and Telecom industries in addition to a in-house big data real-time analytics solution. He is always fascinated by SOA, middleware services and mobile development. Byron is co-founder and Executive Editor at Java Code Geeks.

Get Set view of Hashtable keys example

In this example we shall show you how to get a Set view of Hashtable keys. To get a Set view of Hashtable keys one should perform the following steps:

  • Create a new Hashtable.
  • Populate the hashtable with elements, using put(K key, V value) API method of Hashtable.
  • Invoke keySet() API method of Hashtable. The method returns a Set that contains all the keys in the Hashtable. The key set is backed by the Hashtable, thus elements removed from the key set will be also removed from the Hashtable.
  • Note that it is not permitted to add an element to the resultant key set and java.lang.UnsupportedOperationException exception will be thrown in case we try to,

as described in the code snippet below.

package com.javacodegeeks.snippets.core;
 
import java.util.Hashtable;
import java.util.Set;
 
public class HashtableKeysCollection {
 
  public static void main(String[] args) {
 
    // Create a Hashtable and populate it with elements
    Hashtable hashtable = new Hashtable();
    hashtable.put("key_1","value_1");
    hashtable.put("key_2","value_2");
    hashtable.put("key_3","value_3");
    hashtable.put("key_4","value_4");
    hashtable.put("key_5","value_5");
 
    /* 

Set keySet() operation returns a Set containing all keys in Hashtable. 

The key set is backed by the Hashtable thus elements removed from the 

key set will also be removed from the originating Hashtable. 

Nevertheless it is not permitted to add an element to the resultant key set and 

java.lang.UnsupportedOperationException exception will be thrown in case we try to.
    */
    Set keySet = hashtable.keySet();

    System.out.println("keySet contains : " + keySet);

    keySet.remove("key_2");

    System.out.println("after removing key_2 from keySet, keySet contains : " + keySet + " hashtable contains : " + hashtable);
    
  }
}

Output:

keySet contains : [key_5, key_4, key_3, key_2, key_1]
after removing key_2 from keySet, keySet contains : [key_5, key_4, key_3, key_1] hashtable contains : {key_5=value_5, key_4=value_4, key_3=value_3, key_1=value_1}

 
This was an example of how to get a Set view of Hashtable keys in Java.

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