To perform some operation on all keys: the keys() or keySet() methods : Hashtable Basics « Collections « Java Tutorial

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2. Collections
3. Hashtable Basics

public Enumeration keys()
public Set keySet()

The keys() method returns the set of keys as an Enumeration.

The keySet() method returns the set of keys as a Set object.

Which you use depends upon what you wish to do with the keys.

import java.util.Enumeration;
import java.util.Hashtable;

public class MainClass {
  public static void main(String[] s) {
    Hashtable table = new Hashtable();
    table.put("key1", "value1");
    table.put("key2", "value2");
    table.put("key3", "value3");

    Enumeration e = table.keys();
    while (e.hasMoreElements()) {
      String key = (String) e.nextElement();
      System.out.println(key + " : " + table.get(key));
    }

    System.out.println(table.keySet());
  }
}

key3 : value3
key2 : value2
key1 : value1
[key3, key2, key1]