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How to Iterate Through HashTable in Java?

  • Last Updated : 20 Sep, 2021

HashTable is an underlying data structure where the insertion order in HashTable is not preserved, and it is based on the hashcode of keys. Duplicates keys are not allowed, but values can be duplicated. Heterogeneous objects are allowed for both keys and values. Value null is not allowed for both key and value otherwise we will get RunTimeException saying NullPointerException. It implements serializable and cloneable interfaces but not RandomAccess. Every method inside it is synchronized and hence HashTable objects are thread-safe. HashTable is the best choice if our frequent operation is search operation.

Methods:

There are various ways by which we can iterate through the HashTable which are as follows:

  1. Using Enumeration Interface
  2. Using keySet() method of Map and Enhance for loop
  3. Using keySet() method of Map and Iterator Interface
  4. Using entrySet() method of Map and enhanced for loop
  5. Using entrySet() method of Map and  Iterator interface
  6. Using Iterable.forEach() method  from version Java 8

Now let us discuss the internal implementation of all methods one by one in detail to get a better understanding of iteration through HashTable

Method 1: Using Enumeration Interface



java.util.Enumeration interface is one of the predefined interfaces, whose object is used for retrieving the data from collections framework variable. In a forward direction only and not in the backward direction. This interface has been superseded by an iterator.

Java




// Java Program to Iterate through HashTable
// using enumeration interface
 
// Importing required packages
import java.util.*;
import java.util.Enumeration;
 
// MAin Class
class GFG {
 
    // Main driver method
    public static void main(String[] args)
    {
 
        // Creating Hashtable object where key is of Integer
        // type and value is of String type
        Hashtable<Integer, String> ht = new Hashtable<>();
 
        // Putting key-value pairs to HashTable object
        // Custom input entries
        ht.put(1, "Ram");
        ht.put(2, "Shyam");
        ht.put(3, "Bijay");
        ht.put(4, "Hritik");
        ht.put(5, "Piyush");
 
        // Creating  Enumeration interface
        // and get keys() from Hashtable
        Enumeration<Integer> e = ht.keys();
 
        // Iterating through the Hashtable
        // object
 
        // Checking for next element in Hashtable object
        // with the help of hasMoreElements() method
        while (e.hasMoreElements()) {
 
            // Getting the key of a particular entry
            int key = e.nextElement();
 
            // Print and display the Rank and Name
            System.out.println("Rank : " + key
                               + "\t\t Name : "
                               + ht.get(key));
        }
    }
}
Output
Rank : 5         Name : Piyush
Rank : 4         Name : Hritik
Rank : 3         Name : Bijay
Rank : 2         Name : Shyam
Rank : 1         Name : Ram

Method 2: Using keySet() method of Map and Enhance for loop

The java.util.HashMap.keySet() method in Java is used to create a set out of the key elements contained in the hash map. It basically returns a set view of the keys, or we can create a new set and store the key elements in them. 

Java




// Java program to iterate through HashTable
// using keySet method and enhance for-loop
 
// Importing required packages
import java.util.*;
import java.util.Set;
 
// Main Class
class GFG {
 
    // Main driver method
    public static void main(String[] args)
    {
        // Creating Hashtable object in where key is of
        // Integer type
        // and value is of String type
        Hashtable<Integer, String> ht = new Hashtable<>();
 
        // Putting key-value pairs to HashTable object
        // custom entries
        ht.put(1, "Java");
        ht.put(2, "Scala");
        ht.put(3, "Python");
        ht.put(4, "Pearl");
        ht.put(5, "R");
 
        // Getting keySets of Hashtable and
        // storing it into Set
        Set<Integer> setOfKeys = ht.keySet();
 
        // Iterating through the Hahstable
        // object using for-Each loop
        for (Integer key : setOfKeys) {
            // Print and display the Rank and Name
            System.out.println("Rank : " + key
                               + "\t\t Name : "
                               + ht.get(key));
        }
    }
}
Output
Rank : 5         Name : R
Rank : 4         Name : Pearl
Rank : 3         Name : Python
Rank : 2         Name : Scala
Rank : 1         Name : Java

Method 3: Using keySet( ) method of Map and Iterator Interface

Again we will be using the same method as been implemented in the above example but here for iteration we will be using the Iterable interface 

Java




// Java program to iterate through HashTable
// using keySet method and Iterator Interface
 
// Importing required libraries
import java.util.*;
import java.util.Iterator;
import java.util.Set;
 
// Main class
class GFG {
 
    // Main driver method
    public static void main(String[] args)
    {
        // Creating Hashtale object where key is of Integer
        // type
        // and value is of String type
        Hashtable<Integer, String> ht = new Hashtable<>();
 
        // Putting key-value pairs to HashTable object
        // Custom input entries
        ht.put(1, "Java");
        ht.put(2, "Scala");
        ht.put(3, "Python");
        ht.put(4, "Pearl");
        ht.put(5, "R");
 
        // Getting keySets of Hashtable and
        // storing it into Set
        Set<Integer> setOfKeys = ht.keySet();
 
        // Creating an Iterator object to
        // iterate over the given Hashtable
        Iterator<Integer> itr = setOfKeys.iterator();
 
        // Iterating through the Hahstable object
        // Checking for next element using hasNext() method
        while (itr.hasNext()) {
 
            // Getting key of a particular entry
            int key = itr.next();
 
            // Print and display the Rank and Name
            System.out.println("Rank : " + key
                               + "\t\t Name : "
                               + ht.get(key));
        }
    }
}
Output



Rank : 5         Name : R
Rank : 4         Name : Pearl
Rank : 3         Name : Python
Rank : 2         Name : Scala
Rank : 1         Name : Java

Method 4: Using entrySet() method of Map and enhanced for loop

The java.util.HashMap.entrySet() method in Java is used to create a set out of the same elements contained in the hash map. It basically returns a set view of the hash map, or we can create a new set and store the map elements into them. 

Java




// Java program to iterate through HashTable
// using entrySet method and enhance for-loop
 
// Importing required libraries
import java.util.*;
import java.util.Map.Entry;
import java.util.Set;
// Main Class
class GFG {
 
    // Main driver method
    public static void main(String[] args)
    {
 
        // Creating Hashtable object where key is of Integer
        // type and value is of String type
        Hashtable<Integer, String> ht = new Hashtable<>();
 
        // Putting key-value pairs to HashTable object
        // Custom input entries
        ht.put(1, "Java");
        ht.put(2, "Scala");
        ht.put(3, "Python");
        ht.put(4, "Pearl");
        ht.put(5, "R");
 
        // Storing all entries of Hashtable
        // in a Set using entrySet() method
        Set<Entry<Integer, String> > entrySet
            = ht.entrySet();
 
        // Iterating through the Hashtable object
        // using for-each loop
        for (Entry<Integer, String> entry : entrySet) {
 
            // print ad display the Rank and Name
            System.out.println("Rank : " + entry.getKey()
                               + "\t\t Name : "
                               + entry.getValue());
        }
    }
}
Output
Rank : 5         Name : R
Rank : 4         Name : Pearl
Rank : 3         Name : Python
Rank : 2         Name : Scala
Rank : 1         Name : Java

Method 5: Using entrySet() method of Map and  Iterator interface

Again we will be using the same method as been implemented in the above example but here for iteration we will be using the Iterable interface 

Java




// Java program to iterate through hashtable using
// entrySet method and Iterator interface
 
// Importing required libraries
import java.util.*;
import java.util.Iterator;
import java.util.Map.Entry;
import java.util.Set;
 
// Main Class
class GFG {
 
    // Main driver method
    public static void main(String[] args)
    {
 
        // Creating Hashtable object where key is of Integer
        // type
        // and value is of String type
        Hashtable<Integer, String> ht = new Hashtable<>();
 
        // Putting key-value pairs to Hashtable object
        // Custom input entries
        ht.put(1, "Java");
        ht.put(2, "Scala");
        ht.put(3, "Python");
        ht.put(4, "Pearl");
        ht.put(5, "R");
 
        // Storing all entries of Hashtable in a Set
        // using entrySet() method
        Set<Entry<Integer, String> > entrySet
            = ht.entrySet();
 
        // Creating an Iterator object to
        // iterate over the given Hashtable
        Iterator<Entry<Integer, String> > itr
            = entrySet.iterator();
 
        // Iterating through the Hashtable object
        // using iterator
 
        // Checking for next element
        // using hasNext() method
        while (itr.hasNext()) {
 
            // Getting a particular entry of HashTable
            Entry<Integer, String> entry = itr.next();
 
            // Print and display the Rank and Name
            System.out.println("Rank : " + entry.getKey()
                               + "\t\t Name : "
                               + entry.getValue());
        }
    }
}
Output
Rank : 5         Name : R
Rank : 4         Name : Pearl
Rank : 3         Name : Python
Rank : 2         Name : Scala
Rank : 1         Name : Java

6. Using Iterable.forEach() method  from version Java 8

With the coming of havoc new features in version, 8. It has been Quite a while since Java 8 released. With the release, they have improved some existing APIs and added few new features. One of them is forEach() method in java.lang.Iterable Interface. 

Java




// Java program to iterate through HashTable
// using Iterable forEach()method of java 8
 
// Import required libraries
import java.util.*;
 
// Main Class
class GFG {
 
    // Main driver method
    public static void main(String[] args)
    {
 
        // Creating Hashtable object in where key is of
        // Integer type
        // and value is of String type
        Hashtable<Integer, String> ht = new Hashtable<>();
 
        // Putting key-value pairs to HashTable object
        // Custom input entries
        ht.put(1, "Java");
        ht.put(2, "Scala");
        ht.put(3, "Python");
        ht.put(4, "Ruby");
        ht.put(5, "R");
 
        // Iterating through Hashtable using
        // forEach loop of java 8
        ht.forEach((key, value)
                       -> System.out.println(
                           "Rank : " + key
                           + "\t\t Name : " + value));
    }
}
Output
Rank : 1         Name : Java
Rank : 2         Name : Scala
Rank : 3         Name : Python
Rank : 4         Name : Ruby
Rank : 5         Name : R

 

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