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How to Iterate Through HashTable in Java?
The HashTable is a fundamental data structure that operates on the basis of key hashcodes without preserving the insertion order. It prohibits duplicate keys but allows for duplicate values.
Remarkably, it accommodates a wide range of objects for both keys and values, fostering heterogeneity. Null values for keys and values are not allowed, though, as doing so would cause a RunTimeException called a NullPointerException.
The HashTable implements the serializable and cloneable interfaces in terms of interfaces, but it fails to implement the RandomAccess interface.
Additionally, all the methods within the HashTable are synchronized, ensuring thread safety for HashTable objects.
When it comes to selecting a data structure for frequent search operations, the HashTable emerges as the optimal choice.
Methods Used
We can iterate through the hashtable in a variety of methods, including −
By means of Enumeration Interface
With the help of keySet() method and enhance for loop
With the help of keySet() of the Map and Iterator Interface
Utilising the Map's entrySet() method and enhanced for loop
With the help of entrySet() method of Map and Iterator interface
Utilising the Iterable.forEach() method from Java 8
Method 1: By means of Enumeration Interface
The java.util.Enumeration interface is a pre-defined interface that serves the purpose of retrieving data from variables within the collections framework. It allows for data retrieval in a forward direction only and does not support backward traversal. It's important to note that the Enumeration interface has been replaced by the iterator.
nextElement()
The code creates a Hashtable. The code then populates it with key-value pair. Then,it iterates via the keys to print their corresponding values in descending order of the keys.
Algorithm
Step 1 − Import the required Java classes: "java.util" and "java.util.Enumeration".
Step 2 − Declare a class named "TLP".
Step 3 − Define the main method inside the "TLP" class: public static void main(String[] args).
Step 4 − Create a new instance of the Hashtable class named "ht" to store integer keys and string values.
Step 5 − Add key-value pairs to the Hashtable "ht" using the put method:
Step 6 − Create a new Enumeration object named "e" by calling the keys() method on the Hashtable "ht".
Step 7 − Enter a while loop with the condition "e.hasMoreElements()" to iterate through the keys in the Hashtable.
Step 8 − Within the while loop, retrieve the next key from the Enumeration with the usage of the nextElement() method and assign it to the variable "key".
Step 9 − Print the output message to the console. You do this with the usage of the System.out.println() method.
Step 10 −Terminate the program
Example
import java.util.*;
import java.util.Enumeration;
// Main Class
public class TLP {
public static void main(String[] args){
Hashtable<Integer, String> ht = new Hashtable<>();
ht.put(1, "Apple");
ht.put(2, "Strawberry");
ht.put(3, "Berry");
ht.put(4, "Guava");
ht.put(5, "Peach");
Enumeration<Integer> e = ht.keys();
while (e.hasMoreElements()) {
int key = e.nextElement();
System.out.println("Rank : " + key + "\t\t Name : " + ht.get(key));
}
}
}
Output
Rank : 5 Name : Peach Rank : 4 Name : Guava Rank : 3 Name : Berry Rank : 2 Name : Strawberry Rank : 1 Name : Apple
Method 2: With the help of keySet() method and enhance for loop
keySet()
The code prints the countries and their corresponding capitals stored in the Hashtable, with each pair displayed in the format "Country : [country] Capital : [capital]". The countries and capitals are printed in the order they were added to the Hashtable.
Algorithm
Step 1 − Create a new Hashtable object called "ht" to store string keys and string values.
Step 2 − Add key-value pairs to the Hashtable "ht" using the put method:
Step 3 − Obtain the set of keys from the Hashtable using the keySet() method and store it in the Set variable "setOfCountries".
Step 4 − Iterate through each key in the Set using a for-each loop.
Step 5 − Inside the loop, assign the current key to the String variable "key".
Step 6 − Print the output message. We do this with the usage of the System.out.println() method
Step 7 − End the program
Example
import java.util.Hashtable;
import java.util.Set;
public class TLP {
public static void main(String[] args) {
Hashtable<String, String> ht = new Hashtable<String, String>();
ht.put("India", "Delhi");
ht.put("Russia", "Moscow");
ht.put("Australia", "Canberra");
ht.put("USA", "Washingtin DC");
// getting keySet() into Set
Set<String> setOfCountries = ht.keySet();
// for-each loop
for(String key : setOfCountries) {
System.out.println("Country : " + key + "\t\t Capital : " + ht.get(key));
}
}
}
Output
Country : USA Capital : Washingtin DC Country : Russia Capital : Moscow Country : India Capital : Delhi Country : Australia Capital : Canberra
Method 3: Using keySet() of the Map and Iterator Interface
hasNext()
The code first creates a hashtable of states and their names. Then, it iterates through the hashtable and prints each state and its name.
Algorithm
Step 1 − Create a hashtable called ht and initialize it with five key-value pairs.
Step 2 − Get the keySet() of the hashtable and store it in a set called setOfKeys.
Step 3 − Create an iterator called itr for the setOfKeys.
Step 4 − Loop through the setOfKeys using the itr iterator:
Step 5 − Get the next element in the set, which is an integer key.
Step 6 − Get the name associated with the key from the hashtable.
Step 7 − Print the state and name.
Example
import java.util.*;
import java.util.Iterator;
import java.util.Set;
public class TLP {
// Main driver method
public static void main(String[] args){
Hashtable<Integer, String> ht = new Hashtable<>();
ht.put(1, "Telangana");
ht.put(2, "Karnataka");
ht.put(3, "Delhi");
ht.put(4, "Maharashtra");
ht.put(5, "Rajasthan");
Set<Integer> setOfKeys = ht.keySet();
Iterator<Integer> itr = setOfKeys.iterator();
while (itr.hasNext()) {
int key = itr.next();
System.out.println("State : " + key + "\t\t Name : " + ht.get(key));
}
}
}
Output
State : 5 Name : Rajasthan State : 4 Name : Maharashtra State : 3 Name : Delhi State : 2 Name : Karnataka State : 1 Name : Telangana
Method 4: Using Map's entrySet() method and enhanced for loop
while()
The given code creates a Hashtable and populates it with key-value pairs representing fruits and their corresponding colors. It then iterates through the entries in the Hashtable and prints each fruit with its respective color. The output displays the fruits and colors in the order they were added to the Hashtable.
Algorithm
Step 1 − Create a hashtable called ht and initialize it with three key-value pairs.
Step 2 − Get the entrySet() of the hashtable.
Step 3 − Create a for-each loop that iterates through the entrySet().
Step 4 − In each iteration of the loop, get the next element in the entrySet(), which is a Map.Entry object.
Step 5 − Get the fruit and color from the Map.Entry object.
Step 6 − Print the fruit and color.
Example
package in.bench.resources.hashtable.iteration.ways;
import java.util.Hashtable;
import java.util.Map.Entry;
import java.util.Set;
public class TLP {
public static void main(String[] args) {
Hashtable<String, String> ht = new Hashtable<String, String>();
ht.put("Apple", "Red");
ht.put("Guava", "Green");
ht.put("Lychee", "White");
Set<Entry<String, String>> entrySet = ht.entrySet();
for(Entry<String, String> entry1 : entrySet) {
System.out.println("Fruit : " + entry1.getKey() + "\t\t Color : " + entry1.getValue());
}
}
}
Output
Fruit : Guava Color : Green Fruit : Lychee Color : White Fruit : Apple Color : Red
Method 5: With the help of entrySet() method of Map and Iterator interface
hasNext()
The given code creates a Hashtable and adds key-value pairs representing ranks and their corresponding names. It then iterates through the entries in the Hashtable using an iterator and prints each rank with its respective name. The output displays the ranks and names in ascending order of the keys.
Algorithm
Step 1 − Create a hashtable called ht and initialize it with five key-value pairs.
Step 2 − Get the entrySet() of the hashtable.
Step 3 − Create an iterator called itr for the entrySet().
Step 4 − Loop through the entrySet using the itr iterator:
Step 5 − Get the next element in the set, which is a Map.Entry object.
Step 6 − Get the rank and name from the Map.Entry object.
Step 7 − Print the rank and name.
Example
import java.util.*;
import java.util.Iterator;
import java.util.Map.Entry;
import java.util.Set;
public class TLP {
public static void main(String[] args){
Hashtable<Integer, String> ht = new Hashtable<>();
ht.put(1, "Project Management");
ht.put(2, "Product Management");
ht.put(3, "Software Development");
ht.put(4, "Quality Assurance");
ht.put(5, "Product Strategy");
Set<Entry<Integer, String> > entrySet
= ht.entrySet();
Iterator<Entry<Integer, String> > itr
= entrySet.iterator();
while (itr.hasNext()) {
Entry<Integer, String> entry = itr.next();
System.out.println("Rank : " + entry.getKey() + "\t\t Name : " + entry.getValue());
}
}
}
Output
Rank : 5 Name : Product Strategy Rank : 4 Name : Quality Assurance Rank : 3 Name : Software Development Rank : 2 Name : Product Management Rank : 1 Name : Project Management
Method 6: Utilising the Iterable.forEach() method from Java 8
forEach()
The given code creates a Hashtable and adds key-value pairs representing union territories and their corresponding names. It then iterates through the entries in the Hashtable using the forEach method and prints each union territory with its respective name. The output displays the union territories and names in the order they were added to the Hashtable.
Algorithm
Step 1 − Create a hashtable called ht and initialize it with three key-value pairs.
Step 2 − Use the forEach() method to traverse through the hashtable.
Step 4 − Get the key and value from the hashtable.
Step 5 − Print the union territory and name.
Example
import java.util.Hashtable;
public class TLP {
public static void main(String[] args) {
Hashtable<String, String> ht = new Hashtable<String, String>();
ht.put("1", "Lakshadweep");
ht.put("2", "Delhi");
ht.put("3", "Pondicherry");
ht.forEach((key, value)->System.out.println("Union Territory : " + key + "\t\t" + "Name : " + value));
}
}
Output
Union Territory : 1 Name : Lakshadweep Union Territory : 2 Name : Delhi Union Territory : 3 Name : Pondicherry
Conclusion
There are various methods available to iterate through Hashtable in Java.From the usage of the Enumeration interface, the keySet() method with an enhanced for loop, the keySet() method with an Iterator interface, to the entrySet() method with an enhanced for loop, the entrySet() method with an Iterator interface, and the Iterable, we have discussed it all.
forEach() method introduced in Java 8. Each method offers flexibility and convenience in traversing the Hashtable based on specific requirements.
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