Java Collections – Hashtable Example

The Hashtable maps a key and a value. Hashtable has now been re-engineered to implement the Map interface and is roughly the equivalent of HashMap except that it does not permit nulls and is synchronized. In the example I have detailed, it will represent a mapping of the employee’s name and their yearly salary.

Big-O Notation

According to the Javadocs, this implementation provides constant-time O(1) performance for the basic operations (get and put), assuming the hash function disperses the elements properly among the buckets.

Creating a Hashtable

This was the old-way prior to Generics.

Hashtable employees = new Hashtable();

Generics

If you look closely, you will notice that I am using Generics to limit the type to String for the Key, and Integer for the Value in the Hashtable. Generics add stability to your code, by having the computer detect type incompatibilities during compile-time. These runtime bugs would be more difficult to debug if left unchecked.

Hashtable<String,Integer> employees = new Hashtable<String,Integer>();

Adding elements

Adding elements to the employees Hashtable is done by using the put(Object key, Object value) method. The key of the Hashtable will be the employee name which is a String and the value will be an Integer object. But wait? We are passing an Integer object we are passing an int primitive and Java is not complaining and we did not have to use new Integer(…). Why is this working?

Auto-Boxing and Unboxing

Autoboxing is the automatic conversion that the Java compiler makes between the primitive types and their corresponding object wrapper classes. For example, converting an int to an Integer and vice-versa without the need to cast. In this case, Java is performing the boxing when we put elements into the Hashtable (converting int to Integer wrapper class) and unboxing when we get() elements from the Hashtable (converting Integer to int primitive).

employees.put("John Smith", 85000);
employees.put("David Harvey", 53000);
employees.put("James Young", 103500);
employees.put("Danielle Gray", 94250);
employees.put("Jeff Wang", 76500);
employees.put("Chris Canning", 150000);
employees.put("Mary Anderson", 104100);
employees.put("Tina Mayer", 143700);

Removing elements

Removing elements is just a matter of calling the remove(Object key) method.

employees.remove("Jeff Wang");
employees.remove("Mary Anderson");

Size of Collection

Returning the number of elements in a Hashtable is as easy as calling the size() method.

employees.size();

Iterating through the Collection

Java 1.5 and above provides a foreach loop, which makes it much easier to iterate over the entire collection. This is my preferred way of doing it.

// Loop through the collection of employees
for (String name : employees.keySet()) {
   // Get the salary by passing the employee name as the key,
the salary is Auto-UnBoxed
int salary = employees.get(name); }

Iterating through the Collection with Generics

Hashtable<String,Integer> employees = new Hashtable<String,Integer>();
    
Iterator iterator = employees.entrySet().iterator();
while (iterator.hasNext()) {
  Map.Entry<String,Integer> element = (Map.Entry) iterator.next();
  String name = element.getKey();
  int salary = element.getValue();
  System.out.format("Employee==> %-14s | %8dn", name, salary);
}

Iterating through the Collection without Generics

Hashtable employees = new Hashtable();
    
Iterator iterator = employees.entrySet().iterator();
while (iterator.hasNext()) {
  Map.Entry element = (Map.Entry) iterator.next();
  String name = (String) element.getKey();
  int salary = ((Integer)element.getValue()).intValue();
  System.out.format("Employee==> %-14s | %8dn", name, salary);
}

Please Note

Using a Hashtable does not guarantee the order of the elements.

Full Program Listing

package com.avaldes.tutorials;

import java.util.Hashtable;
import java.util.Map;

public class HashTableExample {

  public static void main(String[] args) {
    // Ensure that the Hashtable only takes String for the key and Integer for the value
    Hashtable<String,Integer> employees = new Hashtable<String,Integer>();
    
    // Populate the employee Hashtable  -- Auto-Boxing 
    employees.put("John Smith", 85000);
    employees.put("David Harvey", 53000);
    employees.put("James Young", 103500);
    employees.put("Danielle Gray", 94250);
    employees.put("Jeff Wang", 76500);
    employees.put("Chris Canning", 150000);
    employees.put("Mary Anderson", 104100);
    employees.put("Tina Mayer", 143700);
    
    
    System.out.println("Display All Employees -- prior to delete operation...");
    displayAllEmployees(employees);
    System.out.println("Number of Employees: " + employees.size());
    
    // See if someone with name 'James Young' is in the employees hashtable
    System.out.println("nCheck to see if James Young is found in employees instance: " 
        + employees.containsKey("James Young"));
    // See if someone with name 'Amaury Valdes' is in the employees hashtable
    System.out.println("Check to see if Amaury Valdes is found in employees instance: " 
        + employees.containsKey("Amaury Valdes"));
    // Is employees instance empty? 
    System.out.println("Check to see if employees instance is empty: " 
        + employees.isEmpty());
    
    // Let's remove employee Jeff Wang & Mary Anderson
    System.out.println("nRemoving Jeff Wang from employees");
    employees.remove("Jeff Wang");
    System.out.println("Removing Mary Anderson from employeesn");
    employees.remove("Mary Anderson");
    
    System.out.println("Display All Employees -- after delete operation...");
    displayAllEmployees(employees);
    System.out.println("Number of Employees: " + employees.size());
  }
  
  static public void displayAllEmployees(Hashtable<String,Integer> employees) {
    int count = 0;
    
    // Loop through the collection of employees 
    for (String name : employees.keySet()) {
      count++;
      // Get the salary by passing the employee name as the key, the salary is Auto-UnBoxed
      int salary =  employees.get(name);
      // Format the output nicely
      System.out.format("Employee==> [%d]  %-14s | %8dn", count, name, salary);
    }
  }
}

Output

Display All Employees -- prior to delete operation...
Employee==> [1]  Jeff Wang      |    76500
Employee==> [2]  Mary Anderson  |   104100
Employee==> [3]  James Young    |   103500
Employee==> [4]  Tina Mayer     |   143700
Employee==> [5]  John Smith     |    85000
Employee==> [6]  David Harvey   |    53000
Employee==> [7]  Chris Canning  |   150000
Employee==> [8]  Danielle Gray  |    94250
Number of Employees: 8

Check to see if James Young is found in employees instance: true
Check to see if Amaury Valdes is found in employees instance: false
Check to see if employees instance is empty: false

Removing Jeff Wang from employees
Removing Mary Anderson from employees

Display All Employees -- after delete operation...
Employee==> [1]  James Young    |   103500
Employee==> [2]  Tina Mayer     |   143700
Employee==> [3]  John Smith     |    85000
Employee==> [4]  David Harvey   |    53000
Employee==> [5]  Chris Canning  |   150000
Employee==> [6]  Danielle Gray  |    94250
Number of Employees: 6

Other Related Posts

Map Examples

List Examples

Set Examples

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