In this post, we will compare and contrast HashMap and Hashtable in Java, and explain their advantages and disadvantages.

A hash table is a data structure that maps keys to values using a hash function. It can allows fast and easy insertion, retrieval, and deletion of key-value pairs. Two of the most widely used types of hash tables in Java are HashMap and Hashtable, which both implement the Map interface.

1. Overview of HashMap in Java

HashMap is a class that implements the Map interface and uses a hash table as its underlying data structure to store key-value pairs. A key is an object that identifies a value, and a value is an object that contains some data. In HashMap, each key must be unique; however, duplicate values are allowed. The keys and values can be of any type. A HashMap offers many benefits:

  • HashMap allows fast access to elements by using the hash code of the keys. It uses an array of buckets to store the keys, where each bucket has a unique index calculated by applying a hash function to the key. When we insert or retrieve an element from the HashMap, we use the key to calculate the hash code and find the index of the bucket where the element is stored.
  • HashMap does not maintain any order of the elements in the map, unlike some other types of maps such as TreeMap or LinkedHashMap. This means that the elements in a HashMap are stored in a random order according to their hash codes.
  • HashMap allows null keys and null values in the map, unlike some other types of maps such as Hashtable or ConcurrentHashMap. This means that we can insert, retrieve, or remove null values from a HashMap without any problem.
  • HashMap provides additional methods to manipulate the map, such as put(), get(), remove(), containsKey(), containsValue(), size(), clear(), etc. These methods allow us to add, update, delete, or check elements in the map.

Let’s see an example of how to create and use a HashMap in Java:

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2. Overview of Hashtable in Java

Hashtable is a class that implements the Map interface and uses a hash table as its underlying data structure to store key-value pairs. In a Hashtable, keys and values can be of any type where each key must be unique but duplicate values are allowed. It has some limitations:

  • Hashtable is synchronized, which means that it is thread-safe and can be shared among multiple threads. However, this also means that it has a performance penalty due to locking overhead. If thread-safety is not required, it is better to use HashMap instead.
  • Hashtable does not allow null keys or null values in the map, unlike HashMap. This means that we cannot insert, retrieve, or remove null values from a Hashtable without throwing an exception.
  • Hashtable does not maintain any order of the elements in the map, unlike some other types of maps such as TreeMap or LinkedHashMap. This means that the elements in a Hashtable are stored in a random order according to their hash codes.
  • Hashtable provides the same methods as HashMap to manipulate the map, such as put(), get(), remove(), containsKey(), containsValue(), size(), clear(), etc. However, it also provides some legacy methods that are not recommended to use, such as keys(), elements(), contains(), etc.

Let’s see an example of how to create and use a Hashtable in Java:

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3. Difference between HashMap and Hashtable in Java

The functionality of HashMap and Hashtable are similar. They both use hashing to store and retrieve key-value pairs quickly. However, they also have some differences and trade-offs that we must understand. Here are some of them:

  • HashMap is a non-synchronized class, which means that it is not thread-safe and cannot be shared among multiple threads without proper synchronization code. Hashtable is a synchronized class, which means that it is thread-safe and can be shared among multiple threads.
  • HashMap allows null keys and null values in the map, whereas Hashtable does not allow any null key or value. If we try to insert or access a null value in a Hashtable, it will throw a NullPointerException.
  • HashMap is faster than Hashtable, as it does not have any locking overhead. However, HashMap may not be suitable for multithreaded applications, as it may cause data inconsistency or concurrency issues.
  • HashMap does not provide any legacy methods that are deprecated or not recommended to use, whereas Hashtable provides some legacy methods such as keys(), elements(), contains(), etc.
  • Hashtable is a legacy class and should not be used if the thread-safe implementation is not required. Javadoc recommends using ConcurrentHashMap in place of Hashtable for thread-safe highly concurrent implementation and HashMap class otherwise.
  • The iterator returned by the HashMap and Hashtable class is fail-fast. That means ConcurrentModificationException will be thrown if the map is structurally modified after the iterator is created without using the iterator’s remove() method. Hashtable is also traversed by Enumerator, which is not fail-fast. The results are undefined if the Hashtable is structurally modified after the enumeration is created.

4. What to use and when?

As we have seen, HashMap and Hashtable are both useful classes to store and retrieve key-value pairs quickly using hashing. Here are some general recommendations on how to choose between them:

  • If you need to store key-value pairs where each key is unique and can be used to access its associated value, we can use either HashMap or Hashtable. Both classes will provide the same functionality and performance for single-threaded applications.
  • If you need to store key-value pairs where each key is unique but also allow null keys or values, we should use HashMap. This class will allow us to insert, retrieve, or remove null values from the map without any problem.
  • If you need to store key-value pairs where each key is unique but also support multithreading, we should use Hashtable. This class will ensure that the map is synchronized and thread-safe and prevent any data inconsistency or concurrency issues.
  • If you need to store key-value pairs where each key is unique but also maintain the order of the elements according to their natural ordering or by a provided Comparator, we should use neither HashMap nor Hashtable, but use TreeMap instead. This class will provide a sorted map implementation that uses a red-black tree as its underlying data structure.

That’s all about the differences between HashMap and Hashtable in Java.