`Hashtable`

This is an associative array: it associates keys (of any type) to values (of any type). HashMap is similar to Hashtable but not thread safe. It is slower.

With Hashtable, performance is reduced by internal synchronization (thread-checking) code. This makes it about 200% slower than HashMap on simple lookups (with containsKey).

Sample usage

This program shows how to create a new Hashtable with String keys, Integer values. We can use any class type for the keys and values. We use put and get.

Start Put() assigns a new entry in the Hashtable. The key is the first argument. The value is the second.

Next Get() looks up a value in the Hashtable from the key. Internally it computes a hash code from the key for faster searching.

import java.util.Hashtable;

public class Program {
    public static void main(String[] args) {

        // Create Hashtable and add two entries.
        Hashtable<String, Integer> hash = new Hashtable<>();
        hash.put("cat", 10);
        hash.put("dog", 15);

        // Get value at this key.
        int value = hash.get("cat");
        System.out.println(value);
    }
}
10

Benchmark, `Hashtable`

This program benchmarks the Hashtable against HashMap. We first initialize the 2 collections with put() method calls.

Version 1 This version of the code uses the Hashtable. It checks the collection for the string key "apple."

Version 2 This code does the same thing as version 1, but uses a HashMap collection instead.

Result The HashMap is faster, for simple lookups, by about 200%. Using HashMap when possible is an optimization.

import java.util.HashMap;
import java.util.Hashtable;

public class Program {
    public static void main(String[] args) throws Exception {

        Hashtable<String, Integer> table = new Hashtable<>();
        table.put("carrot", 10);
        table.put("apple", 20);

        HashMap<String, Integer> map = new HashMap<>();
        map.put("carrot", 10);
        map.put("apple", 20);

        long t1 = System.currentTimeMillis();

        // Version 1: check Hashtable.
        for (int i = 0; i < 1000000; i++) {
            if (!table.containsKey("apple")) {
                throw new Exception();
            }
        }

        long t2 = System.currentTimeMillis();

        // Version 2: check HashMap.
        for (int i = 0; i < 1000000; i++) {
            if (!map.containsKey("apple")) {
                throw new Exception();
            }
        }

        long t3 = System.currentTimeMillis();

        // ... Times.
        System.out.println(t2 - t1);
        System.out.println(t3 - t2);
    }
}53
17

In most programs, a HashMap is the better choice. If we have complex threading requirements, Hashtable may be necessary. HashMap has a significant performance advantage.

Retrofitted

Over the years, Hashtable has been retrofitted to support modern interfaces and generics in Java. And HashMap was introduced to provide an entirely new, faster option.

Hashtable

Sample usage

Benchmark, Hashtable

Retrofitted

`Hashtable`

Benchmark, `Hashtable`