Let’s begin with the similarities of HashMap and HashTable:

• Both store key and value pairs in a hash table
• Both declare an object which are both declared as a key
• The key is hashed, and then this hash is indexed within the hash table it is stored

HashMap vs HashTable

HashMap

• None-synchronised. Not thread safe, unable to be shared between other threads.
• Allows for one null key and multiple null values
• Preferred if thread synchronisation is not needed
• Performance is high as threads are not required to wait
• Introduced in 1.2
• None legacy
• Not good for multithreaded environments

HashTable

• Synchronised. Thread safe, can be shared between other threads.
• Does not allow for null values in key or value
• Preferred if synchronisation is needed
• Performance is impacted as waiting for threads
• Introduced in 1.0
• Legacy
• Good for multithreaded environments

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HashMap Example

import java.util.HashMap;

public class HashMapExample {
    public static void main(String[] args) {
        HashMap<String, Integer> map = new HashMap<>();
        map.put("apple", 5);
        map.put("banana", 3);
        map.put("orange", 10);

        int value = map.get("banana");
        System.out.println(value); // Output: 3

        map.remove("orange");
        map.put("orange", 8);

        System.out.println(map); // Output: {apple=5, banana=3, orange=8}
    }
}

HashTable Example

import java.util.Hashtable;

public class HashTableExample {
    public static void main(String[] args) {
        Hashtable<String, Integer> table = new Hashtable<>();
        table.put("apple", 5);
        table.put("banana", 3);
        table.put("orange", 10);

        int value = table.get("banana");
        System.out.println(value); // Output: 3

        table.remove("orange");
        table.put("orange", 8);

        System.out.println(table); // Output: {apple=5, banana=3, orange=8}
    }
}