Java HashMap is HashTable based implementation of Map. This is the reason why interviewer always asks for the difference between HashMap and HashTable. HashMap is mostly equaled to HashTable except below two differences.

• HashMap is unsynchronized while HashTable is synchronized.
• HashMap permits null while HashTable doesn’t.

Important Property of HashMap

Creation of HashMap

When there is no parameter defined while creating HashMap default Initial Capacity(16) and Default load factor(0.75) will be used. This HashMap can contain up to 16 elements and resize of HashMap will occur when 13th element will be inserted. This is because the load factor is 75%(.75) and this threshold will be crossed when you add the 13th element(12+1).

You can also provide initial capacity and loadFactor. But initial capacity cannot be more than maximum capacity (2 power 30) and load factor cannot be zero or negative number.

Addition of element in HashMap

In order to add an element, you need to provide 2 things, key, and value.

Key: key with which specified value will be associated. null is allowed.

Value: value to be associated with a specified key.

First HashMap will generate a hashcode for given key and then check if there is any value already associated with given key or not. If yes then it will return already associated value. Else it will add value in HashMap with provided key.

Bullet Point

1. HashMap doesn’t give any Guarantee in order of elements in Map(Means Order can change over time).
2. HashMap provides Constant time performance for get & set operation(If proper Hashing algorithm is used).
3. Time requires to Iterate collection is proportional to “Capacity“(Elements it can hold) & Size(Elements it is holding currently) of HashMap.
4. In case of iteration performance is more important then it is advisable to not set initial capacity too high and load factor too low. As performance is directly proportional to Initial Capacity and load Factor.
   • capacity is the number of buckets in the hash table.
   • initial capacity(Default Value is 16) is simply the capacity at the time the hash table is created.
   • The load factor(Default value .75) is a measure of how full the hash table is allowed to get before its capacity is automatically increased.
   • When the number of entries in the hash table exceeds the product of the load factor and the current capacity, the hash table is rehashed (that is, internal data structures are rebuilt).
5. Use “Collections.synchronizedMap()” method to make Map synchronised.
6. Iterators returned by HashMap class is “fail-fast“.
7. HashMap is backed by an Array(Key) and LinkedList(Value).
8. HashMap uses hashcode(Using Key) to identify the exact location where an object should be placed or retrieved in HashMap.
9. In the end, HashCode returns the exact location(Index) in backing array.
10. Backed Array has a fixed size. So whenever an Array is full(Number of keys in this map reaches its threshold). A new Array with new capacity will be created and all element will be added to this new Array.
11. HashCode will be used in both cases(Adding and Retrieving Object) while equals() method may or may not be used in any case.
12. The best candidate for Key in HashMap would be an Immutable Class with properly implement Equals and Hashcode method(Example: String Class).
13. The better hashcode and equals method implementation is better performance of HashMap would be.
14. In such way, String and Wrapper classes of all Primitives will be a great candidate for keys in HashMap.

What is ReHashing

Every HashMap has predefined size (Initial Capacity) and a logic to increment this size(Load Factor) whenever required(When threshold limit crossed).

Example :

Create HashMap with below configuration

Initial Capacity = 16 (Default Initial Capacity)

Load Factor : .75 (Default load factor)

The moment you add the 13th element in given HashMap, Threshold limit is crossed for given HashMap and system will create a new backing keyset array(Size of this array will be double of the previous array). The system will have to again calculate exact bucket where elements from the previous bucket should be placed and all elements from old HashMap will be copied to new HashMap. This whole process is called ReHashing because Hashcode is calculated for each element again.

Because overtime HashMap might be reHashed and order could get change.

Example of HashMap

Here in this example, you will learn below points

1. How to iterate a Map
2. Different ways of iterating a Map

package map;

import java.util.HashMap;
import java.util.Map;
import java.util.Set;

public class HashMapDemo {
    public static void main(String[] args) {
        HashMap<String, Integer> hashMap = new HashMap<>();
        hashMap.put("Kobe", 95);
        hashMap.put("Jordan", 99);
        hashMap.put("Leborn", 98);

        hashMap.put(null, -1);


        for (Map.Entry<String, Integer> entry : hashMap.entrySet()) {
            System.out.println(entry.getKey() + ":" + entry.getValue());
        }

        System.out.println("------key set--------");

        Set keySet = hashMap.keySet();
        for (Object item : keySet) {
            System.out.println(item);
        }


    }
}

null:-1
Kobe:95
Jordan:99
Leborn:98
------key set--------
null
Kobe
Jordan
Leborn