How null key is stored in HashMap Java 8+?

 

In Java, HashMap has specific behavior when it comes to handling null keys and values:

✅ null Key Handling in HashMap

• A HashMap allows one null key.

• Internally, the null key is treated specially and stored in the first bucket (index 0).

• If you insert another entry with null as the key, it will overwrite the previous one.

✅ null Values

• A HashMap allows multiple entries with null values.

🔍 Example:

import java.util.HashMap;

public class NullKeyExample {
    public static void main(String[] args) {
        HashMap<String, String> map = new HashMap<>();

        map.put(null, "first");     // First null key
        map.put("one", "1");
        map.put(null, "second");    // Overwrites the previous null key

        System.out.println(map.get(null)); // Outputs: second
        System.out.println(map);           // Outputs: {null=second, one=1}
    }
}

⚠️ Notes:

• If you use other Map implementations (e.g., TreeMap or Hashtable), behavior differs:

  • TreeMap throws NullPointerException if you use null as a key (unless a custom Comparator supports it).

  • Hashtable does not allow null keys or values.

🧠 How HashMap Internally Handles null Key

When you call:

map.put(null, "value");

HashMap internally routes it differently than regular keys.

🔧 Behind the Scenes (Java 8+):

Here's roughly what happens in the put() method of HashMap:

public V put(K key, V value) {
    return putVal(hash(key), key, value, false, true);
}

🔎 hash(key):

static final int hash(Object key) {
    int h;
    return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
}

• If the key is null, it returns a hash of 0.

So the null key always hashes to bucket index 0.

📌 Special Handling in putVal():

In putVal():

1. If the key is null, it directly checks index 0.

2. If a node with null key is already present, it updates the value.

3. Otherwise, it adds a new Node at index 0.

if (tab[i = (n - 1) & hash] == null)
    tab[i] = newNode(hash, key, value, null);

Since hash = 0 for null key, i = 0, so it stores it in bucket 0.

📥 Retrieval with get(null)

Similarly, in get():

public V get(Object key) {
    Node<K,V> e;
    return (e = getNode(hash(key), key)) == null ? null : e.value;
}

Again, hash(null) returns 0, and it searches bucket 0.

It compares keys using == for null, avoiding equals() calls.

🧪 Quick Test

map.put(null, "value1");
System.out.println(map.get(null)); // Outputs: value1
map.put(null, "value2");
System.out.println(map.get(null)); // Outputs: value2

Each new put(null, ...) just overwrites the old one at index 0.