Java.Core.What means Hash code is cached, making hash-based collections faster. Where is that cached ?

In Java, the hash code of a String is computed only once and then cached, making lookups in hash-based collections (HashMap, HashSet, Hashtable) faster.

---

How Hash Code Works in String

• The hashCode() method in String computes a hash value based on the characters in the string.
• Since String is immutable, its hash code never changes once calculated.
• Java stores (caches) the computed hash code in an instance variable, so it doesn’t need to recalculate it every time hashCode() is called.

---

Where is the Hash Code Cached?

The String class has an instance variable (private int hash) that stores the cached hash code.

🔹 Inside the String class (java.lang.String):

public final class String implements java.io.Serializable, Comparable<String>, CharSequence {
    private final char value[];  // Character array storing the string data
    private int hash;  // Cached hash code (default 0)
    
    @Override
    public int hashCode() {
        int h = hash;  // Read cached hash
        if (h == 0 && value.length > 0) {  // If hash not computed yet
            for (char c : value) {
                h = 31 * h + c; // Compute hash using 31-based formula
            }
            hash = h; // Cache the computed hash
        }
        return h;
    }
}

👉 Key Observations:

• If hash == 0, the hash code is computed and cached.
• If hashCode() is called again, Java returns the cached value, avoiding recomputation.

---

Example: Cached Hash Code in Action

public class StringHashCodeExample {
    public static void main(String[] args) {
        String s = "Hello";

        // First call: hash code is computed and cached
        System.out.println(s.hashCode()); // Example output: 69609650

        // Second call: returns the cached value
        System.out.println(s.hashCode()); // Same output: 69609650
    }
}

👉 Benefit: No need to recalculate the hash every time, making operations faster.

---

Why is Caching Hash Code Useful in Hash-Based Collections?

When you use a String as a key in a HashMap or HashSet, Java frequently calls hashCode() for:

1. Finding the correct bucket (index) in the hash table.
2. Checking for equality in case of hash collisions.

Example: Using String in HashMap

import java.util.HashMap;

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

        map.put("Java", 1);
        map.put("Python", 2);

        // Uses the cached hash code to locate "Java"
        System.out.println(map.get("Java")); // Output: 1
    }
}

🛑 Without caching: Every lookup (map.get("Java")) would require recomputing the hash code.
✅ With caching: The hash is stored inside the string, making lookups faster.

---

Key Benefits of Hash Code Caching in Strings

Benefit	Explanation
Faster Hash-Based Lookups	hashCode() does not need to be recomputed every time.
Reduces CPU Workload	Prevents unnecessary recalculations, improving performance.
Improves HashMap, HashSet, Hashtable Efficiency	Faster retrieval of keys in hash-based collections.

---

Conclusion

✅ The hash code of a String is stored in a private hash variable to avoid recomputation.
✅ This caching mechanism makes HashMap and HashSet operations faster when String is used as a key.
✅ Since String is immutable, the cached hash code remains valid for the object’s lifetime.