How Does a Java HashMap Manage Collisions from Objects with Identical Hash Codes?

How does Java HashMap handle collisions from different objects that share the same hash code?

// Example of a HashMap in Java
HashMap<String, Integer> map = new HashMap<>();
map.put("key1", 1);
map.put("key2", 2);

In Java, a HashMap is a collection that maps keys to values. It uses the hash code of each key to determine its position in an internal array. However, when multiple keys generate the same hash code—a situation known as a collision—HashMap employs a strategy to manage these collisions effectively. This ensures that each key remains distinct even if their hash codes are identical.

// Sample code to illustrate HashMap collision management
import java.util.HashMap;

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

        MyKey key1 = new MyKey(1);
        MyKey key2 = new MyKey(1); // Same hashcode as key1
        map.put(key1, "Value 1");
        map.put(key2, "Value 2"); // This will replace the first value

        System.out.println(map.get(key1)); // Output: Value 2
    }
}

class MyKey {
    private int id;

    public MyKey(int id) {
        this.id = id;
    }

    @Override
    public int hashCode() {
        return id; // Custom hashCode implementation
    }

    @Override
    public boolean equals(Object obj) {
        if (this == obj) return true;
        if (obj == null || getClass() != obj.getClass()) return false;
        MyKey myKey = (MyKey) obj;
        return id == myKey.id;
    }
}

Causes

• Two different objects can produce the same hash code due to the implementation of the hashCode() method (collision).
• Java's default implementation of hashCode() in Object class is not guaranteed to produce unique values for different objects.

Solutions

• Java HashMap uses a linked list or a balanced tree (since JDK 8) at each bucket index to store entries with the same hash code.
• When inserting a new entry, HashMap checks the bucket based on the hash code. If the bucket is occupied, it traverses linked entries to find the correct key using the equals() method to maintain uniqueness.

Mistake: Not overriding equals() method when overriding hashCode().

Solution: Always override equals() whenever you override hashCode() to maintain correct behavior of HashMap.

Mistake: Assuming all objects with identical values will have different hash codes.

Solution: Understand that identical values can produce the same hash code, and be prepared to handle collisions.